Subject: Mirror cover flats with SPIcam

From: Russet McMillan

Submitted: Fri, 29 Oct 2004 01:52:23 -0600 (MDT)

Message number: 849 (previous: 847, next: 850 up: Index)

News of interest to SPIcam observers:

The installation of the new NA2 baffle during summer shutdown has
considerably improved the flatness of calibration images taken off
the mirror covers.  Although twilight flats are still preferable and
superflats taken from object images are best of all, mirror cover
flats are now a desirable alternative when the other kinds are not
possible.  Mirror cover flats will also be slightly redder than the
twilight sky or the night sky, and may therefore be more appropriate
for photometry of red targets through broadband filters.  Drawbacks:
the lamps are quite faint in the bluest filters and will not
reproduce night-sky fringing patterns in the reddest filters (i', z',
I); only object superflats from the night sky can compensate for

Thanks to the new baffle, the illumination pattern of mirror cover
flats now has very little dependence on rotator position, except in
the outer corners of the image.  Observers planning aperture
photometry on small objects near the center of the field should not
have to worry about rotator position for mirror cover flats.
Observers doing surface photometry on larger objects or aperture
photometry of multiple targets throughout the field may want to take
flats at multiple rotator positions and combine them into a master
flat.  This does require coordination with the observing specialist,
since we have to be very careful about having the mirror covers
closed and motors active at the same time -- so be sure to
communicate your plans clearly to the observing specialist.

Here are some suggested exposure times that should give 20K-30K
counts for the Sloan filters.  We do not have a current set of
exposure suggestions for MSSSO or narrowband filters.

filter     lamp     exposure
u'        Brt Qtz    (900s gives about 1400 counts)
g'        Brt Qtz       5s
g'        Dim Qtz     300s
r'        Dim Qtz      90s
i'        Dim Qtz      60s
z'        Dim Qtz      90s

Acknowledgments to Bill Ketzeback for extensive testing.


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