Telescope Reviews: This is driving me nuts; need help!

I use DSS, Images Plus, and Nebulosity.

If I open a Canon raw dark file in Nebulosity, I can see slight banding. I believe Nebulosity is doing a linear stretch of the data. If I ask Nebulosity to demosiac the image, the banding is still there. So I think the banding is there.

Tony, as you are saying, most of us just load our RAW lights, darks, bias, and flats into IP or DSS and let the software do the work. I don't think this is a calibration/stacking issue. The software is doing what it is supposed to do.

If you open up a raw file in ACR (Photoshop), I would assume that some kind of stretch is being applied. There is an auto setting and a default setting. Depending upon the setting, the banding is visible or it isn't. You can adjust the "exposure" settings to see the banding/amp glow increase or decrease.

I guess the question is: Is the banding normal or not? If I take a raw file and stretch it and see banding, the banding is there? The stretching just makes it visible. And it's visibility is determined by the settings that ACR (and other programs) uses in decoding the file. If the settings are set such that that banding isn't visible, we think there is no banding. But it is there. So I conclude that it is normal and we don't have defective cameras. Again, I saw the same banding on my friends 20d as shown above.

Now the whole point of these programs is to remove this noise. For the most part, these programs do a great job. There is nothing defective about the programs. But there are variations in real life that these programs can't fix. So we can take dark frames and apply them to our light frames to remove this noise, but that doesn't mean they will match up perfectly because of differences in temperature, for example.

So I thought that the reason most of us don't use the in camera noise reduction is because of the inconvenience of taking a light and then a dark and then a light and then a dark. All that time wasted taking darks could be devoted to taking lights. Then we can take the darks later (of course the temperature is going to be different), use DSS to statistically combine the subs and try to create a better final image. I'm not sure whether a light that is calibrated for an in-camera dark that is taken right after the light is a better light frame than a light that is calibrated with a median combined dark three hours later? The temperature of the in-camera calibrated light and dark will certainly be closer, but the dark will be noisier because it doesn't benefit from S/N improvement that comes with combining multiple images.

So this is my conclusion based upon this discussion:

1. The banding is normal. I'm sure some cameras have more or less of it. If you apply the same processing to a raw file to bring it out, it will be there in varying degrees.

2. DSS, ImagesPlus etc., will eliminate much of this noise as they are designed to do.

3. Differences in temperature between when the lights and darks were taken will affect the ability to eliminate this noise. I have also found that this noise is harder to eliminate with a six minute exposure as compared to a two minute exposure. I am not sure if this is outside ambient temperature related or due to the camera heating up more and maybe the dark current is less statistically predictable.

Nate, it might be that the Canon 5D is inherently much less noisier, hence superior. I don't know. I guess it comes down to whether you are happy with the final images you get after all the processing. Tony gets great images with his 10D and I think that chip is noisier than either the 20D or 30D.

Please tell me if there is something wrong with the above logic.

Joe

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Tak FSQ 106N, 8" F4 GSO, 10" F4.7 Orion
CCD Labs Q453 (Q8)
TSS Modded Atlas

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http://www.flickr.com/photos/10168933@N08/

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