1. Using Nodal Aberration Theory to Understand the Aberration Fields of Multiple Unobscured TMA Telescopes
Kevin P. Thompson/ORA
Tobias Schmid/CREOL
Kyle Fuerschbach/UofR, Institute of Optics
Prof. Jannick P. Rolland/UofR Institute of Optics
2009 SPIE Annual Meeting
San Diego, CA
August 2, 2009

2. 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

3. 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

4. The Hubble Space Telescope and The James Webb Space Telescope
JWST
HST
6.6m
JWST
HST
2.4m

5. Astronomical Telescopes 0.4 Deg. @ F/20
> 2000
Cassegrain
Ritchey-Chretien
JWST TMA
COMA
ASTIG.

6. JWST A Field Bias, Obscured TMA
Tertiary
JWST
Segmented
Primary
Field Bias used
to implement TMA
Secondary
Tertiary

7. The JWST Telescope is limited by 5th order AST
The offset field of view
for the instruments
of the JWST
ASTIGMATIC
FIELD CURVES
ANGLE(deg) X Y
0.20
0.15
0.10
0.05
-0.5
0.0
0.50
FOCUS (MILLIMETERS)
-0.3
-0.2
-0.1
0.1
0.2
0.3
X Field Angle in Object Space - degrees
Y Field Angle in Object Space - degrees
3rd order
is zero

8. JWST Instrument Format (SPIE 5437, 2004)
ASTIGMATIC
FIELD CURVES
ANGLE(deg) X Y
0.20
0.15
0.10
0.05
-0.5
0.0
0.50
FOCUS (MILLIMETERS)
-0.3
-0.2
-0.1
0.1
0.2
0.3
X Field Angle in Object Space - degrees
Y Field Angle in Object Space - degrees
NIRSPEC Field
NIRSPEC-like
Field ~ NIRCAM
Using 50% less Field of View allows developing a
design using only conics, which is less complex to
explain on a fundamental level

9. The Instrument is rotated about 8 deg
The Instrument is rotated about 8 deg. from the JWST axis about the focal plane
JWST
Tertiary
Instrument
FSM
JWST
Focal
Plane

10. NIRSPEC Layout Xavier Gnata PhD 2007
From JWST
Optical Design by J. Kohler
EADS Astrium(?)
See SPIE 5962, 2005
JWST
Axis

11. An All-Conic NIRSPEC-like Example
Conic+C,D,E,?
NIRSPEC-Like
All-Conic

12. 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

13. 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 degrees
Axis Offset 135 mm
Camera: Field Bias 17.5 degrees
Axis rotated 4.2 deg.
Aperture decentered 140mm

14. NIRSPEC INSTRUMENT TMAs
Relay

15. NIRSPEC INSTRUMENT TMAs
Collimator

16. NIRSPEC INSTRUMENT TMAs
Camera

17. Relay Field Bias Only
NIRSPEC
Image Area
JWST
Axis
Relay
Axis

18. Collimator Field Bias Only
17.5 Deg. Field Bias
Camera Axis
Of Symmetry
System
Optical Axis
Intermediate
Image from
Relay

19. Camera Field Bias and Offset Aperture
System
Focal
Plane
Camera Axis
Of Symmetry
Aperture
Decenter
140mm
System
Optical Axis
17.5 Deg. Field Bias

20. 0.67X Relay 180mm FOV @F/15 Astig. Coma 8 deg. Aperture Astig. stop
RMS
WFE
Astig.
JWST
Focal Plane
Coma
8 deg.
Aperture
stop
Astig.
Coma
400mm

21. Published Predicted Nominal Design Performance - 2005
RMS WFE
vs.
Field
Optical Design of the Near-infrared Spectrograph NIRSpec
Jess Köhler, Markus Melf, Winfried Posselt, Wolfgang Holota, Maurice te Plate
EADS Astrium GmbH, P.O , Immenstaad, Germany
EADS Astrium GmbH, P.O , Ottobrunn, Germany
European Space Agency, Keplerlaan 1, PO Box 299, 2200 AG, Noordwijk, The Netherlands
Optical Design & Engineering II, edited by Laurent Mazuray, Rolf Wartmann
Proc. of SPIE Vol. 5962, (2005)

22. Collimator 17.5 Deg. @F/12.5 RMS WFE Astig. Coma 17.5 Deg. RMS WFE

23. Camera 17.5 F/5.6
w/o
Offset
Stop
FINAL
CORRECTION
Offset
Stop

24. The Final Nominal System Performance w/Four TMAs
Well-Corrected
JWST
Tertiary
Final
Focal
Plane
JWST RMS WFE
At 200mm Off-axis
NIRSPEC-Like
Instrument Model
coma
astigmatism

25. 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