Focus Variation and the Orbital Phase of Observations

Details Focus Variation and the Orbital Phase of Observations Focus Variation and the Orbital Phase of Observations

May 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998nicm.rept....9s&link_type=abstract

Instrument Science Report NICMOS 98-009, 5 pages

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

Scientific paper

We report the effects of cyclic OTA length changes (“breathing”) on focus position for a variety of observational strategies. Special attention is given to the case of NICMOS coronagraphic observations of bright targets. These observations are generally made in pairs, with a large (~30°) spacecraft roll between the observations. We find that if the pairs are forced into the same orbit, there is on the average a 1.6 micron secondary motion between the observations. If the pairs are separated by an orbit, however, on average the focus variation is only 0.75 microns. The difference in average focus variation is even larger (a factor of four) if only the focus variation immediately after a spacecraft slew is considered. This result suggests that coronagraphic observations (as well as others very sensitive to focus) should be put in congruent parts of consecutive orbits.

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