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Instrument Calibration with Stellar Observations

 

Any star within of the ecliptic plane passes within 10 of the sun for about 5 days once per year. There are 10-20 UV bright stars which pass this close. and which have been observed with Voyager and IUE. About 50 stars can be observed with the WLC. They can be used for visible radiometric calibration and co-registration with LASCO.

The position of the star relative to the sun must be accurately predicted in order to be centered in the arcsec field of view of the WLC. The WLC has no capability of tracking the star along its path, since two mechanisms, rolling and mirror rotation, would be involved. Therefore, the best position for observing a star must be predetermined and then the instrument will sit and stare at the star transit. A star will move across the pinhole in about 6 minutes. The successful pointing and detection of a star will permit the spatial co-registration with LASCO. The brightness lower limits to a star detection are given by the brightness of the corona and by the electrical noise of the PMT. Nevertheless, since a background (corona + PMT dark counts) integration can be performed right after the star transit, a background subtraction gives the brightness of the star, which can be used as a second way of calibrating in intensity the WLC, and also for a radiometric cross-calibration with LASCO.

Repeated stellar observations with the EUV channels will track the changes of instrument sensitivity due to detector degradation and other causes. Annual observations of several stars will track sensitivity changes, and comparison with Voyager and IUE observations will check the calibration against an independent intensity scale, which is claimed to be good to about 15%. The best star for this purpose is the sdO star Feige 110. Some B stars are brighter, but some of them may vary in the UV. The stellar spectra will have strong, broad interstellar Ly absorption, so grating positions away from those used for solar observations should be used. Intensity comparison with O VI channel stellar observations will be complicated by the overlap of the primary instrument response and the redundant Ly- response, so that light at two different wavelengths will fall on each pixel. It will be possible, though laborious, to use strong absorption features in the spectrum and multiple grating positions to separate the two contributions to the count rate. Observations of interstellar absorption lines can also compare the wavelength scale to those of stellar instruments. Measurement of residual flux at the bottoms of saturated interstellar absorption lines will place limits on scattered light from the grating. If the pointing can be accurately enough predicted, we can let a star move along the length of the slit to build up counts.

The numbers in the first column pertain to Tau (B3 V, ), which approaches the Sun on June 2. It will have count rates of 18 and 7.3 at 1240 and 1040 , respectively. The last column pertains to WLC observations.

Instrument Calibration with Stellar Observations

** For this example, the slit is placed at 5 at a roll angle of from ecliptic north, and the star drifts across the middle of the slit in about 6 minutes. The start time for this observation must be specified carefully.



next up previous contents
Next: Optical Signatures Related Up: Examples of UVCS Previous: Intensity Calibration of



Peter Smith
Fri Jan 17 12:11:15 EST 1997