Posts Made By: Charles Perrygo

Optimum aperture shape is dictated by many factors, such as image quality requirements, constraints on volume and mass, support structure design, and integration issues. For the Gaia instrument, small star images are only required in one direction, the scan direction, as star positions are only measured along the scan direction. The rectangular primary mirror is long in the scan direction, thus producing diffraction-limited star images that are small in the scan direction. Use of a rectangular primary mirror thus meets image quality requiremenmts while reducing the overall instrument height, which benefits the overall observatory design. The attached drawing shows the anti-Sun side of the Gaia observatory with the instrument thermal covering removed to show the instrument optics. Reducing the height of the instrument allows a reduction in structure mass and in the size and mass of the circular sunshield. The sunshield, which must be deployed after launch, shields the instrument from solar heating, allowing it to passively cool to its -110 C operating temperature.

Chuck Perrygo