Short term focus changes caused by small ( microns) motions of the
secondary mirror of the Optical Telescope Assembly (OTA) of the Hubble Space
Telescope (HST) are referred to as OTA ``breathing''. Though the cause of the
secondary mirror movement is not fully understood, it is now apparent that the
motion is thermally induced (Hasan and Bely 1993). Secondary mirror
variations,
SM, are related to the mean temperature of four sensors in
the light shield around the secondary mirror, denoted LS, and the mean of LS
over the previous orbit, MLS, by the equation
where is an arbitrary constant.
OTA breathing may be only a minor nuisance for now, but could become more serious once the spherical aberration has been corrected with COSTAR and WFPC II. These focus changes can affect the quality of the science data in many ways. The most obvious effect is loss of sensitivity resulting from image blur. However, there are many other ramifications such as the difficulty in performing image enhancement with an unstable point spread function (PSF), or the impossibility of doing photometry with the cameras or the spectrographs. There are also secondary effects due to focal plane shifts induced by the fine guidance sensors (FGS) reacting to focus variation (when guiding in fine lock).
These effects can be very subtle and certainly depend on the type of observation (limiting detection, crowded fields, etc.), and complex simulations may be required in order to pinpoint the level at which a focus change becomes intolerable. In order to make a first order estimate of this ``pain level'' we have determined the PSF at various wavelengths and focus values using the following criteria: