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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.

AUXCAM is a low-fringing, deep-depletion 2k * 4k EEV CCD (15-micron pixels) similar to RED+ on ISIS.

QE outside the wavelength range plotted on ING's 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.

In slow-readout mode, the readout noise is typically 3 electrons, and the gain is 0.9 electrons / count. In fast-readout mode, the noise is 5 - 7 electrons, gain 1.9 electrons / count.

A history of the bias level, readout noise etc. can be found on the CCD quality-control pages.

As is the case for most ING CCDs, the bias can be characterised as B(x,y,t) = f(x,y) + g(t), where f(x,y) is a fairly stable pattern of pixel-to-pixel variation, and g(t) represents the variation with time (typically a few to 10s of counts on a timescale of hours). f(x,y) can be measured by taking a median of daytime bias frames. g(t) is usually measured from the overscan region of each science observation.

Observers sometimes ask what determines the bias level required on a CCD. A nominal value is set in the CCD readout code, but the actual bias level depends on which CCD controller is used. The nominal value is set high enough to ensure that there is no risk of this level going below zero.

Useful area
The CCD is typically windowed to [1:2148,800:3300]. This includes the whole area illuminated during either imaging (0.25 arcsec/pixel) or spectroscopic observations.

The CCD can be binned at least x2, x3 and x4 without problems. Binning up by more than this may yield unpredictable results e.g.:

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.

Overscan region
The region 1 < x < 50 can be used to estimate the bias level.

Cosmic rays

The cosmic-ray rate is ~ 130 per Mpixel per hour. Cosmic-rays typically leave tracks 2 pixels wide, and deposit ~ 1000 counts per pixel. Most tracks are 2 - 10 pixels long. In a typical half-hour dark exposure, the largest events will leave tracks ~ 100 pixels long (~ 100k counts).

AUXCAM is housed in a cryostat with an optically-active window, i.e. the window is part of the ACAM optical train (which greatly simplified the design of ACAM). For this reason, the cryostat cannot be moved, e.g. for focusing. Any focusing required is achieved by moving the WHT's secondary mirror.

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 (Prontor) shutter lies in the light path just after the filter wheels, and just before the main lens barrel. It has a clear aperture of 64 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.

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Contact:  (ACAM Instrument Specialist)
Last modified: 12 Apr 2017