central bright area...vignetting?

Discussion of using SharpCap for Deep Sky Imaging
psy1280
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central bright area...vignetting?

#1

Post by psy1280 »

Last week I was attempting to image the Veil nebula, although clouds came in to ruin my LiveStack, the image that was developing had a circular bright area in the center of the image covering about 1/5 of the frame. I'm assuming this was vignetting, but wasn't sure. And if it's vignetting why is the central area affected? The rest of the image was fine.

8" SCT
f/ 2 with hyperstar
70 light frames (with 70 dark-flat subtraction)
IDAS LP filter
30 second lights / 120 gain
ASI294 camera
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oopfan
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Re: central bright area...vignetting?

#2

Post by oopfan »

Joe,

Your focal length is
406.4mm (that is 8in x 25.4mm/in x f/2)

Your camera's sensor size is
62mm x 36mm

Your sensor is very large for your telescope's focal length. My telescope's focal length is similar but my sensor is only 6mm. Vignetting is almost non-existent but I still take Flats to correct for it. You definitely have vignetting, so Flats are imperative.

The common definition of vignetting is that the "image circle" is smaller than your sensor. With your telescope it is difficult to visualize but if you had a refractor you could pull the camera and point the scope at the daytime sky. Place a sheet of thin photocopy paper where the camera would normally be. You should see a bright disk of light with the edges darker than the center. Why does it occur? I could be wrong, but I believe it is due to a combination of the sensor not being curved in addition to internal light baffling in the optics.

In the old days I had a 3-inch f/16 achromat refractor with baffles. I used a 35mm film camera. It's the same size as your ASI294's sensor. I still have the negatives of M42 and M8 where I can clearly see the "image circle".

Brian
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oopfan
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Re: central bright area...vignetting?

#3

Post by oopfan »

Joe,

You mentioned:
70 light frames (with 70 dark-flat subtraction)
Dark Flats are not a replacement for Flats. I trust that you captured Flats too. However, if you are seeing a bright image circle, then that suggests to me that Flats were not applied.

Brian
psy1280
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Re: central bright area...vignetting?

#4

Post by psy1280 »

Thanks Brian,
I tried to subtract dark-flats from my flats (as this was suggested by some for the quirkiness of the ASI294) rather than bias subtraction...the vignetting remains a mystery to me. I need a another clear night to replicate the situation. This problem has never been an issue before.

Thanks all for responding,
Joe
psy1280
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Re: central bright area...vignetting?

#5

Post by psy1280 »

Also, Brian
I was not accurate in what I wrote in my original post. I should have said 70 flat frames, NOT light frames. Sorry for the miss-statement.
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oopfan
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Re: central bright area...vignetting?

#6

Post by oopfan »

Hi Joe,

I don't LiveStack so I don't know how much help I can be, but I found this reply to you from Robin:
looks like your flat has over-corrected (assuming that it was trying to remove vignetting). This can happen if you do not use darks along with the flat (basically if you do not use darks then the flat ends up correcting the part of the signal that the dark removes, which you don't want as it's not due to light). You should also use dark flats or bias substaction when creating the flats or you can end up with the opposite effect.
I use Bias, Darks, and Flats with AstroPixelProcessor (APP) with very good results, although I recently started to capture Dark Flats for my Hydrogen Alpha filter. To the best of my knowledge Dark Flats are only needed when the Flats exposure is longer than a couple seconds. My Ha Flats need 25 seconds, so it made sense there.

APP spoils me. I just give it the calibration files that I have, and it figures out the rest. I only mention that because of what Robin said in his reply, specifically "You should also use dark flats or bias substaction when creating the flats." Like I said, with APP I don't need to concern myself with those details, but it looks like SharpCap handles it differently. I'd ask Robin for clarification on this procedure.

By the way, your sensor size is 19mm x 13mm, not 62mm x 36mm like I mentioned. Those conflicting dimensions came from the same retailer's website. I verified 19x13 with a calculator. Still, Flats are needed to correct for vignetting.

Sorry that I can't be more help.

Brian
psy1280
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Re: central bright area...vignetting?

#7

Post by psy1280 »

Brian,
First let me thank you for your looking into this problem for me, I do appreciate your help. Let me add a bit to this discussion. Let me start by saying I'm still new to all of this and I'm stumbling my way forward. Trial-and-error (rather than specific knowledge) seems to be my path.

Regarding my flats. For a long, long time I struggled with my ASI294 camera (as have many others regarding flats). One person in the forums with the same camera recommended taking at least 50 flat frames/2 second exposures with dark subtraction. I tried this and was getting encouraging results. The issue I had last week (which had never happened before) with the central "vignette" was following the same protocol, with one exception. Because I saw some bright green pixels in the post-processed image, I decided to take darks. I had never taken darks before. I can't imagine that darks could have been the problem, but what do I know??? I realize that all this could have been an anomaly, and I can't wait to get out and try again to see if the problem persists.

A few additional observations:
1.) my flats were taken at 1 second with a gain of 120
2.) I'm using a Hyperstar f/2 connected to the front corrector plate of my 8"SCt and therefore, when I take dark-flats the only way I could capture these was to place a dark coat over the dew shield (which housed the camera and Hyperstar). I don't know if this in any way could have contributed to the problem.

Reading your responses, I fear it will be a long, long time before I reach the level you've attained. I couldn't even get my Celestron AVX mount to communicate with my laptop! That's the level I'm at...I had to reluctantly go back to my SkySafari in order to align my mount (using Celestron's Starsense alignment). I have all the ASCOM drivers on my laptop...I followed the instructions for connecting my hand controller to the mount and then tried using Carte du Ciel and...you guessed it. Nothing! So you see, I'm at the stage where stumbling is the name of the game. It would have been nice to have been able to plate solve rather than rely on SkySafari to do an auto align with Starsense. But, if you can't get your mount to communicate with ASCOM...well...that's where I am.

Anyway, anything you have to offer will be most appreciated.
Yours,
Joe
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oopfan
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Re: central bright area...vignetting?

#8

Post by oopfan »

Joe,

Progress is agonizingly slow with the terrible weather we have in the NYC region. I haven't been able to take out the scope in over a month.

Something tells me that your Darks are the problem. Using a coat can work but only if the material truly blocks light. I did a quick Internet search. Cloudy Nights says to remove the camera, screw on an opaque dark filter, and capture Dark frames in the comfort of your home. Turn the lights off in your home office for added protection. Don't forget to turn on the cooler!

Capture 30 darks over a range of exposures. By now you have a good idea of what is the most frequently used exposure for your sky conditions. For example, if you commonly use 30-second Light frames then capture a set of 30-second Darks. Then I would do a set of 35-second Darks, 40-second Darks, etc.

Other people may disagree with me, but you don't need to EXACTLY match the Dark exposure with your Light exposure. So, if you choose to capture 38-second Lights one night, then use the 40-second Darks. More than anything, Darks capture the difference in dark current from one pixel to another plus the effects of amp glow.

I've attached the Master Dark of a 50-frame stack of 100-second exposures taken using a Dark Filter to protect against photons from entering the camera. You can see how important Darks are. I used an uncooled Altair 290M CMOS camera in mid-March when the nights were still cold. Your cooled ASI294 will look better but you will still see differences from one pixel to the next.

As an added bonus, I've included a Master Flat. You can see that vignetting still exists with my tiny 6mm sensor. It will be more pronounced with your camera.

Brian
YZ Boo 50x100s Dark.png
YZ Boo 50x100s Dark.png (348.07 KiB) Viewed 1569 times
YZ Boo 30x Flat.png
YZ Boo 30x Flat.png (345.52 KiB) Viewed 1569 times
psy1280
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Re: central bright area...vignetting?

#9

Post by psy1280 »

Hey Brian,
Yes, I did do a series of darks inside...but I believe I have to take it a step further. I took the camera, laid it on the floor and covered it with a make-shift box and winter coat. I did this during the day. Guess I need to be much more conscious of light leaking.

However, I thought darks only affect those pesky pixels, not vignetting. Is that correct?

Thanks again for your support Brian.

Are you near NYC....if so, yeah, the skies have not been kind.

Joe
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Re: central bright area...vignetting?

#10

Post by oopfan »

Joe,

I'm 50 miles north of NYC.

Flats take care of vignetting. Darks take care of Amp Glow and hot/cold pixels and other variations in pixel response.

Darks and Dark Flats are captured when the camera is covered. Lights and Flats are taken when the camera is uncovered. When taking Flats, you are looking to correct for vignetting, dust, and fingerprints. You do that by flooding the telescope with soft, diffuse light using either the Sky, Monitor, or Electroluminescent Panel.

Brian
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