Old 22-12-2013, 12:03 PM
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PI Processing Tip - black spots

I have some quite nice data of M45 taken with the iTelescope FSQ106 in New Mexico. The Luminance data however has black spots - I suspect that I didn't set the L images to dither.

A screenshot is attached.

Any tips from the PI wizards on processing these out?

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Old 22-12-2013, 12:31 PM
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did you get a set of flats?

are they in those?
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Old 22-12-2013, 12:32 PM
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I'd start by trying CosmeticCorrection on the original subs before you stack them. If you create a process icon with appropriate parameters you can invoke it from BatchPreprocessing so that it is done after calibration and before image registration.

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Old 22-12-2013, 05:57 PM
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I just used PS to remove those black spots. Took me 30s with Spot Healing Brush tools. PI might as well have some cosmetic touchup, I am afraid that it may also damage other structures if not masked properly.

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Old 22-12-2013, 08:12 PM
cfranks (Charles)
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I feel you need to tell us what post processing you have done and let us look at the raw subs. The spots look fairly big so it might not be difficult to find the reason.

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Old 23-12-2013, 12:43 PM
Tony_ (Tony)
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Hello Pete,

The spots look like an insect got into your camera. I get this occasionally.
If the spots are in the same position on all of your subs then it probably isn't an insect. If there is a single spot that moves about and some subs have no spots then it probably is an insect. If it is dust on your sensor you should be able to see it (maybe with a magnifying glass).

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Old 23-12-2013, 11:59 PM
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As per Rick's advice, CosmeticCorrection in PI will get rid of them. I wrote a blog article on how to set up a CC template and also how to automatically add it into a custom Batch PreProcessing process icon.
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Old 02-01-2014, 03:50 PM
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pvelez (Pete)
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Thanks everyone for the suggestions.

The images were taken using the iTelescope service in New Mexico. Images can be downloaded pre-calibrated or you can download the calibration frames and calibrate them yourself. I'm pretty lazy so I usually use the calibrated images.

Looking at the master flat more closely, it looks like the flats have not been combined properly or dithered. A screen shot shows the problem - looks like a single sky flat with the stars retained. Alternatively it could be hot pixels.

Anyway, the Cosmetic Correction tool is what I was looking for. I managed to remove most of the dodgy pixels and now have a reasonably clean image. A screen shot of the cleaned up affected area is attached.

I remain a PI novice - but each image encourages me to keep on learning. With this one its been both the CC tool and Star Halo Reducer script. I'll post the full image in the Deep Sky forum.

Thanks again

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Old 03-01-2014, 08:14 AM
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Little black spots, green dots, red dots, blue dots often remain in filtered CCD imaging despite callibration.

The healing brush or clone tool in Photoshop works really well and is very simple to use. Just make sure the diameter of the brush is kept to a minimum and the distance the sample is taken of the background from the spot is not too far and does not have stars etc in it and is similar.

That's for Photoshop though.

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