Help with Donuts

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Europa
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Help with Donuts

Post by Europa » Sat Dec 09, 2017 12:25 am

It appears I may have multiple issues, the first is why I'm getting the donut near Alnitak, if I take longer exposures, I will get these around any bright stars as well as if I move my scope slightly, it will stay with the star. I'm hoping whatever this issue is, it'll resolve the larger ring that looks more like an issue with Flats. I haven't changed anything on my process for flats, so I wouldn't think its that.

This is shot on the Celestron RASA, I had it in for service, so I'm not sure if its something I am doing, or the repair was not completed correctly.

Any help would be greatly appreciated.

Thanks.
Brian
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h2opolo
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Re: Help with Donuts

Post by h2opolo » Sat Dec 09, 2017 8:02 am

Nice shot Brian. Alnitak is a real problem when going after this target. Frankly I don't think I have seen too many images of the Horsehead without some sort of halo around it. That does not concern me near as much as the large circle going around the entire image? That's crazy looking. Flats would handle it I think. Have you checked your back focus distance to ensure that it is correct? What are you using to shoot flats or what program are you using. I'm using a flat light box inside with SGPRO Flat Wizard which gives me really good control on the density of my flats. I would experiment around with them until you find the right exposure to eliminate that ring. How many flats are you using and what processing software are you using to incorporate them into your image?

I think applying the correct calibration to the image will help you a lot and maybe not bother you so much with that bright beast Alnitak.

Keep shooting. Looking good.
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Re: Help with Donuts

Post by Europa » Sat Dec 09, 2017 8:56 am

Thanks for the help.

I use T-shirt flats and I do my pre-processing in Nebulosity4. This was ~60 flats. I’m using a OSC with the Celestron LPS filter mounted on the 4 element lens.(RASA is similar to Hyperstar). My process has not changed, the only change was having my RASA serviced by Celestron, as the lens cell came lose from the front corrector plate.

If you look closer at Alnitak, you will see an additional halo towards 11 o’clock and the vignetting ring is dark on one side and light on the other. I’m thinking this could be a reflection issue. As soon as the clouds go away, I’ll try to remove the filter to see if that does anything, maybe something is not sitting properly????

Thanks again for your input.

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Re: Help with Donuts

Post by h2opolo » Sat Dec 09, 2017 11:04 am

I'm sorry Brian. When I looked at this the first time it was on my phone so I did not make out the reflection. That's definitely a reflection. A bad one too. Can you post a link to the larger file so I can look at it better? I see some collimation issues as well. You have tips on your stars at the 1 o clock portion. I had a hyperstar for a long time and fought with that same problem.

Let me think about it for a while and figure out what is causing that reflection. Does the RASA have a corrector plate on the front like the C11's? If so it could be coming from light bouncing back up against it. Was the moon off to the side perhaps while you were capturing this? That big light-bulb will give you fits with that type of setup.

I don't think you have anything way out of alignment other than the collimation part so I'm leaning towards a bounced reflection. Try pointing at a big bright star and center it. Shoot a few shots like that and see what happens.

T-shirt flats work but it's hard to get control of the exposure for a long period of time and keep it the same. 30 Flats should be sufficient for your setup. Let me know what you come up with.

Good luck.

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Re: Help with Donuts

Post by Europa » Sat Dec 09, 2017 1:16 pm

Thanks Sean, the moon was out, it I also tried this earlier and got the same results.

Yes, there is a corrector plate in the front, the 4 element lens would be similar to the HYperstar .

As soon as the sky clears I will try a bright star, centered. I’ll try it both with and without the LPS filter to try and eliminate that as a problem.

I never had this issue prior to service, I would hope it would have come back perfectly aligned.

Thanks again, maybe I can post it to astrobin when I get the new pix.

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Re: Help with Donuts

Post by gardenfish » Sun Dec 10, 2017 6:07 am

I got similar reflections last week on one night when the moon was out. Didnt think flats would help so I never processed them. I will try a process and see if they go away. I was shooting closer to the moon then I think you were at the time.
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Re: Help with Donuts

Post by Europa » Sun Dec 17, 2017 10:24 pm

h2opolo wrote:I'm sorry Brian. When I looked at this the first time it was on my phone so I did not make out the reflection. That's definitely a reflection. A bad one too. Can you post a link to the larger file so I can look at it better? I see some collimation issues as well. You have tips on your stars at the 1 o clock portion. I had a hyperstar for a long time and fought with that same problem.

Let me think about it for a while and figure out what is causing that reflection. Does the RASA have a corrector plate on the front like the C11's? If so it could be coming from light bouncing back up against it. Was the moon off to the side perhaps while you were capturing this? That big light-bulb will give you fits with that type of setup.

I don't think you have anything way out of alignment other than the collimation part so I'm leaning towards a bounced reflection. Try pointing at a big bright star and center it. Shoot a few shots like that and see what happens.

T-shirt flats work but it's hard to get control of the exposure for a long period of time and keep it the same. 30 Flats should be sufficient for your setup. Let me know what you come up with.

Good luck.

Sean

Sean, I took a 500 Second exposure of Betelgeuse, the Halo is to the right and isn't as badly offset as it was in the Horsehead. I've saved the larger image to https://www.astrobin.com/326189/" onclick="window.open(this.href);return false; No flats, bias or darks were used. Thanks for looking.

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Re: Help with Donuts

Post by h2opolo » Mon Dec 18, 2017 6:11 pm

taking a look at it now. I'll get with ya
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Re: Help with Donuts

Post by Europa » Mon Dec 18, 2017 8:30 pm

h2opolo wrote:taking a look at it now. I'll get with ya

Thanks Sean.

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Re: Help with Donuts

Post by Doug » Mon Dec 18, 2017 9:15 pm

Have you ever seen a giant halo around the moon?

Looks like the same effect here. Could there have been a possibility of high thin clouds that night? OR possibly some dew? Most of the time small dust donuts can be fixed with flats. Hard to tell in the image size, but, it looks like a giant donut around the entire image.

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Re: Help with Donuts

Post by Europa » Wed Dec 20, 2017 10:18 pm

I’m not quite sure what happened with that large ring, I re-shot this again, with new flats and that one is no longer there, but the one at 11:00 of alnitak was still there.

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Re: Help with Donuts

Post by h2opolo » Sun Dec 24, 2017 7:01 pm

I shot the same target so let's see what mine does and compare. Merry Christmas I hope you and your family have a great time.

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Re: Help with Donuts

Post by Europa » Thu Jan 04, 2018 9:55 pm

I think I found the problem, I use 2 extension tubes to achieve backfocus, 1 of the tubes has a longer threaded end. The longer one was up to the camera and was not allowing it to sit correctly. I reversed the order of the tubes, with the shorter threaded end against the camera. I took some test shots and my offsetting halo/rings are gone.

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Re: Help with Donuts

Post by h2opolo » Mon Jan 08, 2018 10:40 am

Right On thats great. I wish my OAG issue was that easy. Good Job
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