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wxastrocapture/SPC990nc - gain/brightness/gamma?

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#1 bugbear

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Posted 21 January 2013 - 05:51 AM

I "fell across" a galileoscope in a charity shop (5.00 GBP) and I have a decent photographic tripod.

I bought a spc880 from Morgan computers, and have flashed it to spc990nc, have the 1 1/4" adapter and I/R filter.

I have installed wxastrocapture (under Ubuntu) on an old laptop, and everything's running "OK".

My question is - what is the interaction between gain, brightness and gamma in the "properties" dialog.

They all seem to do "similar" things, but I'm guessing they don't (really) and that setting them "properly" (or even "optimally") would give benefits.

If anyone can point me at a detailed description of each control action, that would be helpful. Even better would be a detailed guide to setting them.

Note - for my astrophotography will be VERY limited, since I have no tracking mount. A capture like these would make me a happy bunny:

http://www.youtube.c...h?v=BkhMOc4c9tE

http://www.youtube.c...h?v=CxuMzz7JsQU

BugBear

#2 Rick J

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Posted 21 January 2013 - 01:56 PM

This question will get a better response from the Solar System forum since that is what you are trying to do. I can give you just some basics as this type of imaging is not what most of us on this forum do.

The sensor stores a voltage for each pixel that represents a count of electrons received. This is then converted to a digital value ranging 0 through 255 for each color even though the sensor can likely distinguish 20,000 or more different levels. How this is done is controlled by the gain. It works somewhat like ASA for film. The higher the gain the higher the values for low levels fall in the 0 through 255 scale. Set it too low and all the detail will be between level 40 and 41 for example. So you see only a dark grey for all detail. Set it too high and brighter objects are burned in. You want it set so not too much is burned in but high enough that you have the detail you want taking up a good portion of that 0 to 255 range.

Once converted to 256 levels for each color, brightness is like the brightness control on a TV and gamma like the contrast control on the TV. When gain is wrong both will appear to do about the same thing as there's no detail for the gamma setting to display. I prefer more non linear controls that other software provides rather than the more linear control that brightness and gamma provide though used correctly they can do the job. I find curve type adjustments easier to use.

The key is finding the right gain to squeeze many thousands of levels into 256. Once you have that right the rest is rather easy. If you have a tool to see the histogram of the image that will help to find the right gain setting once you understand where in the histogram your detail is hiding.

Rather than a movie you'll get best results by stacking the frames in a program like Registax. A non tracking mount isn't a problem for Registax or similar programs. Stacking reduces the noise greatly allowing aggressive processing to bring out detail not seen in the movie version due to the poor signal to noise ratio.

Rick

#3 bugbear

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Posted 22 January 2013 - 06:34 AM

This question will get a better response from the Solar System forum since that is what you are trying to do. I can give you just some basics as this type of imaging is not what most of us on this forum do.
(detailed reply read and snipped)


Should I repost, or is there a way to get my question moved (by a mod??) ?

BugBear






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