History and Overview of Hubble’s Focus

HST's Cassegrain Optical Telescope Assembly (OTA) is a two mirror system, whose Primary and Secondary Mirrors are both tightly thermostatically controlled. The stable mirror temperatures, combined with a carefully designed and fabricated graphite epoxy optical truss result in a telescope that is quite stable, especially against non-axial element motions (tip, tilt, decenter).

The OTA does however experience noticeable changes in focal length as the separation between the Secondary and Primary Mirrors varies over timescales as short as an orbit and as long as the mission life (Lallo 20062007). Although the Point Spread Function (PSF) delivered to the Science Instruments (SIs) is obviously very stable compared with many other observatories, these variations in focus can be measurable in the images and are considerations in many types of science analyses (e.g. Makidon et al. 2006Sahu et al. 2007Suchkov & Casertano 1997).

To mitigate these effects, active corrections are occasionally performed to maintain long term focus stability, while for shorter term variations, focus and PSF models have evolved and are available to help characterize the science.

HST OTA Diagram

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