Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21460001t&link_type=abstract
American Astronomical Society, AAS Meeting #214, #600.01
Astronomy and Astrophysics
Astronomy
Scientific paper
We discuss the implementation of 'breathing' model for optimal focus positioning of NICMOS camera 1 and 2. HST focus experiences noticeable short-term variations known as 'breathing' on the orbital time scale. These are caused by thermal expansion/contraction of the HST optical telescope assembly (OTA) as the telescope warms up during its orbital day and cools down during orbital night. The thermal model of the breathing effect relates the HST secondary mirror position to temperature variations. For NICMOS the internal focus of all three cameras can be adjusted by moving the Pupil Alignment Mechanism (PAM). For camera 1 and 2 the focus sweep observations consist of a set of 17 images of a crowded star field, both in and out of focus. The encircled energy of at least six stars in a fixed radius aperture is measured at each point in the focus sweep. A fit for points near the best focus gives the location of peak energy in PAM space. Implementation of the breathing model corrects the observed PAM position optimizing the camera focus. While this correction is not used directly in the positioning of the PAM, it decreases the scatter of the measured position of the best PAM position, allowing a more precise monitoring of possible focus changes. Corrective action can then be applied.
Thatte Deepashri G.
Wiklind Thomas
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