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Light areas in images with heavy stretching


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I'm using SharpCap for my EAA, with an ASI585MC and 200PDS and have noticed that in some images, particularly when heavily stretched, that I get areas that lighten much more than others. The pictures below show what I mean, and can be a nuisance when trying to see dim objects, as the light area can swamp the object.

Given that these occur in different places in the image (left, right, corners) for different targets (the camera always has the same orientation relative to the OTA), I don't think that it can be a problem with light leakage in the scope, as it's running in a very dark environment (Bortle 1). I've looked at my flats and darks and they all seem to be what I would expect.

My guess is that it's stray light from nearby bright celestial objects (moon, bright stars) - but I'm happy to be corrected and wonder if there is anything I can do about it.

 

Cheers

 

Geoff 

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It is normal to get a gradient in the sky.  There are tools in some processing software for dealing with this. 
Though l have never had experience  or the good fortune to image in anything better than Bortle 3 let alone Bortle 1 skies  

Carole

Edited by carastro
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A simple background extraction in Siril (free) or Pixinsight (paid) will sort this easily. It's one of the most basic forms of post processing to do on images and it probably has the most effect on the end result, it certainly allows you to histogram stretch the images more to reveal more faint nebulosity.

There's also graxpert fairly new, think it's also free.

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Regarding cause, unless you're imaging during astronomical darkness some element of the sun's influence can exist, but I suspect your issue is to do with lights on the horizon no matter how dim.

The moon is one of the main culprits for gradients especially when RGB/colour imaging, using a narrowband dual/tri/quad filter can help on emission nebula only, for broadband targets, well you have to make the best of what you have. From bortle 1 you should be getting stunning results.

Edited by Elp
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I also see this sometimes with my EAA setup (200PDS + Uranus-C). It seems to vary from object to object and from time to time. I put it down to three things:

  • Stray light

The object might not always be close to the axis of the scope, just in a position where the light from it gets in. I now have dew shields for my Newtonian's in an attempt to stop this happening, and I also switch off all house lighting, just as I do for visual.

  • Cloud

I often find that when I check outside there is very light cloud around the object which I think is reflecting light and lightening the background sky. Not much you can do about this but it's good to know.

  • Dew

Sometimes it gets worse towards the end of a longer session and then I suspect dew build up. I've reduced this considerably by using a dew heater on the primary and hopefully the dew shield will limit any effect on the secondary.

 

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It looks like exactly the sort of problem I get in Bortle 6 with the same camera and an F5 dob.  For me it seems to be caused by 

Stray light - you could try a dewshield. I diy'd one from black EVA foam. Its not pretty but it helps.

The moon - especially if there is high cloud which kind of catches the moonlight. Not much to be done except don't EAA when the moon is bright but personally I like to get out whenever I'm in the mood and its not cloudy, so i just tolerate it.

The Sharpcap gradient settings are definitely worth a go. Linear gradient removal can be helpful but I often stick to Simple offset.

You could also try dropping the gain but and taking longer/more exposures.

Edited by Jonathan_Shields
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Thank you everyone for the various pieces of advice - I suspect it is most likely moonlight. Although I’d read the SharpCap manual pretty much cover to cover while waiting for my camera to arrive, I hadn’t twigged about the background correction, so I think that the gradient removal will be worth a try while live stacking. 
 

And pick appropriate targets on moonlit nights. 

Geoff 

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