ATIK Cameras ACIS Series Manual - Halaman 6

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Dark Frames

To get the most out of your Atik ACIS camera for astrophotography, we

recommend processing your images using dark frames. You can take dark

frames by covering the front of the camera with a lens cap. Dark frames should

be taken at the same temperature, exposure length and with the same gain and

offset settings as your light frames.

A master dark should be generated by averaging a large number of individual

dark frames. This can then be subtracted from the individual image sub frames

during processing. Further information on using dark frames should be available

in the manual of whichever image processing software you choose to use. Dark

frames are not required for High Speed Imaging.

Binning

Binning is a technique where groups of pixels are added together, with the

result being a brighter image. The CMOS sensor in the Atik ACIS only supports

binning in software. This is where the pixel values are added together after

image acquisition, as opposed to hardware binning where pixels are combined

on the sensor.

Gain and Offset

Your Atik ACIS allows you to change the gain and offset settings for the camera.

In very simple terms, this is analogous to the volume control on a radio. The

greater the gain, the easier it is to detect faint signals; however, less detail

might be seen in the brighter ones.

The full well depth (i.e. maximum signal) of the Atik ACIS is around 20,000e-,

which we map to a pixel value of 65,535. By increasing the gain, we reduce the

full well depth, which has the effect of making the images appear brighter. Using

a gain of x2, the full well depth is around 10,000e-. This 10,000e- now gives a

pixel value of 65,535, which makes the image appear twice as bright. The read

noise, or how 'grainy' the background looks, will also increase. However, this

increases a little less than two times.

The useful range for gain is up to x30. Here, the image is 30 times as bright, but

the full well depth is only ~650e-. This means bright objects will saturate very

quickly. Although it may look bad due to the small full well depth, the read noise

will only be about 1/3 of what it was at x1 gain. These kinds of settings are very

useful for very dim objects if you don't mind the bright objects saturating.