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Old 01-10-2013, 03:13 PM
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naskies (Dave)
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Originally Posted by Bart View Post
In image 4 of the OP, on the line that says Source Region in the Overscan section of the Image Calibration tool, what is the 24 and how is this derived and what does it mean? Could you in general explain that section a bit more for me as my camera has an overscan option and am interested to give it a go.

There are several types of overscan pixels, but the ones I was interested in were (according to the KAF-8300M datasheet):

* Dark Dummy Pixels - working pixels that are never exposed to light; it's like normal pixels from a Dark frame but available in every image, and

* Virtual Dummy Pixels - simulated pixels that aren't connected to the vertical shift register (so bias + read noise is present, but no thermal noise or light data); which is like Bias frame pixels but available in every image.

The overscan data download in the SBIG driver in no way correlated with the chip's pixel overscan geometry, i.e. I couldn't match specific overscan pixels to Dark/Virtual Dummy Pixels in the datasheet's diagram (neither could the person I emailed to at SBIG).

I just found a suitable region by trial and error (as per RickS's suggestion). I took a bunch of bias, dark, and light frames and looked at the image with stretching. You'd expect the Dark Dummy Pixels to look identical to a dark frame, brighter than Virtual Dummy Pixels, but less bright than light frame pixels... which looks like the vertical strip on the right edge of the frame.

You'd expect Virtual Dummy Pixels to look identical to bias frame pixels, but much dimmer than dark/light frame pixels - which is the horizontal strip below the rest of the image.

I only specified 60 extra overscan columns/rows in the driver (30 in this 2x2 binned image), but the first 6 rows below the light frame look like Dark Dummy Pixels (it's brighter than the rows at the very bottom)... which leaves 24 rows. I've probably chosen the wrong region, but it gives me much better dark scaling results so I'm happy for now

To find the right region, I'd recommend turning on overscan capture on the camera, and then record a few bias, dark, and light frames. Use screen stretching and the eyedropper tool to have a look at the overscan region to see which bits correlate with Virtual/Dark Dummy Pixels. You want there to be a reasonable amount of dark noise to differentiate between Virtual and Dark Dummy Pixels, so either take really long exposures (the light frame shown in the OP was 60 min), or run the camera warm - say +15 deg C - to exaggerate the difference.
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