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11.1.1 Instrument Design
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The HIPO optical system is reconfigurable to meet its varied requirements. It incorporates two dichroic beamsplitters, one to divert the infrared beam to FLITECAM (if mounted) and one to split the red and blue sides of the HIPO optical paths. Either or both of these may be removed if desired. It is also possible to move either CCD so it is placed directly at the optimal SOFIA focal plane for highest spatial resolution and throughput. The optical design is described in detail in Dunham (2003, SPIE, 4857 62).
The 8-position filter wheels are located near the pupil image formed by the collimator optics. Two positions in the red CCD's filter wheel are normally used for Shack-Hartmann lenslet arrays, but these may be replaced if necessary for a particular observation. We have contemplated adding grism capability to HIPO but have not yet carried this out.
The region between the mounting flange and the gate valve on the telescope can be evacuated to reduce image degradation due to density fluctuations in this region of the optical path. When this is done a window is installed at the location of the gate valve.
HIPO provides a number of CCD readout modes as described in Dunham et al. (2008, SPIE, 7014, 70144Z). The most commonly used are the single frame mode and a frame transfer time series (basic occultation) mode with readout frequency up to 50 Hz. A variety of readout rates are available allowing the observer to optimize the subframe size, speed, noise, full well, and linearity tradeoff for a particular event. This is also discussed together with HIPO data format details in the "HIPO Data Cycle System Software Interface" document.