WISP Instrumentation

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A photograph of the instument hardware mountings, taken at the NASA Wallops Island payload testing facility in 1991. Below is a diagram of the optical path.

Where Does the Light Go?

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Light enters the telescope from the lower left, encountering these elements in order:

• Stressed Waveplate: Four 4-inch square, 1/4-inch thick panes of CaF₂ are mounted in a frame. A programmable pneumatic actuator along one side introduces 1/2 wave retardation at the desired UV wavelength. The polarimetric efficiency is 70-85%. The frame is rotated to provide polarimetric modulation. By having the modulator first, the rest of the instrument does not introduce instrumental polarization.

• Schmidt Corrector Mirror: This almost-flat mirror with an asymmetric asphere figure removes the spherical aberration of the Schmidt primary. The 15 arcsec imagery is far from diffraction limited, but sufficient for a 15 arcsec pixel.

• Brewster Reflector Mirror: A large flat mirror coated with ZrO₂ mounted at an angle of 67.5 degrees to the collimated beam between the corrector and primary. (Detector is in elliptical hole.) Polarimetric efficiency is 85-95%, reflectivity 30%.

• Primary Mirror: F/1.8 sphere.

• Filter: Broadband thinfilm filters: 1640 Å +/- 250 Å and 2150 Å +/- 300 Å, on MgF₂ and Silica substrates.

• Shutter / Field Flattener / Detector: The MgF₂ field flattener is the window of the detector housing. The detector is a Reticon 400x1200 pixel CCD, overcoated with UV phosphor. Estimated UV Quantum Efficiency 15%. Operated at -65 degrees C, total background noise 10 electrons.