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

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 6.6m JWST HST 2.4m

Astronomical Telescopes 0.4 Deg. @ F/20 1890 - 1930 1930 - 2000 > 2000 Cassegrain Ritchey-Chretien JWST TMA COMA ASTIG.

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

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

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

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

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

An All-Conic NIRSPEC-like Example Conic+C,D,E,? NIRSPEC-Like All-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 degrees Axis Offset 135 mm Camera: Field Bias 17.5 degrees Axis rotated 4.2 deg. Aperture decentered 140mm

NIRSPEC INSTRUMENT TMAs Relay

NIRSPEC INSTRUMENT TMAs Collimator

NIRSPEC INSTRUMENT TMAs Camera

Relay Field Bias Only NIRSPEC Image Area JWST Axis Relay Axis

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

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

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

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. 88090, Immenstaad, Germany EADS Astrium GmbH, P.O. 81663, 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)

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

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

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

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