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
Where Does the Light Go?
Light enters the telescope from the lower left, encountering these elements
- 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.