NASA's planned Kepler mission uses a space-born Schmidt telescope to search for Earth-size and smaller planets around distant stars using differential photometry. This paper reports the successful design, analysis and implementation of suspending a large actively cooled (-90C) focal plane array with associated electronics inside the warm (0C) Kepler photometer. Since a Schmidt Telescope requires the focal plane to be in the middle of the telescope, it must be suspended while obscuring only a small portion of the incoming light. The Kepler focal plane is comprised of 21 individual science CCD modules and 4 guidance sensor modules covering an area that is roughly 1200 square centimeters in a telescope with a 0.95m aperture. The Kepler system requires the detector data to be digitized near the focal plane, so a detector electronics box is also suspended behind the CCD array. A total of 65 kilograms is supported by the spider structure inside the telescope and must remain stable through environments and during on-orbit operations. Key to the performance of the system is a stiff, light-weight composite structure that supports the focal plane and electronics above the primary mirror. This spider structure is used to align the focal plane with respect to the primary mirror in the system, and is intentionally over-constrained after alignment. Techniques used to align the focal plane to the optical system are discussed and predicted alignment performance and stability are reported.
Unpolished diamond turned mirrors are common for infrared systems. We report the successful use of unpolished mirrors in a visible spectrum, all aluminum telescope for the planned New Horizons mission to Pluto. The Ralph telescope is an F/8.7 Three Mirror Anastigmat with a 75mm aperture, a 5.7° by 1.0° field of view, and a mass of only 8kg. Key to the performance of the system are a process for reducing the micro-roughness of the off-axis aspheric surfaces to below 60 Ångstroms RMS, and the fabrication of precision diamond turned mounting features on the mirrors and one-piece, thin-walled housing. The telescope achieves nearly diffraction-limited performance with minimal post-assembly alignment, and maintains that performance, including focus, over a wide range about the operating temperature of 210K.
The Multiband Imaging Photometer for Spitzer (MIPS) provides long wavelength capability for the mission, in imaging bands at 24, 70, and 160 microns and measurements of spectral energy distributions between 52 and 100 microns at a spectral resolution of about 7%. By using true detector arrays in each band, it provides both critical sampling of the Spitzer point spread function and relatively large imaging fields of view, allowing for substantial advances in sensitivity, angular resolution, and efficiency of areal coverage compared with previous space far-infrared capabilities. The Si:As BIB 24 micron array has excellent photometric properties, and measurements with rms relative errors of 1% or better can be obtained. The two longer wavelength arrays use Ge:Ga detectors with poor photometric stability. However, the use of 1.) a scan mirror to modulate the signals rapidly on these arrays, 2.) a system of on-board stimulators used for a relative calibration approximately every two minutes, and 3.) specialized reduction software result in good photometry with these arrays also, with rms relative errors of less than 10%.
We describe the test approaches and results for the Multiband Imaging Photometer for SIRTF. To verify the performance within a `faster, better, cheaper' budget required innovations in the test plan, such as heavy reliance on measurements with optical photons to determine instrument alignment, and use of an integrating sphere rather than a telescope to feed the completed instrument at its operating temperature. The tests of the completed instrument were conducted in a cryostat of unique design that allowed us to achieve the ultra-low background levels the instrument will encounter in space. We controlled the instrument through simulators of the mission operations control system and the SIRTF spacecraft electronics, and used cabling virtually identical to that which will be used in SIRTF. This realistic environment led to confidence in the ultimate operability of the instrument. The test philosophy allowed complete verification of the instrument performance and showed it to be similar to pre-integration predictions and to meet the instrument requirements.
Conference Committee Involvement (8)
Optomechanics and Optical Alignment
1 August 2021 | San Diego, California, United States
Optomechanical Engineering 2019
14 August 2019 | San Diego, California, United States
Optomechanical Engineering 2017
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Optomechanical Engineering 2015
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Optomechanical Engineering 2013
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Optomechanics 2011: Innovations and Solutions
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Advances in Optomechanics
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New Developments in Optomechanics
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