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Old 16-09-2017, 12:56 PM
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jwoody (Jeremy)
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Off axis guiding with RC scope - badly shaped stars

Hello all
Could I have some help please.
I use an OAG with my Sharpstar 65Q and it works perfectly, no problems at all and get 30 min subs.

Swap the camera/OAG/filterwheel assembly to the 6"RC and I can't get correctly shaped stars to guide on. If I de-focus the OAG it appears that the stars are semi circles or blobs. It is like the light is being partially blocked by certain internal parts of the RC scope, maybe.

I have altered the distance that the prism protrudes into the light path but doesn't make much difference.
I usually get enough of a part of a star to guide on but it is not ideal.

People obviously guide with much bigger RC's than mine and get results that require good guiding so I assume there is a solution to my problem but I need help at this stage please.

Thank you
Jeremy
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Old 16-09-2017, 01:27 PM
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DiscoDuck (Paul)
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Hi Jeremy,
I'm in the middle of changing over cameras on my scope at the moment, but what I was using until recently that worked OK with my RC8 was the AstroTech field flattener
https://www.astronomics.com/astro-te...rs_p17393.aspx
Not sure if it'd work with the RC6 but may be worth considering,

Without it, the stars in the corners of my QHY8 were a bit egg-shaped. Not tried the OAG without it, but I'd imagine it'd be like you describe. With it, the stars are not perfect, but ok for guiding.

With the RC6, the smaller focal length will make your radius of curvature smaller so the effect might be even worse for you.

Did you have some sort of flattener in the other scope you use?

Paul
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Old 16-09-2017, 01:31 PM
glend (Glen)
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There is a big difference in focal length between those two scopes! Short focal length scopes are much more tolerant of guiding irregularities than long focal length ones. And those budget RCs are notorious for collimation and focuser shifting problems. Of course the light is being blocked in the RC by that large central obstruction. Start with fundamentals, is the RC properly collimated with your image train installed on it? Honestly, i would not worry about a flattener on that RC just yet, they are supposed to be a flat field design, at least flat enough that its not the main cause of your problem.
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Old 16-09-2017, 02:01 PM
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DiscoDuck (Paul)
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Quote:
Originally Posted by glend View Post
There is a big difference in focal length between those two scopes! Short focal length scopes are much more tolerant of guiding irregularities than long focal length ones. And those budget RCs are notorious for collimation and focuser shifting problems. Of course the light is being blocked in the RC by that large central obstruction. Start with fundamentals, is the RC properly collimated with your image train installed on it? Honestly, i would not worry about a flattener on that RC just yet, they are supposed to be a flat field design, at least flat enough that its not the main cause of your problem.
Agree - collimation worth checking first before worrying about buying extras! Collimating an RC is not a fun task - but there are some great guides out there (I found the "hall of mirrors" technique invaluable for getting very close to collimation http://www.davidcortner.com/slowblog/20110815c.php).

Just a side query to Glen - I thought that RCs were coma free, but they still have field curvature (which will manifest as radial differences in optimal focus position). Certainly mine needed a field flattener!
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Old 16-09-2017, 02:18 PM
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jwoody (Jeremy)
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Thanks Paul and Glen

The actual target image on my QHY9 is fine, no problems at all (well to my eyes). Very flat and my collimation is done with a Tak collimation scope and is very good.

In the daylight I remove the guide camera from the OAG and look down to the prism I can see out the front of the scope(as you would expect) and can see the secondary and the reflection back but it is very plain to see that it isn't all lined up properly, not collimated I guess you would call it? Or is that what the "Off Axis" part of OAG is?
Even if I adjust the prism down the shaft as close as I can go to the centre off the scope (more to the middle of the light path if that makes sense)the mis-alignment of everything doesn't change.

I hope that makes sense, I am in the obs now having a close look at everything on the bench. I can take some photo's and that may help explain things.
Thank you again
Jeremy
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Old 16-09-2017, 03:51 PM
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Guide stars taken from the edge of the image can be distorted in shape.
I think you'll find the guiding still works fine. Pick a star that's not over saturated.

Make sure your guide camera is focused as best as you can get it.

If your RC is well collimated it may simply be you are picking up the light out of the illuminated corrected circle of the scope.

Guiding software picks the centre anyway so in my experience whilst I prefer a perfectly round star an odd one still works.

Greg.
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Old 16-09-2017, 04:07 PM
DJT (David)
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Hi Jeremy

I agree with Greg. I have a "budget el cheapo" RC8, no flattener and guide with an OAG. You won't get pinpoint guide stars as you are picking off from the edge of the field where curvature is more noticeable. The curvature is a feature. The flattener is just extra glass that gets in the way.

Still I have no issues guiding long exposures for NB on dodgy looking guide stars but I do pick them from the lower edge of the auto guiding sub where the shape is less Leary when I can. Suggest you give it a go.

Note you are not seeing Coma.

The scope does need to be well collimated though, like any RC.

Enjoy

Last edited by DJT; 16-09-2017 at 04:19 PM.
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Old 16-09-2017, 04:41 PM
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Merlin66 (Ken)
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Jeremy,
The shape of the guide star image...
This should be no worse than the star image at the extreme edge - bearing in mind that the pick off prism is "sampling" the FOV outside your existing camera limits - these will vary with sensor size.
The main issue, I've found, is that the pick-off prism doesn't always allow rotation to bring the guide star image onto the axis of the guide camera.

(Think about it: the prism is just the "end" of a diagonal mirror, the centre of this "diagonal" would sit on the optical axis of the telescope BUT the guide tube is not, it's sitting rearward to suit the body design of the OAG. I have a diagram somewhere which shows this OAG mis-alignment, I'll find it and upload.)
Attached Thumbnails
Click for full-size image (oag_theory_layout.jpg)
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Last edited by Merlin66; 16-09-2017 at 04:45 PM. Reason: diagram added
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Old 16-09-2017, 06:07 PM
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jwoody (Jeremy)
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Thanks everyone for the input.

Unfortunately some high cloud has rolled in here over the northern Gold Coast so I shall have to wait until tomorrow night to have more of a test.

Thanks again for all the help and diagram too Ken
Cheers
Jeremy
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Old 16-09-2017, 06:22 PM
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The other thing that can help is guide using 1x1 binning instead of the usual 2x2 binning advice. I find I get better guiding when I can guide at 1x1.
Sometimes that is not easy if there aren't many bright guide stars nearby.

Greg.
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Old 16-09-2017, 06:25 PM
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jwoody (Jeremy)
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Thanks Greg
I use a QHYLii which is pretty sensitive so I will try that too.
Cheers
Jeremy
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