An aperture cover, opened and closed on command, is used for contamination control, thus preventing dirt accumulation during launch and on orbit when the SBV sensor is not being used. The cover has a secondary mechanism that can open the cover permanently if the normal mechanism fails. Focus quality and stability became an issue early in the development and construction of the SBV sensor. Initial results describing problems with the Hubble telescope were published just before the SBV sensor's Critical Design Review. To maintain focus independent of operating temperature, the telescope housing and mirrors were made of the same type of aluminum, resulting in an athermal system.
All elements expand or shrink at the same rate, keeping the focal plane in a fixed location.
Plate scale and distortion maps are expected to change with temperature, and computer modeling indicated that it was important to keep temperature gradients low, thus making athermal assumptions valid. Thick walls, thick mirrors, thermal isolation from the spacecraft mounting surface, and a multilayer insulation blanket were incorporated into the design to keep gradients well below the design allowable limits of 3.5 C. Focus is specified as an ensquared energy percentage, which is the percent of total energy in the central pixel after optimum centering. A significant challenge for the SBV program is to keep the telescope optics clean up to, during, and after launch. The first two mirrors are superpolished with gold surfaces to maintain low scatter characteristics. Our process to maintain cleanliness covers three areas-design, handling, and operation.