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ACAM detector (AUXCAM) and shutter
The AUXCAM engineering page provides information about the CCD bias level, read noise, gain, readout time and quantum efficiency. For convenience, a summary table of properties from that page (as of July 2018) is reproduced here:
but refer to the engineering page for the latest information.
AUXCAM is a low-fringing, deep-depletion 2k * 4k EEV CCD (15-micron pixels) similar to RED+ on ISIS.
The QE outside the wavelength range plotted on the above-linked page can be found on EEV's pages, e.g. in the red the QE drops smoothly to zero at ~ 1.05 micron.
A history of the bias level, readout noise etc. can be found on the CCD quality-control pages.
looks like a focal-plane filter mount illuminated by light from within ACAM, but is actually a 10x10 binned image of a heavily saturated flat-field.
Bias, overscan region
A nominal mean bias level is set in the CCD readout code, but the actual bias level depends on which CCD controller is used. The nominal value is therefore set high enough to ensure that there is no risk of the actual bias going below zero.
ACAM bias frames should not be obtained simultaneously with biases on ISIS - this results in horizontal features (at a level of a few counts) on both sets of biases. An example ACAM bias affected in this way is shown below (click on image for an enlargement):
At the top left of the field of view (click on the image below for an enlargement) is a single column with only ~ 10% sensitivity, running upwards from (default-windowed) x,y = 566, 1875.
The ~ 100 - 200 black dots visible in the flat-field image are groups of typically 2 - 3 pixels whose sensitivity is low by a few 10s %, as a result of manufacturing flaws (a few of the dark spots might also be due to contamination by small dust particles).
All of the above-mentioned features should flat-field out, but observers may want to avoid placing critical targets near (default-windowed) x,y = 1074, 741.
The white spots in the above late-twilight image are stars.
with a characteristic gradient in the y direction:
The cryostat window is 5 mm thick. The distance between the back of the window and the front of the CCD is 5.9 mm.
The count levels on a series of exposures with requested integration times 2, 1, 0.5, 0.25, 0.13, 0.06, and 0.03 sec suggest that the overhead for opening and closing the shutter is ~ 0.04 sec. Exposures of 0.02 and 0.01 sec are also possible, and yield fewer counts, but it's not clear that the shutter is opening fully. If an exposure time of 0.005 sec is requested, the exposure fails (to clear the error state, repeat with a longer exposure time). We recommend a minimum exposure of 2 sec for observations of photometric standards.
*** During 2019, the ACAM shutter has been showing signs of wear, and on
a few occasions it has not fully opened.
The current plan is to service the
mechanism in late 2019. In the meantime, keep an eye on the photon counts
from objects of known apparent magnitude.
If the counts appear lower than expected,
the telescope operator can check from engineering control the true status of the
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