DESIGN OF THECOUDE ECHELLE SPECTROMETER

FOR THE ESO 3.6 mAND CAT TELESCOPES

D. ENARD

This report is published by the

European Southern Observatory

Telescope Project Division

c/o CER N

1211 Geneva 23

Switzerland

The European Southern Observatory (ESO) is the result of a

scientific collaboration for astronomical research in the Southern

Hemisphere between the six member countries, Belgium, Denmark,

Federal Republik of Germany, France, Netherlands and Sweden.

Copyright © ESO 1979

DESIGN OF THE COUDE ECHELLE SPECTROMETER

FOR THE ESO 3.6 m AND CAT TELESCOPES

D. ENARD

SEPTEMBER 1979

A B S T R A C T

The Coude Echelle Spectrometer for the ESO 3.6 m telescope was

designed as an integral part of the existing and planned spectroscopic

instrumentation of the 3.6 m and the other telescopes on La Silla. It

will provide facilities for spectroscopic observation at very high

spectral resolutions, with the additional features of good photometrie

accuracy and a clean instrumental profile. Since it can be used together

with the 1.4 m Coude Auxiliary Telescope as weil as with the 3.6 m

itself, it is expected to be in year-round operation.

The spectrometer is based on a 20 cm echelle grating with

79 lines/mm, blazed at 63026'. Order separation is effected by a prism

pre-monochromator. In order to minimize reflection losses two

completely separate optical paths are provided, each with its own

monochromator and optimized for the blue and red, respectively. The

main use of the spectrometer is expected to be in the multichannel

mode, where an f/S camera gives a fixed maximum resolution of 100 000

( 40 mR) when used with single-row, 1872-element DIGICON (UV-visual)

or cooled RETICON (red-infrared) array detectors. Considerable

flexibility in the choice of resolution, spectral range, and detector

sensitivity is achieved in the scanning mode, where an f/30 camera in

a straightforward Czerny-Turner arrangement focuses the spectrum onto

an exit slit, where various types of photomultiplier may be used as a

detector. In the scanning mode, tilting a single optical element

changes the instrument into a true double-pass spectrometer, an inter­

mediate slit blocking off the faint outer extensions of the single­

pass instrumental profile. Scanning is accomplished by rotating the

grating on a high-precision, air-bearing turn-table, with possible

scanning frequencies up to S Hz for optimum elimination of seeing

variations. Figs. 1 and 2 show the optical schematic of the instrument.

The instrument is thus expected to meet the most stringent demands of

modern model atmosphere, spectrum synthesis, and detailed abundance

analysis techniques with regard to resolution, spectral purity, and

photometrie aeeuracy, but it should at the same time be a versatile

tool for a wide range of speetroseopie observing programmes. The

estimated limiting magnitude for a l-hour integration in the multi-m

ehannel mode (with the DIGICON) is about 9 , assuming a photometrie

aeeuraey in the eontinuum of 3% per ehannel.

The whole speetrometer is eomputer-eontrolled, and an effort is

made in designing the software to keep routine operation as simple and

eonvenient as possible while retaining the option of non-standard

modes of use. Onee a few parameters have been entered, normal

operation of the instrument or the versatile eontrols for the on-line

display of the ineoming data should be earried out mainly by means

of a few push-buttons, keeping night-time typing to aminimum.

C 0 N T E N T S

5. MULTICHANNEL MODE

3. ORDER SEPARATION - PRE-DISPERSER

4. SPECTROMETER - SCANNER

INTRODUCTION

1. TELESCOPE" ADAPTATION - CAT FOCAL REDUCERS

2 • ENTRANCE SLIT AND SLIT ENV IRONMENT

5.1 Choice of the detector

5.2 Optimization of the camera aperture

5.3 Camera

5.4 Digicon

1

3

4

4

6

7

7

7

8

11

11

15

15

15

16

17

18

18

18

20

21

22

22

23

23

24

25

26

Basic geometrical data

Dispersion

Optica1 quali ty

Requirements

Description

Operation

Performances

Princip1e

Optica1 data of scanner

Detector

System performances

Spectral resolution

Grating turn-tab1e

2.1 Acquisition and s1it viewing

2.2 Ca1ibration

2.3 Sli t

2.4 Decker

2.5 Image s1icers

4.1

4.2

4.2.1

4.2.2

4.2.3

4.3

4.4

4.4.1

4.4.2

4.4.3

4.4.4

4.5

4.6

5.4.1 Description

5.4.2 Predicted Digicon performances

5.5 Reticon

5.5.1 Description

5.5.2 Performances

6. CONTROL AND INSTRUMENT OPERATION

RELATIVE EFFICIENCIES OF DIFFERENT OPTICAL ARRANGEMENTS(ANNEX 1)

HIGH EFFICIENCY COATIN3S (ANNEX 2)

26

27

29

29

29

30

35

37

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INTRODUCTION

A Coude Echelle Spectrograph (CES) was foreseen from the start of

the 3.6 m telescope project to provide a facility for high resolution

spectroscopy. Considering the high cost of this type of equipment, the

long exposure time generally required and the limited observing time

available from the 3.6 m telescope, it was decided to also build a

dedicated auxiliary telescope for coude operation only. This Coude

Auxiliary Telescope (CAT) which is designed and built in parallel to

the instrument is a 1.4 m telescope and although much less powerfu1 than

the 3.6 m telescope will have the great advantage of being avai1ab1e

every night.

The foundation of the spectrograph was laid down by a review team

who met the first time in June 1976 and adopted a general proposal

estab1ished by Guy Ratier (at that time an ESO Staff Member) .

The review team in charge of fo11owing up the development of the

instrument ti1l its comp1etion is composed of the fo1lowing astronomers:

Dr. J. Andersen, University Observatory Brorfelde, Denmark

Prof. Ch. Fehrenbach, Observatoire de Haute Provence, France

Dr. H. Nieuwenhuijzen, Sterrewacht "Sonnenborgh", Nether1ands

Dr. E.H. Schröter, Kiepenheuer Institut für Sonnenphysik,

Federal Repub1ic of Germany

Dr. J. Solf, Max-P1anck Institut, Federal Repub1ic of Germany

The instrument is now (in October 1979) near its final comp1etion

and is scheduled to be availab1e for visiting astronomers at the end of

1980.

The work presented in this report is indeed the result of a common

effert ef severa1 peep1e among them:

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B. Amrhein, responsible for the electronics,

R. Clop, responsible for the mechanics,

M. Le Luyer, responsible for the optical calculation,

P. Rossignol, responsible for the software.

Although it is difficult to mention all those who also participated

in the project one should also cite B. Forel, F. Franza, M. Wensveen

and also thanks to the astronomers who have a particularly long

experience with this type of instrument and from whom very much was

learned:

Dr. A. Baranne, Observatoire de Marseille, France

Dr. E. Richardson, Dominion Astrophysical Observatory, Victoria,

Canada

Dr. R.G. Tull, University of Texas, USA

- 3 -

1. TELESCOPE ADAPTATION - CAT FOCAL REDUCERS

The CES can be fed by the 3.6 m telescope and the 1.4 m Coude

Auxiliary Telescope (CAT).

The f/32.3 aperture of the 3.6 m is well adapted to a large

spectrograph and the beam is used directly. The CAT has an aperture

of f/120 and requires adaptation. The aperture adaptation could have

been achieved by a single optical element; however, we preferred a

mo~e rational solution which provides for both telescopes the same

aperture and the same pupil position. A condition specific to the

CAT was that the position of the focal plane before and after the

focal reducer should not move more than the focusing range of the

telescope.

The optical parameter at the entrance of the CES:

. ~._&_.- ---------APERTURE PUPIL FOCAL LINEAR

POSITION LENGTH SCALE---------_. --- - -- --- .-

3.6 m 32.3 -35.7 m 115.3 m 0.576 mm/arcsec

CAT 32.3 -35.7 m 45.22 m 0.226 mm/arcsec

The optical design of the CAT focal reducers is based on the

minimallens combination which uses one simple lens followed by a

doublet (Fig. 3). Unfortunately this combination is rather clumsy.

The two optical elements are mounted on barrels fixed at both ends

of a tube passing through the wall of the slit room.

Each barrel has three positions. Two for the blue and red elements

of the focal reducers and one for the 3.6 m telescope: a glas shutting

plate may be used if the effects of the turbulence between the slit

room and the dome caused by thermal exchange, appear to be important.

Barrel posit1oning 1s manual but position is 1ndicated to the computer

by switches.

- 4 -

Dry nitrogen may be circulated within the tube, to prevent water

vapour condensation on the windows, due to a temperature difference

between the slit and the dome.

Fig. 4

Fig. 5a + b

Illustration of the focal reducers' support.

Illustration of the theoretical spot diagram for different

wavelengths and distances from the axis. At the edges of the

field the quality is far from perfect, but is sufficient

for star acquisition. On axis, it is possible to focus the

telescope according to the wavelength used and to obtain

a diffraction limited image. In practice, turbulence will

Fig. 6a + b

seriously limit this possibility.

Illustration of the chromatism of position. Even considering

the small aperture it is wise to focus the telescope using

a filter corresponding approximately to the spectral region

concerned. Focusing in infrared is impossible, because

the T.V. camera which has an S20 photocathode is not sensitive.

In that case a fine focusing can be achieved by using an

offset value.

2. ENTRANCE SLIT AND SLIT ENVIRONMENT

This part of the instrument provides the following functions:

TV (or eye piece) large field acquisition,

TV (or eye piece) guiding on the slit,

Spectral and photometric calibration,

Remote control of slit width and decker position,

-, Use of Richardson's type image slicers.

2.1 ACquisition and slit viewing

Fig. 7 shows the optical schematic of the slit environment. A

mirror of nearly 450 inclination deflects the telescope beam towards

- 5 -

a field mirror. The image of the sky is then projected onto a TV

camera (QUANTEX QX 26 - ISIT tube). When the star is recognised and

centered on the screen, the mirror can be rotated out and the beam

reaches the entrance slit and the decker. The beam is reflected by the

slit and the decker and an objective and mirror send it into the same

optical path as for the acquisition. Also foreseen is a rear viewer

which, with a similar system, images the slit seen from the back on the

TV camera. It is not shown on Fig. 7 for more clarity.

The different fields of view given by this system are:

- - - .. ....._. -- .. _..- '.- ...._-_ ..

3,6 m CATr-.---- ...... -

.. ~ield of view 1,1 x 1,45 2,8 x 3,7arcm~n

-- - -- ---------- ---- -- -- - . -- ----- - _..- .._---_. ....Acquisition

Scale on 'IV 0,144 mm/arcsec 0,056 mm/arcsecphotocathode 9 lines/arcsec 3,6 lines/arcsec

1---- ..---- ... ... ._- .. -.. -

Field of view22 29 57 76x x

Slit arcsec

viewingScale TV 0,42 mm/arcsec 0,165 mm/arcseconphotocathode 27 lines/arcsec 11 lines/arcsec

Field of view 6,6 x 8,7 17 x 23

slitarcmin

Rearviewing

Scale on TV 1,4 mm/arcsec 0,55 mm/arcsecphotocathode 90 lines/arcsec 36 lines/arcsec

Expected limiting magnitude in acquisition mode is about 15 with the

3.6 m and 17 with the CAT.

A manual turret of 6 filters is set before the camera. 3 coloured

filters (blue, green, red) and one UBK7 plate are foreseen, two other

positions are available.

The transfer objective can be focused from the instrument console.

- 6 -

In case of fai1ure of the TV camera, an eye-piece can be used,

without co10ured filters. An F = 16 rnrn eye-piece provides about the

same fie1d of view as that of the diagonal of the corresponding TV

fie1d of view.

TV camera, 1arge fie1d mirror setting and objective focusing are

contro11ed from the conso1e independent1y of the computer which

neverthe1ess knows the mirror position. Switching from eye-piece to TV

and filter changes are manual.

2.2 Ca1ibration

The ca1ibration unit images an aperture onto the entrance slit.

The pupi1 imaging is identica1 to that of the te1escope to minimise

errors due to a different illumination of the grating during observation

and ca1ibration. Severa1 ca1ibration sources are disposed around this

aperture. A 450 mirror rotating around the optica1 axis se1ects one

of the sources whose emittive area is projected onto the aperture.

A 450 mirror set on the te1escope axis projects the ca1ibration beam

onto the entrance slit and, when turned off, c10ses the ca1ibration

unit in such a way that the sources can be switched on during observation

for warrning up. Provision has been made for 8 sourees. So far the

fo110wing have been attributed:

l. Fe-Ne Ho110w cathode 1amp,

2. Th-Ne Ho110w cathode 1arnp,

3. Ne 10w pressure 1amp,

4. Hg 10w pressure 1amp (for a1ignment),

5. Quartz iodine 1amp BV/50W (28000 K) , for

photometrie ca1ibration,

He-Ne Laser ))

Hg 198 Isotopic 1amp )

8. avai1ab1e

both for instrument profilerecording

- 7 -

Switching and selection of the sources and setting the 450

mirror are controlled from the console. Selected source and mirror

position are known by the computer.

2.3 Slit (Fig. 8)

Two symmetric jaws provide an aperture variable between 50~

and 5 mm. The maximum height is 30 mm. The two jaws are coated with

a silica-protected aluminium layer. The unit is remote controlled and

although the width is normally set by the computer, a manual setting

is possible.

2.4 Decker

A reflecting mask is set at a short distance from the slit

(0.5 mm) - total stroke is 50 mm. The mask being reflecting, the

complete field of view can be observed without removing the mask.

The present mask is shown in Fig. 9. An inclined slot allows

centering of the spectrum onto the detector. Considering the different

scales of the two telescopes, two slots are provided which give a

4 arcsec spectrum height with each telescope. This mask is adapted

to single array detectors; it can be easily changed for other detectors.

The decker position is also remote controlled from the computer in the

same way as the slits.

2.5 Image slicers

The slit and the decker can be removed and replaced by an image(1)

slicer of fixed aperture. Two I.S. of the RICHARDSON type are

foreseen. One will be dedicated to the blue spectral range, the other

to the red. They are well adapted to array detectors. The characteristics

of these I.8.'s are:

Number of slices = 3

81it width 0,2 mm (0,89 and 0,35 arcsec)

Projected area on the sky: CAT: 2,6 x 6,6 arcsec3.6 m: 1 x 2,6 arcsec

- 8 -

3. ORDER SEPARATION - PRE-DISPERSER

The separation of orders of an echelle grating is quite crucial.

A very classical solution consists to use a cross disperser - generally

a grating - to disperse the light in a direction perpendicular to the

echelle spectrum. The result is a set of parallel spectrums corresponding

to the different orders of the echelle grating. All the spectral

information is recorded if a two-dimension detector is used. The

spectrums are slightly curved and the spectral lines correspondingly

inclined due to the combination of the two dispersions. Therefore,

processing of data becomes delicate. It was a deliberate choice in

h . f . t 1 (2)t at ~nstrument to avour accuracy aga~nst spec ra coverage •

A pre-filtering, having a band-pass smaller than the free spectral

range of the echelle grating has been preferred to a cross dispersion.

Then the final spectrum is rectilinear easy to process but limited in

length to the free spectral range of the grating. Another advantage

is that reducing the total quantity of light introduced in the

instrument, stray light is reduced and spectral purity improved.

This filtering must isolate a spectral band inferior to the free

spectral range of the echelle grating which with a 79 grimm varies .

from 50 Rat 350 nm to 350 Rat 900 nm. Several solutions have been

examined.

Interference filters would have been an elegant solution if

their transmission was greater in V.V. and blue. They have not

been considered a valid solution.

Grating monochromator. Although a compact and straightforward

solution, gratings have the disadvantage of being less efficient

than prisms, when angular dispersion is not a critical parameter.

Prism monochromator. We rapidly came to the conclusion that this

was the correct principle to apply. In order to choose a convenient

arrangement we took into account the following constraints:

- 9 -

have the minimum of active surfaces,

avoid refractive solutions which always introduce

additional problems of chromatism,

keep the exit beam with approximately the same orientation as

the telescope beam (because of the coude room dimensions) ,

avoid as far as possible a change of orientation of the

exit beam when varying the wavelength,

put the pre-disperser spectrum parallel to the echelle's.

(A cross dispersion would give a spectrum with a variable

position difficult to compensate) .

The dispersion necessary depends on the wavelength and on the

slit width.

f 01.. hI (5X ~s t e

should have

Fig. 10 shows how the output from the exit slit looks.

reciprocal linear dispersion of the pre-disperser one

S2 + SI < tHA~

OX

>6Ü

S2 - SI~

OX

where 611. is the free spectral range, 621. is the useful spectral range,

and SI - S2 the entrance and exit slit widths. With an entrance slit

of 1 arcsec (with the 3.6 m), the 79 grimm echelle grating and a 30 mm

long detector with an f/S camera, one is led to a minimum dispersion

of about 47 R/mm and at 3850 Rand 320 R/mm at 9300 R. However, higher

dispersion could increase the possibility of working with a larger

slit on a different camera/detector combination.

The final choice was for a sYmmetrical mirror arrangement using

a Littrow prism as a dispersive element. The relative low dispersion

of a prism was compensated without severe inconvenience by increasing

the collimator focal length.

- 10 -

The final characteristics of the monochromator are:

focal length of collimators:

relative aperture:

prism material:

prisn angle:

angle between beams:

2 meters

f/32,3

LLFl glass

22 degrees

3 degrees

Considering the low priority put on UV below 3500 i, a dispeLsive

glass was preferred to silica. Therefore, a smaller angle gives a

sufficient dispersion and incidence angle on prism is kept reasonable,

thus limiting light losses by reflexion.

Figure captions

Fig. 11:

Fig. 12:

I

Fig.13:

Fig. 14:

Reciprocal linear dispersion of the monochromator.

Schematic of the optical arrangement. In order to deviate

the beam back towards the slit room, an additional mirror

was necessary. This mirror is in fact a spherical mirror

(R = 10,6 meters) which acts as a field mirror. Thus no

additional element is necessary to image the pupil on the

grating •. The output aperture is slightly modified and is

f/29. The split into red and blue paths is obtained by

exchanging collimators mounted on a 4-position turret

(normally only two are used). From there, the two paths

are superposed and each optical element is duplicated:

there are two prisms, two camera mirrors, two field mirrors

and two slits. The two field mirrors are oriented in such

a way that the beams converge at the same place on the

spectrometer collimators (Fig. 1).

Prisn glass transparency for the average thickness

(double pass).

Spot diagrams showing the theoretical optical quality of the

monochromator. Inclination of astigmatic line is compensated

in the spectrometer.

- 11 -

Control: Exit slits are identical to the entrance slit and controlled

by the computer in the same way. The exit slit width is calculated by

the computer according to the following:

k is a coefficient experimentally determined which makes sure that

no light outside the spectral range makes its way through. This

coefficient takes account of the monochromator quality, of the prism

positioning accuracy and of different misalignment and defocusing

effects.

The two prisms are set on the same rotating table which is

controlled by a lead screw driven by a OC torque motor and a linear

encoder (Fig. 15). The position of the encoder is experimentally

determined for several reference wavelengths and the control program

interpolates between those positions to determine the prism position

corresponding to the requested central wavelength.

Position of collimator turret is read by the computer from

position indicators.

4. SPECTROMETER - SCANNER

4.1 Principle

The quality of echelle gratings has improved considerably during

the last decade. However, they are not completely ghost-free, and

it has been considered essential to have the possibility of working

with a very pure instrumental profile. This is only possible with a

double pass scanning mode in which the diffracted beam is focused on an

intermediate slit before being diffracted off once again by tl1e grating.

The ghosts and grass of the echelle are then almost completely removed,

but the efficiency of the system is low, limiting its application to

very bright objects.

- 12 -

Consequently, the spectrometer ought to be provided with a multi­

channel mode which remains the basic working mode. It relies on multi­

channel detectors whose best example is the spectrographic plate.

However, the instrument has been optimized for electronic detectors

whose efficiency is higher although being of limited dimensions, the

spectrum'length is correspondingly limited.

The main design parameters which have been taken into consideration

are:

a) easy switch from multichannel to scanner, and from single pass

to double pass.

b) high efficiency. Thus, limited number of optical elements.

c) no (or very limited) refractive elements to ensure a large

spectral range.

d) no necessity for adjustment in operational use (implying design

parameters should not be over-critical) •

e) should fit the coude beam position (mean height 2 meters from

ground) •

f) leave a possibility for future improvement (larger echelle grating

or mosaic of gratings) •

The optical arrangements are in practice very limited (Fig. 16):

"Czerny-Turner perpendicular" (often called Littrow): The incident

and diffracted beams are separated along a direction perpendicular

to the dispersion. The main inconvenience is that it introduces a

li~e rotation which is the visible part of the more general line

curvature. On the other hand, this is the arrangement where the

grating is considered to be the most efficient.

- 13 -

Czerny-Turner: This is the classical mounting which gives a fully

coma-corrected field and no line rotation. The efficiency of the

echelle is reduced depending on the arrangement (see annex 1) •

This arrangement can be set up in a horizontal plane thus

facilitating mechanical design and alignment. A greater mechanical

flexibility can also be expected.

Baranne: The white pupil mounting projects an image of the pupil on

the camera, thereby eliminating the vignetting and improving the

optical quality of the camera.

The "white pupil" is of little advantage if one considers that

electronic detectors have limited dimensions, and that consequently

the field of view of the camera is small and the vignetting not a very

great problem. On the other hand, the double pass scanning mode is

difficult to achieve with this arrangement, and the efficiency is

reduced by the number of optical elements which is significantly higher

than with a classical mounting. It has not been considered an

interesting solution for this particular case.

The choice between the first two arrangements is somewhat difficult

since it depends on the priority one gives to the different design

parameters. After some hesitation the Czerny-Turner arrangement was

finally selected. It offers more mechanical flexibility becau~e space

is available below and above the beams. For instance a change of the

grating size or of the detector may be more easily accomplished, and

it was considered essential that a Coude Spectrometer be kept as

flexible an instrument as possible.

There are two different ways of using the same in-plane mounting,

depending whether the diffraction angle is larger or smaller than the

incidence angle. A discussion and experimental results are given in

annex 1 and results in a preference to have the diffraction angle

smaller.

- 14 -

The two beams coming from the two exit slits of the pre-disperser

are converging at the same place on the collimator. There are two

exchangeable collimators and two camera mirrors, one for the blue, the

other for the red. Each is adjusted in such a way that the beams

follow exactly the same path up to the exit slit.

When using the double pass mode, the camera mirror is tilted and

the beam deflected towards a total reflexion prism before reaching the

intermediate slit, after which another prism deflects the beam again

on to the collimator and grating. In order to optimize the anti­

reflexion coatings of the prism to the spectral range, there are, in

fact, two prisms, one blue and one red, set up on each side of the beam.

The system is entirely static, except for the collimators and the

camera mirrors, each of them having two positions (single or double

pass) .

Scanning: In order to limit the noise introduced by the atmospheric

turbulence and the guiding errors, it is essential to scan the spectrum

on the exit slit with the highest possible frequency. Considering the

mass of the grating, it would be desirable to scan a smaller optical

element. Unfortunately, the double pass requirement limits the

possibilities, and if one wants to avoid systems with 2 moving elements

which are terribly difficult to synchronise, there is no other choice

than to scan with the grating. So it is set on a turn-table which rotates

the grating to a defined position and wobble at a given amplitude and

frequency. Fig. 17 provides a schematic of the scanner mode.

Multichannel: This mode is obtained by setting a camera/detector unit

on the: diffracted beam before the scanner camera mirror. No optical

element needs to be removed or changed.

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4.2 Optical data of scanner

4.2.1 Basic geometrical data

"In-plane" Czerny-Turner arrangement.

collimator focal length F =(equivalent focal length with

pre-disperser)

5800 mm

6460 mm

entrance beam aperture

collimator diameter

f/29

300 mm

(f I 32,3 beforepre-disperser)

grating: echelle 204 x 408 mm

blaze angle

ruling

incidence angle (center of blaze) ~ =diffraction angle (center of blaze) ß =angle between beams

scanner camera mirrors

4.2.2 Dispersion

According to the grating equation we have

the angular dispersion

630 26'

79 grimm

660 17'

60°35'

50 42'

F' = 5800 mm

dß 1dA = r Sin ~ + Sin ß

Cos ß

the reciprocal linear dispersion

dAF'dß

=A • Cos ß

F' (Sin ~ + Sin ß) R/mm = 0,237 R/mm at 5000 R

the mean free spectral range (mean value between free spectral

range calculated towards greater and shorter wavelengths) •

- 16 -

In these relations:

a is the incidence angle,

ß is the diffraction angle,

A the wave1ength in Angstroms,

F I the camera foca1 1ength,

m the grating order.

Fig. 18 shows the reciproca1 linear dispersion in single pass

and the free spectra1 range versus the wave1ength and grating order.

In double pass the linear dispersion is simp1y doubled.

4.2.3 Optical qua1ity

Fig. 19 shows the geametrica1 spot diagram of the instrument in

single pass. Due to the smal1 aperture, spherica1 aberration is

neg1igib1e but there remains an important astigmatism due to the off-

axis angle of the spherica1 mirrors.----.The astigmatism corresponds in the worst case to 2,8 arcsecs

(with the CAT te1escope in double pass), and one can say that a10ng a

spectrum the mounting is diffraction 1imited.

Because of the aperture, the focusing is not critical and one can

conceive that no adjustment other than.periodica1 shou1d be necessary.

Fig. 20 shows the theoretica1 modulation transfer function where

diffraction p1ays the major ro1e.

F~g. 21 shows the 1ine curvature which is complete1y neg1igible

even for 1arge distances from the axis.

- 17 -

4.3 Spectral resolution

The effective resolution of the scanner depends of the width of

the slits (entrance and exit) and of the quality of the optics.

The best definition of the imaging properties of an optical system

is given by its modulation transfer function. However, MTF is somewhat

difficult to handle and one often uses a simpler criterion which, in

most cases, is only a particular point of the MTF, i.e. the modulation

at a given frequency, or the frequency corresponding to a given

modulation.

We have chosen as a reference the frequency corresponding to 20%

of modulation. This definition is arbitrary; however it corresponds

to a practical limit since the frequency content of the signal generally

decreases with the frequency and the response of the system over that

frequency is generally not important. Moreover, MTF of well corrected

systems often show linear characteristics in the medium frequencies

followed by a much more progressive decrease of the slope. The limit

of the two parts corresponds often approximately to 20% of modulation.

Therefore, we define the resolution of the instrument as the periodicity

of a sinusoidal object whose contrast in the image plane would be 20%.-1Let us call this frequency Vmm , the spectral resolution is

dA = 1V

xdAdx

dAdx ' is the reciprocal linear dispersion in the image plane.

In the case of the scanner, the calculation is very simple since,

if the exit width is set equal to the entrance slit (corrected for

the grating anamorphic effect), the MTF of the system is:

2

Sin TI~V

TI~V

where S(V) is the MTF of the optics.

- 18 -

Assuming S(V) is known (theoretica11y or practica11y) one can

determine for each va1ue of ~ the frequency corresponding to M = 0,20.

The function ~ = f(V) thus obtained is ca11ed the "slit function" and

is used to determine the slit width corresponding to a given spectral

resolution.

Fig. 22 shows the resolving power ~A (for A = 5000~) plotted

versus the slit width. It has been calculated from the theoretical

MTF of the optics. Comparing the angular equivalent slit width on the

sky to the average seeing of 2 arcsecs, one can see that the instrument

efficiency is a1ways determined by the slit width and rough1y proportional

to it.

4.4 Grating turn-table

4.4.1 Requirements

The scanning frequency although not a fundamental requirement

ought to be as high as possible. Considering the mass of the grating

(around 40 kg with its cell) a frequency of 1 Hz was taken as a

realistic target. The scanning amplitude depends on the spectrum length

one wants to cover. A maximum length of half an order seems a reasonable

goal and leads to a scanning amplitude of ! 30 arcmins. The minimum

amplitude is set to + 5 arcmins. Velocity should be kept constant

along the scan and small variations must be random in order to be

averaged. Acceleration and deceleration periods at the two ends of the

scan have to be minimized, the efficiency being determined by the ratio

of the usefu1 scanning time (with a constant velocity) to the scanning

period. It should not be less than 0,80.

4.4.2 bescription (Fig. 23)

It consists of a 300 mm diameter plate onto which the grating

cell is mounted.

- 19 -

High precision air bearings provide support for the platen

consisting of an opposed thrust bearing and a journal bearing. These

air bearings, in addition to providing a very low axial and radial

run-out, ensure that operation is completely 'rumble' free and

virtually frictionless making very small increments possible.

The drive system for the table consists of a De motor and tacho­

generator mounted directly onto the spindle and provides a stepless,

gear-free assemb1y. This avoids mechanical irregu1arities and al10ws

for precise contro1 of rotation in either a c10ckwise or counter­

clockwise direction.

Exhaust air is contained within the table thus avoiding turbulence

effects on the optical paths. This exhaust air provides coo1ing for

the motor and reading heads.

The table position is measured with a radial Moire fringe grating

system with 64,800 1ines per revolution. Twin reading heads are fitted

1800

apart to provide compensation for errors due _to grat.~.!l.9>.....

The reading heads give a fringe sub-division ratio of 200, which

corresponds to aresolution of 0.1 arcsec. An absolute datum reference

point is included.

The position counter operates in binary code. A microprocessor

is used to convert the binary reading into a display of angular position

in degrees, minutes and seconds.

The microprocessor, which is a 16 bit high speed bipolar unit also

generates the position servo error signal, generates the scanning

function and delivers synchro signals which are used to sample the

signal from the detector.

- 20 -

A linear OC servo amplifier is used to drive the directly-mounted

OC torque motor. Servo stability is ensured by the use of a directly­

mounted OC tachogenerator.

4.4.3 Operation

The control panel allows selection of the on-line or off-line

mode. The off-line mode permits initialising and positioning of the

table to the absolute datum point, positioning to any pre-selected

position and scanning to any frequency and amplitude. Lights indicate

the detection of a failure (air failure, datum loss, lamp off etc.) •

When the on-line mode is selected, the control panel operation is

inhibited, but still provides indication of the different parameters

(position, scan etc.). The signals at the CAMAC interface are as

follows:

Input: Position

Control

(1 bit corresponds to 0,1 arcsec) •

fault - on-line - datum loss - scan

acknowledge - position acknowledge.

End of data pulse.

Measuring pulse.

Output: Central position request

Amplitude

Frequency

Interval between measuring pulses

Auto-datum

Move to position

Scan

Enable data pulses

- 21 -

The amplitude used is the usefu1 amplitude in which velocity is

considered constant. It i8 divided by the int~rval between two

measuring pulses to obtain the number of channels. At the end of a

scan an "end of data pulse" is generated and allows a software control

of the procedure (Fig. 24). All the parameters are generated by the

software from the observation requests introduced by the observer

4.4.4 Performances

The turn-table was one of the first parts of the instrument to

be contracted and delivered. The final performances obtained are

slight1y different from the initial exp~ctations. The scanning

frequency appears to be limited not by the servo itself but by thermal

drift of the central position when the heat dissipation inside the table

becomes too important. It also appears that the angular measuring

device introduces a periodic error of 20 arcsecs periodicity and of

maximum va1ue + 0,3 arcsec. The effect of this error on the pixel

position is negligible but the angular width of channels for a high

sampling rate varies considerably; this is the equivalent of having

a different integration time for each pixel. This effect is not

completely eliminated by a prior calibration because random noise due

to velocity variations and very slight variations of the measuring

device are still present. The solution found was to use the measuring

pulses in two ways. Firstly for sampling the PM signal, secondly for

sampling a reference signal given by a clock. The division of the two

signals gives a photometric signal corrected for both velocity variations

and encoder errors.

The different figures summarize the performances:

Fig. 25

Fig. 26

is the macro calibration of the turn-table over

360 degrees;

shows the velocity variations at 1, 3, 5 Hz for a

scanning amplitude of + 20 arcmins;

Fig. 27

Fig. 28

4.5 Detector

- 22 -

shows the dramatic effect of the periodic error of

the measuring system on the raw data for a very high

sampling rate of 1 arcsec;

shows the usable frequency range and the recommended

frequency versus the amplitude.

A RCA Quantacon photomultiplier has been selected. This PM has

a high quantum efficiency over a wide spectral range and a very low

noise at low temperature.

A compressor-cooled housing keeps the operating temperature of

the PM at _50oC with astability of + lOC.

Extensive tests of normal and selected tubes have shown that

there is no appreciable modification of the responsive quantum

efficiency (at least until 800 nm) at low temperature while the dark

current is drastically reduced. A selected PM gives a typical dark

count of 0,5 count/sec at -SOoC.

Figs. 29 and 30 show the measured responsive quantum efficiency

and the dark count versus temperature for a

normal and selected PM.

4.6 System performances

Although the characteristics of the photomultiplier are very good,

the overall efficiency is reduced by the non-simultaneous detection

of the spectral elements. Assuming a turn-table efficiency of nearly

100% the detected signal in each channel is

S =RQE • ~T • Np

n

and the total noise2 RQE· ~T • Np DK • ~Ta = - +

n n

- 23 -

where

RQE is the relative quantum efficiency;

!Y.T is the total integration time (exposure) ;

Np the photon flux;

n the number of channels;

DK is the dark count.

Fig. 41 gives the signal to noise ratio corresponding to 500

channels and one hour of integration compared with other detectors.

One should note that efficiency in double pass 1s approximately

the same as in single pass: the dispersion being twice as high it is

possible (for the same final resolution) to widen the slit, but since

the beam passes a second time on the mirrors and the grating, this

does not result in any gain.

5. MULTICHANNEL MODE

5.1 Choice of the detector

The astronomical programmes envisaged with the CES (2) require

a high spectral purity, i.e. an instrumental profile as far as possible

free of ghosts and wings, as weIl as a high sensitivity.

Although severa1 types of detectors may be considered, our choice

was oriented towards array and intensified array detectors whose

usefulness in this kind of work has been remarkably demonstrated by(3) (4)

R. Tull at the MdDonald Observatory •

Two multichannel detectors are therefore foreseen: one Digicon

and one Reticon. Both will use basically the same array (1872 diodes,

15 x 7oo~) and will fit equa11y the instrument. It might well be that

in the near future CCD arrays could supersede both of these detectors,

but the pixel size being nearly the same, no modification of the basic

instrument will be necessary.

- 24 -

The 1872 diode array is a modified version of the dual Reticon

936 diodes. The two rows of diodes of the standard array are too

close to each other to take full advantage of the two arrays. Hence

it is preferable to use a single array with twice the height and twice

the number of diodes. The loss introduced by the non-simultaneous

observation of the background is partly recovered by the increase of

the detector resolution because the same final spectral resolution is

obtained with a wider entrance slit.

5.2 Optimization of the camera aperture

The classical rule of thumb to optimize spectrographs consists of

matching the projected slit width with the detector resolution. Although

this method does give good results to determine roughly the spectro­

graph characteristics, it does not say anything about the real resolution

which could be expressed in terms of MTF or full width at half maximum

of the instrumental profile. This is demonstrated in Fig. 31 which

compares photographie and FTS spectrum of Acturus: despite the projected

slit width about half the resolution of the FTS spectrum, much more

detail appears on the latter.

Therefore, the projected slit width is not a correct definition

of the resolution. Modulation transfer function may be seen as clumsy

by the astronomer but it is very convenient for the designer, because

final MTF can simply be obtained by multiplying MTF of slit, optics

and detector, and moreover the instrumental profile can be easily

calculated from the MTF.

As for the scanner (see 4.3), our criterion of resolution is the

frequency corresponding to a modulation of 20%.I

For a given detector and a given camera focal length, one can

compute the resulting MTF for different slit widths. The MTF of the

optics is approximated in that case to a linear function whose contrast

is 50% for 40 lines/mrn. Detector resolution is assumed to be determined

by pixel size. The curves of Fig. 32 show how the resolution varies

- 25 -

with the slit width for different camera apertures. When the slit

becomes narrow, the resolution is entirely determined by optics and

detector whilst the slit influence predominates when it becomes larger.

With an array detector (or when sampling an analog signal) one should

pay attention to a possible aliasing at low frequencies caused by the

repetitive structure of the detector. Because the optical MTF is

generally steeply and continuously decreasing and is not that far from

the perfectly adapted filter, this effect can generally be ignored as

long as limiting frequency, as defined above, is kept below or around

the Nyquist frequency (equal to half the detector pixel frequency) .

It is also possible to use these curves to determine the camera

best adapted to a given resolving power. If one defines a relative

factor of luminosity as being the ratio of the slit width to the camera

aperture (this factor is proportional to the number of photons collected

by one pixel), one obtains the curves of Fig. 33 from which the most

adapted aperture for a given resolving power can be easily determined

(Fig. 34).

A relative aperture of fiS has been selected to give a good

efficiency for resolving power of 70 to 120'000. At a R.P. of 100'000

the projected slit width corresponds to 42 m~.

5.3 Camera

A relative aperture of fiS with q field of 30 mm does not lead

to a complex design; nevertheless, a simple spherical mirror is

inadequate and a corrector must be added.

An off-axis Schmidt system would have the advantage of not having

a central occultation but would be a very expensive item. A MAKSUTOV

system with a Newton focus has therefore been selected. All surfaces

being spherical, the price is kept low and the central occultation is

only worrying with an image slicer in which the loss is limited to 9%.

- 26 -

After final optimization the real aperture of the camera becomes

4.7, the focal length 942 mm, and reciprocal linear dispersion 1,46 ~/mm.

Fig. 35

Fig. 36

Fig. 37

shows the optical and mechanical scheme of the camera.

illustrates the optical quality. The spot diagrams

are computed taking account of the complete instrument.

Astigmatism is introduced by the off-axis angle of the

collimator.

(a and b) shows the MTF of the spectrometer for different

wavelengths and its variation with the focusing.

The camera is manually set up on two pillars located between the

grating and the scanner camera mirror. The detector is hung under the

camera with a 3-point attachment allowing orientation and focusing of

the detector.

,The camera and the detector can easily be removed and replaced

by another one. In the first phase only one camera will be built.

Optical elements will be coated with aluminium and single layer anti-

reflexion coating. It is a wide-band camera and two cameras with

specific coatings for blue and red bands would be more efficient by

about 25%.

5.4 Digicon

5.4.1 Description

The Digicon tube (Fig. 38) is a magnetically focused one-stage

intensifier where detection is achieved by a diode array working in

EBS mode. If an important number of channels is required, the diode

array ~annot have a parallel output and therefore practically excludes!

a pure photon counting process. Nevertheless, the performances of this

type of detector are not notably different from those of a photon­

counting system, with the considerable advantage of greater simplicity.

- 27 -

The Digicon selected for the CES has one Reticon diode array of

1872 diodes 15 x 700~ and a S-20 photocathode. The camera field lens

is cemented on the front window, to reduce light losses and possible

ghost images.

The accelerating potential is 20 to 30 kV and focusing achieved

by a coil. Orthogonal deflection coils produce perpendicular magnetic

fields which allow deflexion of the electronic image along and

perpendicular to the array.

In order to limit the thermal leakage of the diodes, the array isocooled down to -80 c, thus allowing integration times of up to 60 seconds.

Cooling is achieved through a closed loop refrigerator with direct

expansion of coolant in the refrigeration probe.

Pre-amplifiers are set directly in the back of the tube, thus

limiting the length of the wires to the array.

Analog circuitry including video processors, clock drivers and

digitizer are contained in an "analog chassis" located near the detector.

All the digital control circuitry and power supplies are located in a

remote chassis outside the coude room. The electronics is interfaced

on a standard input/output CAMAC module; data are sent to the computer

through a Direct Memory Access channel immediately after a read-out

request.

5.4.2 Predicted Digicon performances

Photocathode has approximately an S 20 response. Dark emission

of this type,of pho~ocathode is reported to be 100 p.e./sec/cm2, which

corresponds to 10-4 p.e./sec/pixel, in practice a negligible value.

- 28 -

Major noise contribution is the read-out noise which can be

limited with very good amplifiers to 1000 e-h/read-out/pixel RMS.

Typical gain of the tube being 3000, this corresponds to 0,3 p.e/

read-out or 1,5 ph/read-out/pixel.

Another important noise contribution is the thermal leakage of

the diodes, which is the equivalent of a dark emission. At a temperatureoof -80 C the normal leakage of a Reticon array may be considered

negligible for integration times of several minutes. Unfortunately,

the leakage current increases considerably when the diodes are exposed

to an electron bombardment. Leakage current 10 to 50 times above(3)

the initial value has been reported but good performances can be

retrieved through a special annealing procedure which must be done

from time to time depending of the electron dose received by the array.

A reasonable average value of leakage is 16'000 electrons/sec. If

one takes only into consideration the major contributions, noise is

described by the equation:

0 2= (RQE) . Np n 6t + 2 . nOR 2

GNL+ 2 . n x 6tG2

where RQE responsive quantum efficiency

Np = photon flux

6t = elementary integration time on target

n = number of cycles

0R = read-out noise

NL = thermal leakage

G = tooe gain

A cycle is assumed to be composed of one integration 6t on the

object ~d one identical integration on the background. The back­

ground 1s primarily composed of the detector fixed pattern signal

and possibly sky background.

- 29 -

With an integration time of 30 seconds, one hour of observation,

and a RQE of 20%, a S/N of 10 would be obtained with a photon flux

of 0,32 ph./sec/pixel.

5With a resolving power of 10 and an expected instrument/telescope

efficiency of 0,01 (which takes account of a seeing disc of 2 arcsecs),

this would correspond with the 3.6 m telescope to a star of magnitude 12.

5.5 Reticon

5.5.1 Description

The detector itself is weIl known and has been used in astronomy

for several years (4) (5) (6) (7) (8). The Reticon system used with the

CES has been primarily developed by ESO for near infra-red spectroscopy.

Mechanical schematic is shown by Fig. 39. The cooling, down

to -130oC, is achieved by a liquid nitrogen cryostat, array temperature

is regulated with a resistor. Vacuum ensures thermal insulation

and protects against moisture. The analog electronics is located

around the cryostat in order to limit the length of wires.

Fig. 40 shows the diagram of the electronics. It can cope with

double arrays as weIl as single arrays of up to 4 video lines. Charge

amplifiers are from ORTEC and a microprocessor ensures local control

and interfacing with the computer.

5.5.2 Performances

Contrary to the Digicon, the Reticon performances are almost

entirely determined by the read-out noise. A careful adjustment of

electronics gives, like for the Digicon, an RMS read-out noise of

1000 e-h/pixel/read-out but because there is no amplification it

corresponds to approximately 2500 ph/pixel/read-out if one takes an

averaged RQE of 40%.

- 30 -

Thermal leakage is less than 1 e-h/sec at a temperature of

-130oC and practically negligible for normal exposures.

The noise then becomes:

0 2 = RQE· Np . n . 6t + 2 . n . OR 2

Considering the high value of R, one will try to have only one

exposure, then n would equal 1. Ideally one should make one exposure

on the background and one on the object with the same integration

time. In fact, one can compromise, if the sky background is negligible,

with one exposure of a few minutes to record the detector fixed pattern

signal, which will be latter subtracted from the object signal. Care

should be taken that the detector parameters remain stable.

Fig. 41 shows the signal to noise ratio of the Reticon compared

with the scanner and the Digicon; for one hour exposure time, a5 0 (4)

resolving power of 10 and A = 4500 A. As shown by Tull ,the choice

of the detector depends primarily on the object; with a bright object

the Reticon becomes more efficient than the Digicon because of its

higher quantum efficiency.

6. CONTROL AND INSTRUMENT OPERATION

The schematic of the instrument control system is shown in

Fig. 42. The design is aimed at facilitating the maintenance of both

hardware and software. To that effect, the control is divided into

functions assumed by physically independent modules; most of them

are standard ESO design or already commercially available. In a

similar way software intelligence is distributed at different levels

depending on the degree of interdependence and of the timing of thei

functions.

- 31 -

1. The Centra1 Coude Instrumentation Computer (HP21MXE) is used for

data storage and processing and works under RTE system. Standard

software routines may be used such as the ESO Image Handling and

Processing Software (IHAP) which is now a standard system

imp1emented at La Si1la on severa1 instruments. It may be used

during observation with the current observation or the result of a

previous observation. Peripherals are one disk, one magnetic tape

unit and one printer-plotter.

2. The stand-a1one instrument computer. This is again an HP21MXE

computer dedicated to the instrument and working under BCS, whose

duty is to ensure data acquisition and macro-contro1 of the

instrument. A CAMAC system is used as an interface with the

instrument.

3. Local contro1 is generally achieved by micro-processors performing

specific tasks. For instance, the turn-table, the Reticon'and the

Digicon have their own micro4Processors while motors (both DC and

stepping motors are used) are controlled with micro-processors

10cated in CAMAC modules. One module may control up to 4 DC motors

and 4 stepping motors. The power stages as weIl as PM pre-amplifiers

and HV power supplies are located in a NIM module.

The computer terminal is located in the control room and cons~sts

of one alphanumeric display and one graphie display; a printer-plotter

serves as graphie hard copy and observation log: the graphie display

content is printed at any time, by pressing a key.

In addition a touch-sensitive transparent panel is set on the

standard display. This panel possesses 16 software keys triggered

by a simple finger touch; the display identifies the function

corresponding to the touch.

- 32 -

The program can dedicate each key to different functions with

practically no limitation. This leaves only a few hardware functions

to be controlled from the instrument control panel.

The touch sensitive panel is only used to select functions;

numerical parameters are entered with the keyboard through a "form­

filling" technique. With a conventional terminal, it is possible

to operate character-by-character as a completely interactive mode

or by sending at once a complete block of data. This second transmission

mode, called "form-filling", allows a user's dialogue which is both

powerful and convenient and is used as support of the keyboard

utilisation.

A form is a set of text lines displayed together on the screen

and divided in two parts:

the text fields (defined as protected fields) provide a clear

identification or a general comment. These fields are not

transmitted to the computer.

the data fields (defined as unprotected fields) are filled in by

the user in the normal mode with the keyboard. The cursor indicates

which field is selected.

After reaching the end of a data field, the cursor moves auto­

matically to the beginning of the next data field; the "TAB" key

can also be used to move the cursor from one field to the next one.

The characters typed on the keyboard are not transmitted to the

computer as soon as they are typed. So the user can fill the form"

completely, check and if necessary correct the values before sending

"the screen" into the computer by pressing the "ENTER" key.

- 33 -

As soon as the data flow (i.e. the ASCII character string) has

been received and parsed by the computer, each parameter is checked:

valid character: for example, no letters in a numerical field.

valu~ within the range defined at the creation of the FORM.

If an error is detected, a message is displayed at the bottom of

the screen and the cursor is automatically positioned at the beginning

of the data field containing an error.

A"HELP" command is available for the user resulting in a self

teaching dialog with the computer. The character "?" typed in a data

field will result in a short description of the parameter to be

displayed on the screen.

As an example, Fig. 43 shows a touch panel form used for

manipulating buffers (star, background, calibration). Fig. 44 shows

a form used to set the Digicon parameters.

The different forms used are schematically illustrated by

Fig. 45. They are subdivided in three phases.

instrument setting: setting of wavelength, resolution, spectrum

length, scanning frequency, etc.

detector setting: integration time, exposure time, dweIl cycle, etc.

The form corresponding to the detector is automatically selected.

observation control: a first form is used for general commands:

start, stop, abort, etc. A second form is used to select the

buffer to be displayed: star, background, star minus background,

calibrated spectrum, etc. The third form controls the graphic

display: roll, zoom, etc.

- 34 -

Another set of forms is used to introduce the fixed parameters

of the instrument. Those are:

grating paramete~s: ruling frequency, limits of orders.

turn-table parameters: offset angle; maximum, minimum and

optimum scan frequencies for several amplitudes.

prism calibrations: prism position for several reference wavelengths

from which the actual position is determined.

slit function: for the different modes and detectors.

Thus the interface between the observer and the instrument is

quite versatile and easy to operate and the necessary know-how to

operate the instrument is reduced to aminimum.

* * *

- 35 -

A N N E X 1

RELATIVE EFFICIENCIES OF DIFFERENT OPTICAL ARRANGEMENTS

The use of eehelle gratings has been diseussed by several authors

(8) (9) .. We will discuss here the relative effieieneies of a "Littrow"

arrangement eompared with the two sYmmetrieal Czerny-Turner arrangements,

in the partieular ease of the CES.

The eamera foeal length is assumed to be adapted in order to provide

always the same reciproeal linear dispersion. If the optieal quality

of the instrument is supposed stationary when the eamera foeal length

is slightly modified, the projeeted slit on the deteetor should be the

same in order to obtain the same resolution. This can be expressed by

the relation:

We also have:

.R.proj = .R. xCos aCos ß x

FeamFeoll e

te

implying

Then

dA A Cos ß te- = = edx Feam (Sin a + Sin ß)

Feam te= e

Cos ß

.R. x Cos ate

e

Let us eall the effieieney of the grating E, h the grating height

(assumed equal to the beam diameter) and L the grating length.

When the projected length of the grating on the incident beam is

smaller than the beam, there is a loss which,with a reetangular pupil

like the one given by an image slieer is

1 -L Cos a

h

- 36 -

If we only consider the latter case, the efficiency of the

mounting can be expressed by the relation

E = k x ~ x L Cos ah

or after elimination of constant terms:

E = k' x E

x E

Then the mounting efficiency is proportional only to the grating

efficiency.

The geometrical theory predicts a loss of light caused by

obstruction of diffracted and incident light by the secondary groove

face when a is smaller than ß.

Experimental measurements performed on the real echelle grating

did not confirm this and the two SYmmetrical mountings were found

equally efficient while the Littrow mounting was found approximately

6% more efficient.

Dur conclusion is that "Littrow" would be preferable from the point

of view of efficiency but if "in-plane mounting" is selected there is

no pertinent reason to choose one or the other mounting. However, if

one considers the case of anormal circular pupil, the loss introduced

by the grating coverage would only be about half of the one taken into

consideration, thus giving a preference to a mounting where a > ß.

One should also notice that apermutation between the two mountings

with tite same camera is equivalent to changing the camera focal length.

This pdssibility can be used to modify the optimization of the

instrument for another resolving power. It is also worth noting that

the grating efficiency measurements are representative of that particular

grating and might not be extrapolated to other gratings.

* * *

- 37 -

A N N E X 2

HIGH EFFICIENCY COATINGS

One of the drawbacks of echelle gratings is the order separation.

If a second grating is used as a cross-disperser, the loss of efficiency

is at least 40%. This is why a prism monochromator has been thought

more efficient. However, this higher efficiency may be reduced if a

larger number of optical elements are used. With aluminium coated

mirrors and normal single layer anti-reflexion coatings, the transparency

of the CES monochromator would not be higher than 55%. This is why

efficiency of coatings is essential for this kind of instrument.

High efficiency coatings are nowadays available with a limited

spectral bandpass • Considering these limitations, the total spectral

range has been divided into two parts. The blue range corresponds to

0,34 to 0,53 microns, the red one covers 0,5 to 1,2 microns.

The coatings are multi-dielectric type and provide a very good

mechanical resistance. Hence mirrors can be cleaned up without risking

damage to the surface. This is mainly why, for the red mirrors, this

type of coating is preferred to a protected silver coating which has

almost the same efficiency. The total transparency in single mode

scanner is estimated to be 70% without the echelle grating whose

average efficiency is around 50%.

With the Digicon camera which has only classical coatings, the

transparency is reduced to 55%. Figs. 46, 47, 48 show the efficiency

of the multi-dielectric coatings we have selected.

As a result of the small oscillations of the reflectivity of mirrors,

a "flat field calibration" of the spectrum becomes an absolute necessity

with the instruments utilizing these types of coating.

- 38 -

Nowadays, only medium size components of up to 300 mm can be

coated with sufficient uniformity. This excludes the coating of large

telescope mirrors or classical coude mirrors. A reduction of the

coude focus aperture of the 3.6 m telescope would allow the use of

smaller mirrors as proposed by E. Richardson. High efficiency mirror

coatings would then improve the efficiency of the 3.6 m telescope at

the coude by 55%. Another possibility would be to coat coude mirrors

with a protected silver layer giving the same gain but with a limitation

towards short wavelength to about 4000 R.

* * *

- 39 -

REFERENCES

(1) E.H. Richardson, ESO/CERN Conference Proceedings, 2-5 May, 1972

(2) D. Enard and J. Andersen, 4th Colloquium on Astrophysics,

Trieste, July 1978

(3) R. Tull, J. Choisser, E. Snow, Appl. opt., Vol. Nb. 5, 1975

(4) S. Vogt, R. Tull, P. Kelton, Self-scanned photodiode array,

lnt. Report, Univ. of Texas, Austin

(5) G. walker, lAU Colloquium, No. 40, Sept. 1976

(6) W. Livingston, lAU Colloquium, No. 40, Sept. 1976

(7) J.C. Geary, lAU Colloquium, No. 40, Sept. 1976

(8) M. Dennefeld, B. Guttin, P. Rossignol, ESO int. report 1979

(9) Burton and Reay, App1. Optics 1970, Vol. 9, No. 5

(10) D. Schroeder, Appl. Optics 1977, Vol. 6, No. 11

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_._._._ ..~ ;_..:......:.""---..1 "-"':::::::~.._

._"-1 a___. ' .om.~ ..~ .._.. .._-_.. +. 3.6morCAT

_._._.Inl"-'"Grating tum-\able ! \ Exit Photomdt~ierslit

DO'btc pass system

Blult and rltdcolllmalorS and earllltrc1 mirrors,

~-=r==~ 0_" ,,",,,"f/5 ImuUi-c:haMct camcr&

COUDE ECHELLE SPECTROMETERSCHEMATIC OF THE OPTICAL ARRANGEMENT

Fig.l

COUDE ECHELLE SPECTROMETER

1 Light trom telescope 10 Pre-dlsperser eXlt sllts2 M,rror (acqulsltlon vlewlng) 11 Exposure meter3 Mirror (sllt vlewlng) 12 Collimators4 TV camera 13 Gratlng and turn-Iable5 Eye-plece 14 Mullichannel camera6 Calibration uM 15 Mullichannel deteclor7 Enlrance sill and decker 16 Scanner focuslng mlrrorsB Coilimalors and focuslng mlrrors 17 Scanner double-pass system

of pre-dlsperser 1B Scanner eXil sill and deleclor9 Prlsms

FIG.2

31

IE1011­I .....IIII

III1I

ff)Cl::WU:::JoWCl::

-.J

()f2t-<{U

W:r:t-C")

l.L.cn0-'=

U

~2:w:r:uff)

-.J<{UI­a..o

I1

11\ I\ ~ /~ :( /I

:'I\ /\ :/ I\ :/ I\ :( /I :/

\ 1'1 /I I /I '(I , /I ,

I 'II , /I , 1\I: /~n/

Vn/1 I" ,

/' I, ,, I

/I I

: I ', I

I I I II I

'I 'I I It I

\ "':\n)I I

I I \ :11 \ :\I ~ ':11 ,I,

I 1

Turret for 2ndoptical element(doublet)

~~I---t-+ Orienlationadjuslmenl

from CAT.......

Turret forfront lens

·----{±i===!=.=t==;!!rll~ Adjustment of foot position

Bellows sealing unitwood canvas construction

towards -­stit -"'11--- ---++- -+-11-- -I---+- -lIl---:+---l-t +--- ----ft --i_

SUPPORT OF FOCAL REDUCERSfig.4

::Lo<D .,....~ ---f-+-+----c;..'..:.

::L...<D .~.':' :CO---+-iH----iP.-i~~ .:..:.. ...

::LCOU') .... : :

~ ---H~,.1+-••••+..--0•• :

.'.'

0::WU::>ow0::.....J

~j'0

<.9l.L.l.L.

W::>.....JCO

r­«u

::Lr--~ --~:+-i:~'~'1----~ .,;.:..

::LoLt'l<D-'1ft"l'-+-iH----C""! '••

WWC'OC =.A 070' =e

.'...

WWS'~Z =,A oCO' =e WWO=,A oo=e

E:::L.E00~ JI

JI ::::

:- 'EJI Vl

Vl

O~

:L.

§---+--+--+-- (:

::LN ••' ..c:oln.---+--+--+-.r.P-io' "

~. \. "

a:wu::>owa:-J

.c<tLOU'0

~LL.lL. O

Wa:

~u

::LCD\S---+-~~~-

ww~n~ =,1.. 0€0'=e WWO=.1.. 00=9

15

10

5

o

-5

-10

dX'mm

.40471-' .43581-'

(-7.92)

.48611-'

Blue focal reducer Chromatism of positionfig. Ga

(12.22)

l\

.65631-'

c0.-

o+J

'tn0a.....0

EUl.-

o+JrtSE

:L - ··et'--N '"": ..c .0(0IJ') <D U<.DtL:! I- .Ql

\-....

~u:J"0~\-

-rtSu0...."0~

0:

:l.-(0a)

-.:r:

-.- :L- a)IJ')('f')

~ - -.:r:.-NM

>< ('f')

"C -0 0 0 0 0('t) N - .-

I

Slit

Field mirror

.~amera

?v/_~.\\d~Coloured filters

-\.\---\:

\~, Eye Piece·

OPTICAL SCHEMATIC OF SLIT ENVIRONMENTfig.7

/ from CAT

/.///'/

./

'-~

0.

~ eil'0 ~

.s::;u

'- :t::

tl)0 ~-.!n0

:J E "fg0.'-

0 0\ ::::

.~~c: 0.'ä. 0

-tl) 0. ...CUc: tl) :§.~ tl) --0\ eil -.!:!!:t::c:

@

@

@ ~ CO.Cl.-

~

l.L.

~u eil

eil ~'0cu

,-tl)

~- üi0

(S)

eil~'0 @=iji::11

~

DECKER MASKF1G.9

-, I"II II II

I II II I

L

51 5.1.

/ 1\52 -51

52. +51

PRE-DISPERSER EXIT SLiT OUTPUTfig.10

1

0.5

700 nm600500400OL-L----..I.-----""""--------L-------I,____300

LLF1 PRISMS INTERNAL TRANSMISSIONfig.13

dAdlA-mm

250

150

50

i\

300 400 500 600 700 800 900 1000

FIG.11Reciprocal linear dispersion of predisperser

Ä

entranceslit

LLF1prism (22°)spherical

mirrorR =10.000

P.M ofcountmeter

centre-holedmirror

exit slit

mirrorf=2000

cotlimatorf= 2000

FIG.12PREDISPERSER OPTICAlSCHEMATIC

'.

~"

EE-+

o.g....~

"'C

EE-I

l/lE~ClCU.-

"'C...o0.

(/)Io:~w­(/).o:~WI..L.a..(/)

CiLJJ0:a..

Prism level\ing system

Manual drive

Prism orientation

/

Tacho generator

I ,~Torque motor

Linear encoderMechanical end stops

FIG. 15Predispezrser prism drive

Prism turn-tabte

~

limit switches

Planetaryroller screw

a) Czerny - Turner ~endicular

b) Czerny -Turner

ECHEL EC 152

c) Baranne mounting,

FIG.16 Possible basic optical configurations

Collimators single pass__- double pass

Entrance s\\ts

-------~Echellegrating /

00/ Intermediate slit

/ Exit slit

--=:::--- =:::--- :::::0--

- - .:::::::--=--=-=:::::"...

---- :::::----.

----------=~~~~~----- -----. -- ~ ---

J::>,

blue

Camera mirrors( 2 positions forsingle and doublepass)

COUDE ECHELLE SPECTROMETERSCANNING MODE

Fig.17

t< 8~

0 8 0 0 C<J 0 0 00 0

~ lt) ~ ('f") N - E '-u~ 'E'0'-

.....,0 t<,(""') -, N,

~,

~,,

Ln 0')0-- , N d~ \~

,,~

,~

\ ,Cl

~ \ co C

~ \ d :;::, t1S

~ 0 '-\ ('f") (!)

Cf>\

~\ --,~ \ t': ~.r;

~ \ 0 co u~ \ - w

\ Ln,

,~~ ~

\ ('f") .r;\(6

~\U. +J

~ "l 't-

0 0,~ (6\~

y S c-16' 0

'b ~''in

~'-

~\ ~

"ö~\ ~ Ln a.

~ 0 tn,-

~\ c0

~\ lt)

\\ 't.\ 0

0

\ (D

\1<:1 x \ g'0'0

M

Ln ~ rld

E ~ -d d 0 d

o~

E ••E •....,. • • • • •

+ • • • • • • •• • • • • • •• • • • •• : • •

E:::l..oo..-

o.uo..-~"0

EE....,.I

EEcoI

••• • • • ••

• • ••• • •• •• •• •• •• ••

• •

•

.....o0-

U)I

U)U)O")«..­CL •w~....Jl.L.<!)ZtnCl::wzz5tf)

- Ln.

aLn

a-

aa

zo-....uz::::>IJ..

0:WIJ..lf)Z<{0:t-

Zo-~.....Ja::>No.o<.!)~G:I

lf)lf)

~W.....J<.!)ztn0:WZZ<{Ulf)

vt Slit height

I

I ----I­- lO lJm

1--­10fJm

·x

r-smm

I

SCANNER SINGLE PASS - UNE CURVATUREFIG.21

R150000

100000

50000

20000o .1 .2 .3 .4 .5 .6 .7mm

SCANNER SINGLE PASS - RESOLVING POWER VERSUSENTRANCE SLiT WIDTH

Fig.22

earthquake clampsand dampers

D-,\;:"S>"\:'\:'\:'\:',j I J' ff I air bearing

GRATING TURN-TABlE SCHEMATICFIG.23

"

Datum point

UsefulAmplitude f >' ",

Measuring pulses

End of datapulse

Velocity variationduring a scan

FIG.24TURN -TABlE-OPERATING PRINCIPlE

doe., are sees

300 360Angular indication

4

3

2

1

o _120- --

~1

-2

FIG.25 ANGULAR CALIBRATION OF THE TURN-TABLE(dClC, is the ~ifferenee between true and indicated position)

Ul.~

«E

r~-------_--l~OCUN+

-

--------

LOd

LU--Im

~za::::>I-

LU:r:I-

l.L.olf)

zo 0

~-a:~

>I-Uo-JlJJ><0N

(!jLL

~0'

1.0 ii i

0.5

o

'RAW SPECTRUM" FROM A REFERENCE CLOCK SHOWING THE DRAMAT1C EFFECTOF THE ENCODER PERIODIC ERROR

FIG.27

are sees

Frequeney (Hz)

5

4

3

2

1

,...- ......../ """, "-

" ........ ...... ....... .......

......... ------------ Reeommendedfrequeney

oo 150 300 600 1200 1800

Half amplitude(are sees)

FIG.28 USABLE FREQUENCIES VERSUS AMPLITUDE

0/0

40

30

20

10

k",_" FOB6400-.;;::.~-. --<lt--__ -iC .x .Jl

--- ~,"''2.....:..: ·-';:':·::...ci:--- Jl::--__ ..x.... ---.JS ~__--- _.... 095104

-----....:.-. -" -. =::::B.... F RCA typical" .............--..._----- ------N 63948 ~

~

I800700600500

RQ E of three quantacon photomultipliersFIG.29

400300

'-'

Dark count rate versus temperatureFIG.30

I I I I I I I I I I I I I I I I I I I I I I I

ARCTURUSATLAS

projected stitwidth 23mA

2.7 hrs

9 5160 2 4 6 8 5170 2 4 6 8 5180

ARCTURUSFTS

resolving power91.0002.5 hrs

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

5150 2 4 6 8 5160 2 4 6 8 5170 2 4 6 8 5180 2 4 6

COMPARISON OF RESOLUTION OF PHOTOGRAPHIC AND FTS SPECTRUMSFIG. 31

43

2

543

2

o 0.5 1.0 slit width mm

Entrance slit width versus resolving powerfor different camera apertures

FIG.32

N987654

x105

~ -_-_-_-_-:0.6

~===-~=:==~==~~~~~O.81.01.251.5

321

O~--"""'---L-.----'---r...a-_-,----r----'r------r--_--.

1.0

Q.5

Relative efficiency versus camera aperture forvarious resolving power

AG. 33

4.7 N

9876542,·· 3104 ~--_----,r----__--+--.----__--or---__----.

1

Camera aperture versus resolving power for bestefficiencyFIG.34

.-

.JI

~I

"C

~-

iJU--

«0:l1J~«u.....Jl1JZZ«J:u~:J~LOM(jlJ..

._-.-

I I

~58::::lI/l

~

~I-~ 1_ r-l-L---~I____"=-"

_.1---'-- --'-'- .f-c EI­'t=:::====:j:=:====::=:::;r-..u.Ji....--~ 11 .r:b.

'\ \. \, ,,~ "... ' \, .....' .. I" :'-, , \ " " '\" \ \ " ". \ \ \ \ \ \\ '\ \" , -:\ ~ , , I. \\" \'

I \ .. " \" I' '. "~i ... \ '" "\\ \ ,"' ':

\. .. \. \, .. \ '\ \ \. .... \ \ \ "\ \ \ 'I \' I '-. ':

V .' ~Ä 'm /r ~[....,.......-;-,/~,T/T"iA~r(i910]jrr;X;;J1 ?;1",.11/7;.,L,;.. I~:-'-'17....,/.L/+:k/.-!.-,-/~-I1 _

~ ~ ~ -oW ~ l:~

--------------------

• ••

y'= +15

y'=+ 10

y'= Omm

y'= -10

y'= -15

A=390nm A=550nm A=750nm

-t--t--t- ~=20~

FIG.36Spectrometer with f/5 camera-

Image quality at the best, tangential focus

Modulation

1.0

.75

.5

.25

A390nm550nm

750nm

o 10 20 30 40 50 60 p.lknm

FIG.37aSpectrometer with f/5 camera- Modulation Transfer function

Modulation1.0

390 nm550nm750nm

focus

0.1 0 0.1 0.2 mm

FIG.37bSpectrometer with f/5 camera .Ima~~ modulation at 40p.Ymm

versus focus posItIon

~>

: I I I I I"//7 /T7 TT7 ~////

anti coronawindowfield lens

photocathode

nI

\\

u

~~

refrigerationprobe

/,.,

- reticon array I -

-ä;U~=t~- preamplifier

~~

I-.;;:

focus coil deflection coils pottingcompound

thermal insulation feam

SELF-SCANNED DIGICON TUBEFIG.38

from clock drivers ,

ceramic printed board

Diode Array

Fi!Zld lens ./'wlndow ~

Resistor '

from temperature controller

Preamptifiers

RETICON CRVOSTATFig.39

Vacuum Detector

VacuumI 'pump

~

, ,~Valve,e ,

DETECTOR II

CLOCKTYPE I+5V STATUS I

BIAS~--

-

CLOCK (f)I-

I +START PULSE

t-- TIMING L-.I SYNC. (2f)

VADDRESS &."-..Il ARRAY A 11 I,,-CONTROL /

t •BUS

0 -MC 6800SYNC r--UNE SINC PTA

EDC PTIMING LOGIC T MICRO-

~ tSTART 0 PROCESg)Rn PULSE

I1 ARRAY B Il GENERATOR ARRAY SELECT C"0 V DATA "-..INTEGR'/HOLC CONVERT EOC U

f" I BUS /

t • PDEMUXDEMUX L

ADDRESS E ADDRESSl-

RS

"-OMA~

.....Y> HINTEGRATE r t1 ADC I , CONTROL

DAND HOLDJ I JI

HINTEGRAT~r :-lADC I GI

ANALOG AND HOLD I TCHARGE AMPLIFIERS MPXR AH INTEGRATEr [j ADC r- L

~AffiAY

TRISTATEM --:7'" ".l

DRIVERS.......... AND HOLD IDATA"P,,/

X '---r--..... H INTEGRAT~r L-j ADC l- RV AND HOLDJ I J '--, " '

ARRAV DRIVER AND VIDEO SIGNAL PROCESSOR

RETICON' ELECTRONIC DIAGRAMFig.40

DETECTOR CONTROLLER AND DATA BUFFER

1000

100

10

1

signal to noise. ratio

A=4500AR= 105

1 hour exposure3.6m telescope

ph/sec/pixel

13 12 11 10 9 8 7 6 5 4 3 magnitude

Fig.41Comparative expected performances of Reticon /Digicon

GRATINGTURN­TAßLE

COOLING

DIGICONand

ANAlOG. ElECr.

SUTSFILTERS

, , LAMPSELB:T

I :

I ! I PMT I SCANNER

I I I I n T lEXPOSUREi .. METER

DIGICONCONTROL

21 MXE(BCS)

HP 7920DISC UNIT

r--------------------------------, r-----------,I II I

LrIIIIIIIII ITURN-TABLE I' II PANEL

I IL J

SCHEMATIC OF CES CONTROL SYSTEMFig.42

FIG. 43

FIG.44

Instrumentsetting

General setting

Scanner Multi-channel

Detectorsetting

Observationcontrol

SCANNER

BUFFERHANDLING

DIGICON

OBSERVATION

RETICON

GRAPHICDISPLAY

INSTR.STATUS

OBSERV.STATUS

INSTRUMENT SOFTWARE OPERATIONFig.45

uncoated silica

TO/o

100 I-~:::::::=====================:::=l_

90 -V80 -

70 -

60 ­

50 ­

40 -

uncoated silica

30 -t----r-.--"T'.----,.---r-.---rt----..r-----I300 350 400 450 500 550 600 650

i\Cmp)1°10

100 .,----r=======::::==--~=======--=====l

90

80

70

60

50

40

900800700600 '000 '100A( mIJ)

Typicalefficiency of blue and red anti reflexioncoating's (Transmission of a silica plate with both

faces coated.)FIG.46

30450 500

protected aluminium

20

10

O---------r------------.--------~

80

70

60

50

40

30

RO/o100 -r--------------------_90

300 350 400 450 500 550i\(m~)

Typical efficiency of blue reflective coatingFIG.47

v- Y -(

~-7 r---- ..... ----~- protect~d alum nlum

-

---

-

--

40

30

20

10

o

50

R 0/0

100

90

80

70

60

500 600 700 800 900 1000 1100 1200A(mJJ)

Typical efficiency of red reflective coatingFIG.48