Using Nodal Aberration Theory to Understand the Aberration Fields of Multiple Unobscured TMA Telescopes

Kevin P. Thompson/ORATobias Schmid/CREOL

Kyle Fuerschbach/UofR, Institute of OpticsProf. Jannick P. Rolland/UofR Institute of Optics

2009 SPIE Annual MeetingSan Diego, CAAugust 2, 2009

Motivation

• The James Webb Space Telescope (JWST) is a Three Mirror Anastigmatic telescope (TMA), corrected for the primary 3rd order aberrations; spherical, coma, and astigmatism

• In one instrument path, NIRSPEC, there are in-fact 3 additional unobscured TMAs in a row, all with higher order aspheres – this may be a first

• The motivation for this paper is to begin to study the interaction of multiple TMA telescopes from the perspective of nodal aberration theory

Topics

• The JWST TMA form

• The TMAs in the JWST NIRSPEC instrument

• The evolution of the final aberration field for a NIRSPEC-like configuration

• Plans for future study

The Hubble Space Telescope and

The James Webb Space Telescope

JWST

HST

HST

JWST

2.4m

6.6m

Astronomical Telescopes0.4 Deg. @ F/20

Cassegrain Ritchey-Chretien JWST TMA

COMA

ASTIG.

> 20001930 - 20001890 - 1930

JWST A Field Bias, Obscured TMA

JWSTSegmentedPrimary

Secondary Tertiary

Tertiary

Field Bias usedto implement TMA

The JWST Telescope is limited by 5th order AST

ASTIGMATIC

FIELD CURVES

ANGLE(deg)

X Y0.20

0.15

0.10

0.05

-0.5 0.0 0.50

FOCUS (MILLIMETERS)

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3

X Field Angle in Object Space - degrees

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3Y Field Angle in Object Space - degrees

3rd orderis zero

The offset field of viewfor the instruments

of the JWST

JWST Instrument Format(SPIE 5437, 2004)

ASTIGMATICFIELD CURVES

ANGLE(deg)X Y0.20

0.15

0.10

0.05

-0.5 0.0 0.50FOCUS (MILLIMETERS)

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3X Field Angle in Object Space - degrees

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

Y Field Angle in Object Space - degrees

NIRSPEC Field

NIRSPEC-likeField ~ NIRCAM

Using 50% less Field of View allows developing adesign using only conics, which is less complex to

explain on a fundamental level

The Instrument is rotated about 8 deg. from the

JWST axis about the focal plane

JWSTFocalPlane

Instrument

JWSTTertiary

FSM

NIRSPEC LayoutXavier Gnata PhD 2007

From JWST

JWSTAxis

Optical Design by J. KohlerEADS Astrium(?)

See SPIE 5962, 2005

An All-Conic NIRSPEC-like Example

NIRSPECConic+C,D,E,?

NIRSPEC-LikeAll-Conic

The Principle of Multiple TMA Design

• TMA designs are rotationally symmetric systems that use a combination of – Field Bias– Offset (Eccentric) Aperture

• Field Bias does not affect the aberration field, only the part of it that is used

• Aperture Offset is used to move the correction zone for the 3rd order aberrations to the offset field point

TMA Characteristics

• Here– JWST: Field Bias 7.2 arc-minutes

• Instrument Axis rotated 8 deg. about JWST focal plane (3 TMAs)

– Relay: Axis Offset ~ 425 mm

– Collimator: Field Bias 17.5 degreesAxis Offset 135 mm

– Camera: Field Bias 17.5 degreesAxis rotated 4.2 deg.Aperture decentered 140mm

NIRSPECINSTRUMENT TMAs

Relay

NIRSPECINSTRUMENT TMAs

Collimator

NIRSPECINSTRUMENT TMAs

Camera

RelayField Bias Only

JWSTAxis

RelayAxis

NIRSPECImage Area

CollimatorField Bias Only

17.5 Deg. Field Bias

Camera AxisOf Symmetry

System Optical Axis

IntermediateImage from

Relay

CameraField Bias and Offset

Aperture

17.5 Deg. Field Bias

Camera AxisOf Symmetry

System Optical Axis

SystemFocal Plane

ApertureDecenter140mm

0.67X Relay180mm FOV @F/15

8 deg.

RMSWFE

Astig.

Coma

Aperturestop

400mm

Astig.

Coma

JWSTFocalPlane

Published Predicted Nominal Design

Performance - 2005

Optical Design & Engineering II, edited by Laurent Mazuray, Rolf WartmannProc. of SPIE Vol. 5962, (2005)

Optical Design of the Near-infrared Spectrograph NIRSpecJess Köhler, Markus Melf, Winfried Posselt, Wolfgang Holota, Maurice te PlateEADS Astrium GmbH, P.O. 88090, Immenstaad, GermanyEADS Astrium GmbH, P.O. 81663, Ottobrunn, GermanyEuropean Space Agency, Keplerlaan 1, PO Box 299, 2200 AG, Noordwijk, The Netherlands

RMS WFEvs.

Field

Collimator17.5 Deg. @F/12.5

17.5 Deg.

Astig.

Astig.

Coma

Coma

RMSWFE

RMSWFE

Camera17.5 deg. @ F/5.6

OffsetStop

FINALCORRECTION

w/o OffsetStop

The Final Nominal System Performance w/Four TMAs

Well-Corrected

JWSTTertiary Final

FocalPlane

JWST RMS WFEAt 200mm Off-axis

coma astigmatism

NIRSPEC-LikeInstrument Model

CONCLUSIONS

• The NIRSPEC Instrument on the JWST represents one of the most complex mirror systems ever attempted

• Initial independent work with a system with a similar geometry and all conics has indicated new aberration field behavior, not observed previously, particularly as it relates to higher order terms, like trefoil (elliptical coma) – it appears the higher order aberrations are moving to follow the lower order

• Nodal Aberration Theory (NAT) and Full Field Displays (FFD) provide a unique, independent method to understand and analyze the performance of these groups of TMAs that can be extrapolated to developing insight into what is needed to verify alignment

• While this model is based on conics, the actual design use higher order aspheres and operates 50% farther off-axis than this example