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WUPPE and Orion


WUPPE Detector

The WUPPE detector is optimized for bright sources and large dynamic range. It consists of two parallel 1024-pixel Reticon photodiode arrays coupled to a single-stage ITT intensifier which converts the ultraviolet light to green phosphor light detectable by the photodiodes. In the intensifier, a Cesium Telluride photocathode on a MgF2 window provides a solar-blind spectral coverage from 135-330 nm. A single straight channelplate is proximity focussed to the photocathode to provide an electron gain of 1000; these electrons are accelerated onto a P-1 phosphor (P-1, which does not have the persistent phosphorescent afterglow of the more frequently used P-20, is more suitable for bright sources). The phosphor output of the intensifier is then fiberoptically coupled to the photodiode array. The diode pixels are 25 microns x 2.5 mm, and the arrays are separated by 3.5 mm, thus matching the spectrometer format. The system peak efficiency is 4% (see table below for approximate efficiencies of each optical element). The combined maximum spectral resolution of the system is 0.6 nm. The detector is cooled to -35 degrees Centigrade by a thermoelectric cooler and operated in analog integrating mode, with integration times from 0.2 sec to 128 sec, giving an input photon-limited dynamic range from 0 to 14th magnitude stars. Because of the very high signal/noise required for polarimetry, most WUPPE targets were in the range 0 -8 magnitude: readout integration times ("frames") were less than 8 seconds, so that a typical observation integration is the sum of many short frames. The zero order of the grating is relayed to an intensified CCD camera operating in the visible. This is used for acquisition and for monitoring of the pointing during an observation.

      Element   Effic   Wavelength(nm)
    =========== ======  ==============
    Mirrors (3)  0.75   140 (min)
                 0.90   200 (max)
    Waveplate    0.91   Flat
    Prism        0.50   140 (min)
                 0.95   300 (max)
    Grating      0.68   200 (max)
    Detector     0.10   190 (max)
    ----------- ------  --------------
    System       0.04   200 (max)

The instrument is controlled by a Motorola 6809 microprocessor, which commands Reticon and CCD readouts, mechanism motions, and science and housekeeping telemetry. An extensive library of observation sequences controls observations, while allowing for intervention by the Payload Specialist on board or by ground command. The output of the zero order camera is relayed to the shuttle aft flight deck video monitor for monitoring of the progress of the observation.

An instrument of similar design, but with sensitivity to the visible wavelengths 320-760 nm (recently upgraded to cover 320 -1000 nm) was constructed for use on the University of Wisconsin Pine Bluff Observatory ("PBO") 36 inch telescope. This has been used in conjunction with WUPPE to develop science goals, verify the calibration by overlap at 320 nm, and to obtain time dependence information and simultaneous observations of targets observed by WUPPE.

-taken from Nordsieck et al. 1994, SPIE, Vol. 2010, p.2.