Possible Evidence for NICMOS Focus "Precessional Breathing"

Details Possible Evidence for NICMOS Focus "Precessional Breathing" Possible Evidence for NICMOS Focus "Precessional Breathing"

May 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998nicm.rept....7s&link_type=abstract

Instrument Science Report NICMOS-98-007, 9 pages

Physics

Optics

Hubble Space Telescope, Hst, Space Telescope Science Institute, Nicmos

Scientific paper

Combining the results of measurements of NICMOS focus and temperature (outside NICMOS dewar), we have found a correlation between detrended NICMOS focus and detrended temperature. The correlation suggests that there is a thermally induced (quasi)periodic component in NICMOS focus variation which has a time scale resembling the HST precessional period. This component may thus be due to HST temperature variations caused by changing radiation flux from the Sun as the telescope attitude at the same pointing (same target of the focus monitoring proposal) changes with respect to the Sun because of precession. The mechanism of focus variation may be similar to the one causing the well-known HST orbital focus "breathing" except that in this case it may be associated mostly with light path variations in NICMOS fore optics. The data seem to be consistent with some amount of "precessional breathing" in the Optical Telescope Assembly (OTA) as well. The amplitude of "precessional breathing" has been found to correspond to ~5 micron of the Secondary Mirror (SM) shift. This is to be compared to ~2 micron of the regular "orbital breathing" and ~5 micron of the actually adopted tolerance with respect to the HST long term focus drift.

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