At the time of this writing, the SOHO schedule does not include a time period for characterization of the complete UVCS/SOHO instrument. If such an opportunity is not found, then it will be necessary to rely on laboratory characterizations of the following: optical components, the spectrometer assembly (SPA), and a mock-up of the white light channel. It may be possible to include some laboratory UV tests of the white light mock-up, which would provide some indication of the performance of the UV telescope channels.
In addition to the laboratory tests, some characterization can be done in space. The flight portion of the characterization will utilize instrument mechanisms to determine variations of instrument characteristics across the detector and as a function of the filled aperture (i.e., vignetting function). Stars can be used to determine spatial co-registration of the instrument channels, and as radiometric transfer sources. A calibration flight of the Spartan 201-1 instrument could be used for an in-flight radiometric calibration and stray-light performance checks at a few wavelengths. The UVCS sun sensor can be tested by scanning the UVCS across the solar disk. Characterizations of the pointing control and roll system can be done in a similar fashion. It will be possible to get some information about the telescope mirror mechanism in this way also. The XDL flat field response and pulse height distributions (i.e., gain) can be determined in space.
In all cases the laboratory optical component testing is being done with hardware considered
to be very similar to the flight articles. The tests include characterizations of the OVI grating,
the Ly-
holographic grating, the surfaces of the sunlight trap, and the telescope mirrors.
Laboratory characterizations of the flight XDL detectors will be done at Berkeley.
A laboratory characterization of the SPA is planned. It will include a fairly complete characterization of the white light channel, and somewhat abreviated characterizations of the UV channels. The SPA characterization will include, in priority order, measurements of: light leaks, off-band sensitivity, transmittance of neutral density filters, detector dynamic range, grating angle calibration, slit size, detector flat field, radiometric efficiency, spectral plate scale, and spatial plate scale. Obviously, the SPA characterization cannot include characteristics of the telescope mirrors and other telescope assembly items.
The stray light properties of the instrument are affected by the scattered light properties of the telescope mirrors. The scattered light level can be affected by contamination, and so tests of a mock-up or other model do not provide a good indication of the flight article performance unless care is taken to ensure that all mirrors are similarly clean. Nonetheless, a mock-up of the white light channel has been built and is being characterized. If possible, the characterization of the white light mock-up will include some UV tests. The tests are performed in a test chamber that simulates a solar illumination of the instrument at the UVCS wavelength ranges of primary interest. Stray light in the test chamber is controlled in order to simulate a near zero intensity corona. In this condition, the instrument response to the simulated solar illumination is a measure of the instrumental stray light.