PUSHING THE ENVELOPE: CCD FLAT FIELDING Roberto Bartali

INTRODUCTION

CCD imaging revolutionized astronomical research due to the impressive

sensitivity, efficiency and the linearity of the silicon sensor used, giving unexpected fine details and reaching extremely faint objects, compared to photographic plates (figure 1 and 2). But this is not without paying some price, very complex software routines and high

precision electronics are needed to get out information from the electrons stored in the CCD chip[O’Connell, Sterken 1995, Massey 1992].

Figure 1 120 minutes exposure on photographic plate.

Figure 2 45 minutes exposure on CCD sensor.

Figure 4 After processing theraw image of figure 1((raw-bias-dark) / flat)we obtain the Scienceimage.

To obtain a science image (figure 4), the Raw image (figure 3) must be corrected with a series of calibration frames that do not contain the object of interest, but they are a representation of all kinuntil they reach the sensitive area converted to a science image.

This essay is a discussion about Flaobtain them.

We will discuss also, someweaknesses. WHAT IS A FLAT FIELD?

Scientific CCDs, even whenidentical, there are slight differencesof defects) this is a problem speciall

A Flat Field is a frame repr

spectral response, it contains also

Figure 3 Raw image of M101. 3minutes exposure onTakahashi Mewlon 300 (12”diameter f7.8) telescope anda 1024 x 1024 pixel xxxxCCD.

d of noise and imperfections encountered by photons of the CCD; without them, the raw image can not be

t Fields, why they are so important and the way we can

alternative techniques, giving their advantages and

they are almost perfect devices, do not have all pixels in sensitivity and spectral response, (among other kind y when we do photometry and spectroscopy.

esenting the pixel to pixel difference in sensitivity and , information about optical imperfections, vignetting,

interferences and dust particles deposited on the CCD protective glass or on any optical surface in the light path (figure 5).

If the CCD is exposed to a perfectly uniform light source which general spectrum is similar as the background sky where it is the object of interest, then all the possible differences in sensitivity and spectral response, as well as imperfections and dust, became visible (and can be corrected): this is the perfect Flat Field; as we will see later, it is almost impossible to obtain it [Oliver 2004, Gullixon 1992].

Figure 5 Flat Field example of a 1 Mpixel Loral CCD. Clearly visible are dust donuts and difference in sensitivity and illumination.

Dark current and Bias frames, are needed to correct the Flat Field for thermal generated noise and for intrinsically noise generated by the internal electronics of the CCD. The goal is to obtain a uniform frame that contains differences of no more than 1% and, applications like photometry and spectroscopy, requires a uniformity of

0.1% or better.

Figure 6 Flat Fields obtained through different filter on the Gemini telescope. From left to right and up to down: g, i, r, z filters. Clearly visible their different patterns.

Flat Field frames must be

instrument (filters, focus, lenses,

we use filters, we have t

taken without moving or changing anything on the

etc) because we need that the light from the object of interest follow the same path and encounter the same dust particles or optical imperfections.

If

o take a Flat for every one we are planning to use (figure 6), because each filter has its own imperfections, dust or particles deposited on the surface and the spectral response may be a little non uniform through the area. Filters and CCD must be parallel to a very high degree of accuracy, if not, the transmission or blocking factor may change, as the focus and the relative position of dust and imperfections. Changes in the temperature of the instrument can change the position of the focal plane and the position of the image on it, so flats must be taken (as near as possible) at

the same temperature of the raw image.

2

Figure 7 flector for flat field exposure at the 4

he signal/noise ratio (S/N) of the Flat must be as high as possible and must be achieved

Combining and averaging at least 5 (10 or more is better) exposures, best if obtained with di

OME FLAT FIELD TECHNIQUE

This technique consist on exposing the CCD camera to a uniformly white matte painted

ectors

Due to the brightness of the reflector or the light box, very short exposure are

needed

Painted screen reflectors t on point the telescope to a white matte painted screen placed at

some d

dvantages: implement and install.

y time

• ort exposure times (less than a

Drawbacks:

reen must be parallel to the telescope focal plane, plain and smooth.

Twith short exposures. This can be done using an illuminated screen brighter than any object we wish to observe (called Dome Flats) or some part of the sky (called Sky Flats), explained in the following sections. Each pixel of the flat must contains at least 30% of photoelectrons of the full pixel well capacity, normally 10,000 to 20,000.

fferent methods, we have a Master Flat and it can be used for a short period of time only if the instrument and sky conditions remains almost constants, otherwise it is necessary a flat for each observation. D

screen inside the observatory (figure 6) illuminated by the twilight sky or some lamps which spectrum resembles the Solar spectrum (white light). There are 2 different methods employed:

• screen refl• light boxes

to obtain required S/N ratio, but the finite and slow action of the shutter (0.1 to 0.2s) can create shadows on the image [Zhou 2004].

This method consisistance from it in the observatory dome (figure 7). Illumination may be from a

projector lamp, a series of coloured lamps, a special kind (like quartz) lamp or by the twilight sky. Reflector’s illumination may be indirect with lamps behind the screen, or direct with light in front or aside the screen. Some filters are placed in front of the lamp to match the wanted wavelength. A

• easy to• exposures can be taken at an

before or after the observation, even with bad weather, because the dome is closed. Very sh

Screen remeters Kitt Peak National Observatory.

minute).

• the sc

3

• Difficult to obtain a uniform illumination over a sufficient area.

obtain better control of both uniform

Figure 8 ion incide the box

illumination are difficult to

• y. bject of

ight diffuser boxes

nt the telescope to a screen, this method uses a box placed at the

This method is more

ity and spectral response.

KY FLAT FIELD TECHNIQUE

Instead of expose CCD camera to an artificially illuminated screen, the dome is

eld

dvantages:

nating spectrum matches better with that of night sky.

rawbacks: rightness and colour varies too fast.

cities. dome flats.

ay appears in frames.

• External light may contaminate or generate reflection. • If the light is from twilight sky, uniform and constant

obtain and the reflection of unwanted spurious light is difficult to avoid. Illuminating source wavelength do not mach the required colour of the sk

• The focus of the camera when exposing flats is not the same as for the ointerest.

L Instead of poitelescope aperture end. The box has one translucent diffusing plate facing the focal plane of

the telescope. Inside the box there are one or more lamps (monochromatic with filters or multicoloured) attempting to give a uniform illumination to the diffusing side (figure 8).

Illuminatis obtained with various lamps.

complicated, but it has the advantages that spurious light contamination is fully blocked and it is easiest to The same disadvantages are

almost as described above, plus that it is difficult to implement for large aperture telescopes. S opened and the telescope is pointing and tracking to some area of the twilight sky or at:

• zenith • blank fi• star fields

A• Illumi

D• Sky b• Fringes appears at some wavelengths. • Moon light contamination. • Light pollution from nearest• Exposure times longer compared to • Due to the high sensitivity of CCD, some stars m• Internal reflections.

4

Figur

the dis

Typic

wilight sky ter sunset or before sunrise (normally 15 minutes, figure 9) the brightness and

Fringe patterns (figure 10)

A part

requir

Zenith

lank Field are some areas on

able 1 k Fields areas.

appeasky fl

T Just afthe spectrum of the sky is almost uniform and there are sufficient light to obtain high S/N

ratio frames with relatively short exposures, but the brightness varies rapidly, so there is no time to take exposures for each filter. Even when professional astronomers uses [ESO manual] twilight flats, this is best suited for amateurs, because they uses a reduced (or none) number of filters.

appears because of internal

wavelengths emission lines, pro[Tyson 1988].

reflection of light inside the

Flat fi

The best place for Flat Fiethe sky and the brightness gradien B There stars and without extended objectsbecause their general spectrum is selected Blank Fields. TSelected Blan

FIELD NAME FIELD ID Davis Blank 1 raham aham blan

Davis Blank 2 Davis Blank 3 Davis Blank 4

G Gr k

Figure 9 Twilight sky over Antarctica.

e 10

advantage that needs very long exposure times to obtain sufficient S/N ratios, so, useful and expensive telescope time must be spent for calibration frames. To avoid this, we can produce a clean Flat directly from the images of the observed object, but this work ber of frames (thousands).

al fringes

duced by Oxygen, passes t

icular disadvantage is that twilight light is polarized. ave

e a large num

e Zenith because it is the darkest part of

the sky that appears as “blank” with few and very faint

ring on ats.

CCD and when long hought narrow band filters

elds produced using Zenith, Blank and Star fields h

ld exposures is thts are minimal [Chromey 1996].

, called Blank Fields, they are ideal for Flat Field frames exactly the one we need. Table 1 contains a list of some

R.A. DECL. EPOCH 1950

16:49:42.0 -15:21:00 1982 19:19:09.0 12:22:05 1950 21:26:54.4 -8:51:41 1950 23:54:08.9 59:28:18 1950

4:25:46.0 +54:09:03

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Figure 11

scope which is stationary. The exposure time is obtained by the movement of the CCD and by the reading time (figure 11). Each pixel of the observed object is moving on the CCD columns (figure 12). It need a carefully orientation of the CCD on the East-West

direction, otherwise pixels are shifted on adjacent columns and the resulting image is smeared.

In drift scvan, stars e moves on the opposit

direction respect to the CCD movement.

telescopes or LMT.

Figure 12 is formed moving across the CCD and row by row as the pixels

tar field it is not possible to point the telescope to a blank field, another area of the sky

Combining, averaging and median filtering, stars are borrowed obtaining a clean aster

SCAN-DRIFT TECHNIQUE

This method consists on reading the CCD while moving it across the field of view the tele

the object is not on the celestial

curved path, the amount of the trail and the trail rate

dvantages:

stationary, no tracking errors. r large zenith

• lat fields (0.1%).

els in each column are eliminated.

he CCD.

S If, can be used, but has to be far form the Milky Way and galaxies. Even it can be used a star cluster, the number of exposures is proportional to the number of stars in the selected field. Each frame must be done moving the telescope no more than 30% of the field of view. To avoid high photoelectron quantity due to long exposures, telescope tracking may be off, so short star trails with much less brightness appears [Manfroid 1996, Toussaint 2003, Tyson 1990]. m flat.

Ifequator, every pixel moves following a is also a function of the distance of the

object respect the equator. This effect can be eliminated or reduced by continuously reorient the CCD or the array of CCD, even with a special corrector (offsetting and tilting lenses) [Hickson 1998] or with a non parallel CCD array {Zaritttsky 1995].

.

The image are read. On the left the scanning sequence, on the right the complete reconstructed image.

A• Telescope• Easy mechanical implementation, good fo

More uniformity of F• One dimensional flats. • Non uniformities of pix• No needs of sky or dome flats. • Adequate for survey. • Most efficient use of t

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ength of the area can be as long as th

Figure 13

is the image on the focal plane as the planet rotate. Light from each pixel is integrated through all the column (figure 13). To avoid smearing, the CCD must be read at exactly the sidereal rate.

Each image píxel is shifted

ated until

sed on pixel coordinate.

isadvantages: short exposure times.

uator. ht on the equator.

CD data and storing on

• nd electronic control systems to move the CCD.

IME DELAY INTEGRATE TECHNIQUE

This technique is the easiest (in principle) to implement, because the only thing is moving

The area covered (on

( the orientationl e

dvantages ed of complex mechanics and electronics control systems for tracking.

ity due to the averaging of each pixel in a column.

ents.

isadvantages

posure time, depending on the CCD size.

along its column and photoelectrons are continuously integrthe shift register send them tothe computer.

depending on

• Easiest way to do Astrometry ba• Useful for satellites. D

• Relative • image stationary only on the eq• star trail curved on the poles and straig• Need very fast reading and storage electronics; reading C

computer memory at same speed. Complex and precise mechanical a

• Increased cost respect to traditional CCD imaging. • Elongated PSF on East-West. • Image Smearing.

T

one axis) is equal to the pixel scale on the focal plane multiplied by the number of columns or rows ) of the CCD, but the

dark sky allows for. A

• No ne• Adequate for quick surveys. • Improved sensitivity uniform• Efficient use of CCD. • Ideal for transit instrum• One dimensional flats. • Useful for satellites.

D• Short ex• Enormous quantity of data collected on one night. • Precise synchronization with sidereal rate. • Elongated PSF on East-West.

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CONCLUSIONS

A spectrally and spatially flat frame is very difficult to obtain, but not always we need to

From the best calibration frames we obtain a science image containing all the data we are

Each of the techniques described have many advantages and disadvantages, so there is not

ther methods are implemented trying to solve the problem of flat field difficulty, but eve

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a better than another method for taking flat fields, the choice depends on the application, observing purposes and availability of time and instrumentation, many times just on personal experience. A better approach is to combine different techniques taking the advantages of each one obtaining a Super Master Flat.

On when they performs well on some applications, they have their own drawbacks.

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Wong H.S. et al, TDI charge coupled devices : design and applications, IBM research and Development, 1992. IMAGE CREDITS Figure 1:

Photographic plate image, O´Connell, Astronomy 511, Lecture 7. Figure 2:

CCD image, O´Connell, Astronomy 511, Lecture 7. Figure 3:

Raw image of M101, Bartali R., 2003. Figure 4:

Processed image of M101, Bartali R., processed by Rossner A.,2003. Figure 5:

Flat Field, Howell S., Handbook of CCD Astronomy, Cambridge, 2000. Figure 6: Sample of filter flats,

http://www.gemini.edu/sciops/instruments/gmos/gmosSflats.html Figure 7:

KPNO screen reflector, http://www.noao.edu/kpno/mosaic/dflats_new.html Figure 8:

Diffuser box, Tulloch S., Design and use of a novel flat field illumination light source, Technical note 108, Royal Greenwich Observatory, 1996.

Figure 9: Twilight sky, http://sethwhite.org/images/ross%20island/ross%20island%20scenery/twilight%20from%20ah%203.jpg

Figure 10: Fringe pattern, http://www.astrosurf.com/cavadore/chungara/marconi47-10/flat.gif Figure 11: Drift scan, Poor Meadow Dyke Observatory Sky Survey. Figure 12: Scanning sequence, Gorjian V. et al, Drift scanning using infrared arrays: a powerful method for background limited imaging, 1997PASP 109:821-826, 1997. Figure 13:

TDI scanning, Wong H.S. et al, TDI charge coupled devices : design and applications, IBM research and Development, 1992. Table 1:

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