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Nikon Coolpix 950 dark frame (under construction)

Most CCD has bad pixels and all CCD has noisy pixels after a long exposure because of dark current. Dark current is the electrons of the CCD acting as though light has reached the CCD chip. This response is generated by the effects the temperature has on the silicon that makes up the CCD chip. In general, the CCD would be saturated with heat generated electrons within a few seconds, completely obliterating any image.

Hotter the camera is, more CCD noise you'll see. Since the camera's temperature not only depends on how much you was using it, but also depends on the surrounding temperature. For astrophotography, one should take a dark frame right before or after each picture one has taken with the exact timing setting so that later we can "cancel" out the CCD noise.

In a more serious astrophotography, one should also take a flat field. The flat field is an exposure taken with the CCD uniformly illuminated. The image identified the irregularities in the response of the CCD since each pixel is not necessarily as sensitive as its neighbor. Taking a flat field exposure gives you a map of the CCD's irregularities. During later image processing, you can divide the desired image by the flat field to correct the irregularities in the CCD chip.

Here is the dark frame I took right after I took the picture of the M5 globular cluster.

Not too bad. Can't really "see" much noise.

Exposure Mode: Aperture Priority Metering: Center Weighted
Shutter: 8 sec Exposure: F2.6
Focal Length: 7.2 mm Focus: INF
Exp +/-: 0.0 Sensitive: +2.0
Using the Photoshop to "Stretch" the meaningful viewing brightness level.

There are the noisy pixel I was talking about.

Here is the M5 globula cluster with the same "Stretching" of the viewing brightness adjustment.

By comparing this picture with the dark frame, you know there are not that many stars. :)

Subtract out the noise and do a little more "Stretching".
Here's the final image.

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 Copyright © 1999 Wesley Chuen

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