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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