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NICMOS optical aberrations: coma and astigmatism

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.