Atik Cameras

Author Topic: Horizon OSC...gradient or vignette?  (Read 355 times)

CraigG

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Horizon OSC...gradient or vignette?
« on: January 12, 2019, 01:36:37 PM »
So my leaning curve with the Horizon is gradually flattening out as I get time to work with the camera! While waiting to hear from Atik on the horizontal banding question, I've noticed what I would call a vertical gradient (vignette?) effect in my images. I've attached an over processed stacked image to highlight what I'm seeing.

I was hoping that those of you that have worked with the Horizon and other cameras for way longer than I have could share your thoughts on this effect. Is this the camera's 4/3rds sensor merely telling me that there's a back focus issue or is it something else? I use a .8x ff/fr that's made for my 735mm refractor and I know that I'm about 2mm longer than the 55mm that manufacturer recommends for spacing. Before I order up an alternative spacer combo to address it, it would help to know if that will solve the issue or is the problem bigger than that? This is a simple imaging train with no diagonal, Barlow, etc. in the mix.

Thanks for your comments as always!
Craig

theoatc

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Re: Horizon OSC...gradient or vignette?
« Reply #1 on: January 12, 2019, 07:37:21 PM »
Amp glow Maybe? Change the orientation, around 90 degrees. See if it change orientation as well. Then it is amp glow.
If it doesn't, shot with hydrogen alpha filter. Check if it shows up. If not, then it is stray light

CraigG

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Re: Horizon OSC...gradient or vignette?
« Reply #2 on: January 13, 2019, 03:48:09 AM »
I forget to mention that I do have a Baader 2" UV/IR block filter mounted on the scope side of the FF/FR.
-Craig

theoatc

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Re: Horizon OSC...gradient or vignette?
« Reply #3 on: January 13, 2019, 11:19:41 AM »
I forget to mention that I do have a Baader 2" UV/IR block filter mounted on the scope side of the FF/FR.
-Craig
only ha filter blocks out completely stray lights. UV/IR makes stars looks less bloated.

bwa

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Re: Horizon OSC...gradient or vignette?
« Reply #4 on: January 14, 2019, 02:51:54 AM »
...
I was hoping that those of you that have worked with the Horizon and other cameras for way longer than I have could share your thoughts on this effect. Is this the camera's 4/3rds sensor merely telling me that there's a back focus issue or is it something else? I use a .8x ff/fr that's made for my 735mm refractor and I know that I'm about 2mm longer than the 55mm that manufacturer recommends for spacing. Before I order up an alternative spacer combo to address it, it would help to know if that will solve the issue or is the problem bigger than that? This is a simple imaging train with no diagonal, Barlow, etc. in the mix.

Thanks for your comments as always!
Craig
Wow, I've never seen a vertical gradient like your example!

I'm pretty sure it isn't a FF/FR problem.  In that case the gradient would be circular.  FF/FR backspace is pretty flexible and your stars appear nice and round all the way out to the edges, so don't worry about spacing.

It doesn't look like amp glow.  It normally shows up in a corner.  If I remember correctly, the lower right corner on the Horizon and Infinity.

Is there a chance you have lights off to the side(s) of your imaging setup?  Side light can sometimes cause a similar problem.

If this is the only time you've seen the problem, it might just have been a gradient in the sky from clouds?

Sorry I can't be of much help...

bwa

CraigG

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Re: Horizon OSC...gradient or vignette?
« Reply #5 on: January 14, 2019, 01:44:09 PM »
Thanks bwa and theoatc-

Well I'm glad to know that it's not a spacing issue so I can concentrate my efforts elsewhere. Unfortunately, I don't think the cause is from stray light or seeing/transparency variations. I'm imaging from Bortle 3 rural skies with no lights anywhere around me. The image train components are tight and sealed. I have noticed the gradient in other images on different nights and noticed it again last night in some really good seeing and transparency conditions. Maybe it relates to the horizontal banding issue? The gradient does line up with the sensor's long axis.

Sigh, getting pretty frustrated with these issues that keep me from fully enjoying an imaging session but I am thankful for everyone's good council. Bwa, I'm appreciating your earlier comments about using this camera for EAA more and more each session.

Thanks guys!
Craig

dpaul

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Re: Horizon OSC...gradient or vignette?
« Reply #6 on: January 15, 2019, 10:17:29 PM »
Hi Craig,

Maybe it is just light gradient in the sky, I get it all the time.

I did a very simple gradient extraction using Pixinsight (Dynamic Background Extraction) in Pixinsight. Then also a simple mask for the galaxies and reduced the background granulation. Finally a quick tweak with the Canon debanding tool. The attached result was only 5 mins work.

Just trying to be helpful showing that very simple post processing can make a difference.

One thing that is definately evident is 'undersampling' so the star looks quite pixelated. If you tried without the focal reducer you should see better detail (but less field of view of course). I have the opposite problem with an F3.5 mirror and a long focl length so my number of pixels per star is 'over-sampled'. I can't use a FR because it would create coma.

Hope this is helpful

David






CraigG

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Re: Horizon OSC...gradient or vignette?
« Reply #7 on: January 16, 2019, 02:33:16 AM »
David-

You are ALWAYS helpful! Wow, your PI tweaks really made a difference. Just one more reason to take the plunge and go with PI.

Thanks for your comments on undersampling of the stars. I do use the "Raw Pixel" setting when stacking in Infinity so maybe that's a factor. I wouldn't have guessed that undersampling would be an issue with this sensor in my setup but I'll try your suggestion!

Anyway, your post gave me encouragement just when I needed it. Hope I can return the favor some day!

Best wishes,
Craig

dpaul

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Re: Horizon OSC...gradient or vignette?
« Reply #8 on: January 16, 2019, 02:12:11 PM »
Hi Craig,

No problem at all, just trying to help - be warned Pixinsight takes a lot of learning but even using 10% of its capabilities will get great results. I do the actual image stacking and post processing with Pixinsight.

Regarding undersampling vs oversampling, the following link should be useful:

https://www.atik-cameras.com/news/rules-1-2-arc-second-per-pixel-and-when-to-break-them/

I'm actually using about 0.6 arcseconds per pixel but the recommendation if 1-2. However as the link says you can break the rules sometimes. I think yours might be too much the other way, check the table on the link, you can calculate it.

Regards

David