2
QUIRC User Guide
in caseswhere the background has been changing rapidly. It may also give better resultsif the dome
flatwas not evenly illuminated (it is difficultto achieve even illuminationat the 0.6m telescope). The
dark should be subtracted from the sky before division. The disadvantages of this technique are that
the sky flat showsthe response of the detector to the OH airglow + thermal emission. In particular,
fringing may be present in certainconfigurations (e.g. 2.2m f/10 1:1), and fringing is something
which should be subtracted, not divided.
Thenumber of bad pixels usually dictatesa special technique for observing in which several exposures
are made of the field being studied, with each exposure shifted slightly from the others (dithering).
Whenthe images are combined, the bad pixels in one image can be “filled in" with good pixels from
a shifted image. This technique also improves flat-fielding relative to a single long exposureat the
sameposition.
It is recommended that at the start and end of each night d omeflat s and darks be taken. T hed arks
should ideally be exposures of the samel engthas the object exposures. Even if the darks are not
directly used in the reduction, theywill serve to show which pixels have high dark counts so that
thesepixels can be included in a bad pixel mask. Dome flats are generally taken asa lights on/lights
offpair. Usingthis strategy results in a differenceimage (ON – OFF) which represents the detector’s
flat-field response to a source withcolor temperature of a few 1000 K, which is roughly the same
temperatureas some of the sources being studied.
The shutter is a leaf type shutter, meaning than the center part of the aperture is open slightly
longer than the outside. Recent tests showed significant center-to-edge illumination differencesfor
integrationtimes less than 1 second. Therefore,shortexposures should be avoided, particularly when
exposingdome flats—it is far better to dim the lights with the domelight dimmer switch and use an
exposureof a few seconds than to use the dome lights at full intensity and an exposurewhich is less
than a second (this can introducespurious radially varying structure into the flat-field). Thereis an
uncertainty in the timing of the shutter ofthe order of 10 millisec. Therefore, short standard star
exposuresshould also be avoided—onthe 2.2m, the Elias standards may need to be slightly defocused
to allowreasonable exposure times in the broad filters.
At the 2.2m telescopethere is a slightrotation in the nominal cassegrain rotator position (270). The
rotationwas measured in February 1996 to be 0.883 degrees CCW (e.g., N is rotated 0.883 deg E of
verticalwhen displayed in the normal way). One could attempt to adjust slightly for thisby changing
the rotator position, or adjust for itlater during data reduction. If the precise rotation and scale is
importantto the observations, one must measure this carefullyduring the run since the exact rotation
value is likely to change slightly betweenruns when the instrument is taken off the telescope and
remounted.
2.1 Detector Linearity,Saturation, Read Noise, Dark Current
Hardsaturation of the detector occurs at 50,000 ADU’s. The total gain of the system results in a scale
factor of 1.85 electrons/ADU. Recent tests (2/96) s howed the device to be linear to better than 1% for
valuesup to about44,000 ADUs. However,the gain and illumination is variable across the array so
caremust be taken so that parts of the arrayare not saturating when the average ADUvalue is getting
close to the non-linear region. The average value should be kept below 40,000 ADUs to ensure that
one is not saturatingareas of the array. Theaverage detector dark current is 0.8 electrons/sec, and
the readnoise is 15 electrons rm s.