NICMOS optical aberrations coma and astigmatism

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Coma and astigmatism of NICMOS cameras have been derived using phase retrieval … Coma and astigmatism in NICMOS cameras are generally small, producing the …

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NICMOS optical aberrations: coma and
astigmatism
A. Suchkov, G. Galas
June 10, 1999
ABSTRACT
Coma and astigmatism of NICMOS cameras have been derived using phase retrieval
analysis of the data from the NICMOS focus monitoring program. Analysis of these aberrations
has revealed that y-coma in all three camera experienced a systematic change over
the NICMOS lifetime in Cycles 7 and 7a. A similar variation has been found for x-astigmatism
in camera 2. In camera 3, the x-astigmatism increased dramatically after the FOM
had been given a y-tilt of 16 arcsec. X-coma and y-astigmatism in all cameras did not
show substantial systematic variations over the same period of time. The results of this
study should help to understand the origin of the NICMOS optics instability. They should
also facilitate interpretation of possible changes in NICMOS cameras when they are back
to operations after installation of the cryocooler in Cycle 10.
1. Introduction
NICMOS optical characteristics experienced noticeable variations since the start of the
instrument orbital life in early 1997. These include not only the changing focus position
(Burrows & Krist 1997; Suchkov, Bergeron, & Galas 1998) but also coma and astigmatism.
A study…