Old 02-08-2014, 11:44 AM
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Rc8 odd rings

Hi Everyone,
I have caused an image defect on my RC8. Pictured are the rings that appear in each of my images no matter the direction I am facing. I am happy that I know the cause of the elongated stars, but the rings confuse me.

My imaging system has undergone several changes simultaneously so it is difficult to isolate a potential candidate. But here is the list of changes so far;

New Orion off-axis guider
New Moonlite 2 inch to t-thread adapter
New Bob's Knobs for secondary

I am using an Orion parsec 8300 Monochrome, with 2 inch filters. This is the same configuration as 2 years ago when I last imaged without the aforementioned defects.

I know that a flat will illuminate the issue but I would rather deal with the cause.

I believe that the secondary might be too far from the primary as a result of installing the Bobs Knobs, but I am unsure on how to prove this either way. Any help would be greatly appreciated.


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Old 02-08-2014, 12:08 PM
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alocky (Andrew lockwood)
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Hi Trent, the worst you could do with the secondary being in the wrong position is cause vignetting or slightly hurt your contrast, since the scope has still come to focus - same with the new T-adapter. If your secondary is not well positioned your collimation will also be problematic, but won't cause the effect you're showing. Is there anything reflective in the OAG? This really looks like a stray light issue to me, so some shiny metal exposed in the imaging train - and - a new source of light off axis.
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Old 02-08-2014, 09:06 PM
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Paul Haese
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I would put out an ask for someone to measure their secondary to primary baffle. That will give you the correct distance. Take a mean measurement from several scopes.
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Old 03-08-2014, 07:48 AM
DJT (David)
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Hi Trent
I also put an oag on my Rc8 last week and now have a fine set of rings very similar to yours.

Am assuming flats will sort it out
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Old 03-08-2014, 11:36 AM
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Thanks everyone.
As it turns out it was because of reflections. During the day I used matt paint on my 2inch - t adapter. Additionally I used a "Sharpie" on some areas of the OAG this resolved the issue.

Thanks for your advice. My focal length measurements for the rc8 as seen on a solved pinpoint image is 1608mm. I hope this might help someone else.

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