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OAG Odd Star Shapes


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Hi Everyone,

I'm using a ZWO OAG with my QHY5III174M guide camera. I can see stars in PHD2 when the exposures are looped, but their shape seems odd.

Here's a screenshot from PHD2:

Screenshot_2018-07-16-02-48-40-1.png.e9d6e9ea0fc7bc41603b2dbad88fa307.png

It looks a bit like only half the star is showing. The stars are definitely out of focus. I did try to adjust focus but could only move the guide head so far before it butted up against the filter wheel. I can never seem to get the top of the OAG aligned with the top of my filter wheel with my threaded connection.

Any advice on what could be causing the star shapes would be gratefully recieved. 

Andy.

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I do not use an oag (yet) but could the mirror be too far up. No really sure on aligning the oag but do you no have a threaded ring you can add to lock it off in the position you need. 

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With my OAG I had to fiddle with how far it was in the light path to get a good round star.

Try sorting it out in daylight with a distant object - also a pencil is a good friend mark the drawtube whrn the main scope i in focus then mark it again when the OAG is in focus and that is the size of spacer needed.

 

BTW OAG stars do not have to be spot on in focus, with one scope i had seagulls as my guide stars and it still worked fine , not saying it was ideal but it orked.

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I think you're both right the prism must not be far enough into the light path. I thought I'd got the positioning spot on, but it maybe the locking screw wasn't quite tight enough. Will do as suggested and try sorting it out in daylight. I think the star profile is definitely not right. I don't think FWHM should be negative.

Andy.

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Lots of oag users have rugby shaped guidestars..think about what a point of light is going to do on angle such as the pick off prism..ive not had the experience yet as my camera isnt seeing much at the moment

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  • 4 weeks later...
On 17/07/2018 at 21:34, newbie alert said:

Lots of oag users have rugby shaped guidestars..think about what a point of light is going to do on angle such as the pick off prism..ive not had the experience yet as my camera isnt seeing much at the moment

 

On 17/07/2018 at 21:45, Davey-T said:

I get banana and seagull shaped stars with me OAG it doesn't seem to bother PHD though.

Dave

I can second both of these, I've had some weird and wonderful shapes from my OAG...but little impact on the image. Don't worry about it too much.

 

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  • 2 years later...

Is your pickup prism optimally aligned with the chip? It should come into the middle of one of the long sides so it can come deeper into the light cone and so get more light and less coma.

Once you've done that you've done all you can and the others above say it works fine anyway.

Olly

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