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Two Issues With T-ReCS Optical Performance

5 March 2005

Two recent issues regarding T-ReCS optical performance have recently come to our attention.

Filter red leaks

The Si-3 9.7 um and Si-4 10.4 um filters have red leaks in the 18-24 micron range, probably due to a shift in the cutoff wavelength of the BaF2 blocking elements at the ~10 K operating temperature. The leaks appear to be small, but can significantly affect photometric measurements of very red sources. As a temporary work-around, we are using these two filters in series with the N-band filter to provide satisfactory long-wavelength blocking. A permanent fix will be to replace the BaF2 blockers with CaF2.

Qb filter plate scale

The plate scale with the Qb filter / KRS-5 window has been found to be significantly different than with the other filters:

N and Qa average / KBrC window:  0.08976 +/- 0.00021  arcsec / pixel
Qb / KRS-5 window             :  0.08633 +/- 0.00013  arcsec / pixel

The difference is about 5%. The values are derived from measurements of the binary star Alpha Cen. Please note that the FITS header WCS assumes a value of 0.089 "/pixel that is not adjusted for the different filters.

Last updated 2005 March 17; Tom Hayward

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