unequal brightness in image

Discussion of using SharpCap for Deep Sky Imaging
psy1280
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unequal brightness in image

#1

Post by psy1280 »

I've distorted this image in Photoshop to highlight how the right side of the image is much brighter than the left side (I see this on all my images), any idea why this always happens? I'm using an Evolution SCT 8" al/az with Hyperstar and ZWO294 camera. This distorted shot was 15 second exposures with a gain of 120. I used a master and dark flat.

Thanks,
Joe
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admin
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Re: unequal brightness in image

#2

Post by admin »

Hi,

Could it be vignetting? Normally would expect the darkening to be in the corners but if your image was somehow offset from the centre point that it might look like this.

Cheers, Robin
psy1280
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Re: unequal brightness in image

#3

Post by psy1280 »

Thanks Robin,

Could you elaborate as to what you mean by and image being offset?

Joe
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Re: unequal brightness in image

#4

Post by admin »

Hi Joe,

Imagine that somehow your camera is not set in the middle of the imaging circle of the telescope – that it is offset to one side a bit. If that happened you would tend to see vignetting just to one edge of the image. As well as the problem occurring if physically moved the camera to the side, it would also potentially occur if primary collimation was out in a Newtonian – that would push the imaging circle off to one side. Not sure if it's possible to be badly collimated with your setup though.

Cheers, Robin
donstim
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Re: unequal brightness in image

#5

Post by donstim »

Any chance you could have an outside light source reflecting in? What happens if you change camera orientation? What do your flats show?

Don
psy1280
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Re: unequal brightness in image

#6

Post by psy1280 »

Don & Robin

Robin, yes, with my Hyperstar and camera on the front of my SCT I don’t think this would be a collimation problem (according to Hyperstar, poor collimation would produce elongated stars at the edges of the image, which I”m not seeing)

Don, I would agree this looks like it could be a stray light source, however, I’m doubtful this is the problem since my camera is covered with my dew shield.

I do appreciate your brainstorming on this....

Joe
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oopfan
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Re: unequal brightness in image

#7

Post by oopfan »

Joe,

Is that the Western Veil Nebula?

What time was it taken? I'll plug those numbers into my planetarium software to see what direction you were looking and elevation above the horizon.

Are you taking these from your balcony? I am guessing that your view is limited by your neighbors balcony above? If I remember correctly you live in Jersey near the Geo Washington bridge? Are you south of the bridge or north?

My guess, just a guess, is that your images contain "light domes" from looking across the river towards brightly lit Manhattan/Bronx/Yonkers. Does your balcony face east? Cygnus rises directly east so it makes sense that your telescope is pointing across the river.

If this is the case then you will need to learn how to remove light pollution gradients from your images using PixInsight/Photoshop. Are you using a light pollution filter?

Brian
psy1280
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Re: unequal brightness in image

#8

Post by psy1280 »

Brian,
You are quite the detective! Yes, I live across the river in a north Jersey suburb, about 8 miles north of the G.W. Bridge. Yup, looking east into the dreadful Manhattan skyglow (I’m using a IDA LPS-D2 LP filter) This was the Western Vail Nebula, about 30 degrees above the horizon, around 11:00 PM. Single family home, no one above me. I’m familiar with Photoshop in general, but just getting started applying it to astrophotography. Any beginner tutorial suggestions?

I’m going to give this same target a shot tonight (hopefully). This was taken with 15 second exposures/ 120 gain. I did use a master flat and dark.

Thanks for exploring this Brian, I appreciate your help,
Joe
psy1280
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Re: unequal brightness in image

#9

Post by psy1280 »

Brian,
Just a followup to get your opinion. Since the Vail Neb is due east and the skyglow in my image is to the right (Manhattan is to my right), could this simply be the glow? If so, I guess, as you say, my only recourse may be in post-processing. What say you?

Thanks in advance,
Joe
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Re: unequal brightness in image

#10

Post by oopfan »

Joe,

Ah, so you are probably in the Alpine, NJ area. Nice. A light pollution map says that south Alpine is Bortle 7 and north Alpine is Bortle 6. That's not bad. I was afraid you were in Fort Lee!

Do you have a 35mm DSLR and a tripod? If so can you point it in the direction of Cygnus at the time you are LiveStack'ing? If possible try to capture part of the horizon (i.e. treetops, rooftops) so as to give us an idea of the dome of light? Use a wide angle lens if you have one. Take a series of snapshots with different exposures. Post these to the forum if you can.

One more test if you can fit it in afterwards. Point your scope straight up to the zenith or as close to it as you can get. Capture a half dozen individual frames using the same exposure and gain setting that you used for the Veil Nebula. I guess that would be 15 second exposures/ 120 gain. No need to LiveStack. We just need the individual frames.

Let us know how you make out and then we'll go from there. I hope the skies clear up.

Brian
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