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Old 13-09-2009, 05:59 PM
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8" GSO and canon 40D strange stars

Using a canon 40D and an 8" GSO RC I took the following image - I have never seen stars quite like this before. I suspect the scope needs collimation, but would this alone cause the image to appear as it is?

Regards,
Grahame.
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Old 13-09-2009, 06:11 PM
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The collimation looks ok, the big problem is coma
http://www.oldham-optical.co.uk/images/Photo_coma.jpg
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  #3  
Old 13-09-2009, 06:15 PM
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Coma? isn't an RC designed to be coma free?
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Old 13-09-2009, 06:21 PM
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Quote:
Originally Posted by Grahame View Post
Using a canon 40D and an 8" GSO RC I took the following image - I have never seen stars quite like this before. I suspect the scope needs collimation, but would this alone cause the image to appear as it is?

Regards,
Grahame.
Graheme....how did you mount the camera? Was this an afocal image? Did you use any sort of focal reducer?

Rgds
Peter
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Old 13-09-2009, 06:27 PM
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Ahh thanks david - I suspected this and so added a Field Flattener into the imaging train (with the result attached to this post) But my eyes still say there is something wrong with the image.... (lower and upper left in the image)
Could it be that this is caused by using such a large sensor for imaging?

Grahame.
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Last edited by Grahame; 13-09-2009 at 06:40 PM. Reason: spelling
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  #6  
Old 13-09-2009, 06:32 PM
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Looks like you may have a collimation problem. One side of the field is correct while the other is over stretched. But your first picture just shows massive coma and no miscolimation evidence. This is very confusing for an RC.
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Old 13-09-2009, 06:34 PM
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Gday Peter,

There was no focal reducer in use when the first image was taken. The second image was using a taka Field Flattener.

Grahame.
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Old 13-09-2009, 06:37 PM
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Do you have the standard focuser? Is it possible the weight of the camera has put the cam off center?
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Old 13-09-2009, 06:47 PM
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David,
As with all things GSO I have replaced the focuser with a Starlight Instruments Feather Touch focuser (so much better!)
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Old 13-09-2009, 06:57 PM
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Quote:
Originally Posted by Grahame View Post
Gday Peter,

There was no focal reducer in use when the first image was taken. The second image was using a taka Field Flattener.

Grahame.
Graheme..I have to say this looks odd. Have you adjusted the collimation at all? (I'm wondering whether the secondary is being stressed somehow...fix: loosen all three adjustment screws and gently re-collimate)

I'd also play with the focus. RC's have curved focal planes...often mistaken for coma. I always use stars about 2/3rd's out from the center.

rgds
Peter
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Old 13-09-2009, 07:31 PM
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Hi Peter,

I have not adjusted the collimation at all since the scope arrived from an un-named dealer (which has since lost all my business) Also when I was focusing I used a bright star in center of image - will adjust for next time being 2/3 way out from center - Thanks Peter!

Regards,
Grahame.
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  #12  
Old 13-09-2009, 07:39 PM
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I'm taking notes on all of this, should I get one of these babies.

Dave
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Old 13-09-2009, 07:42 PM
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I'm taking notes on all of this, should I get one of these babies.

Dave
Me too. It is actually the second similar post I see about this problem on IIS. I was thinking of getting one but now I'm hesitant to go this way.
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  #14  
Old 13-09-2009, 07:55 PM
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Have to say the Grahame's initial result surprised me

The test image I took (below) was in no-way fudged.

No flattener, focus at 2/3rd out from the centre. APS field

http://www.atscope.com.au/GSO/GSOomega.jpg.
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Old 13-09-2009, 08:00 PM
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Wow Peter! what camera was this image taken with?

Last edited by Grahame; 13-09-2009 at 08:01 PM. Reason: .
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Old 13-09-2009, 08:03 PM
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Hi Marc and anyone else thinking of getting one of these scopes,
dont let this thread diswade you from getting one of these babies! It is a great scope and defiantly serves its purpose. Just get it from someone like Peter for arguments sake.

Grahame.

Last edited by Grahame; 14-09-2009 at 08:44 PM.
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  #17  
Old 13-09-2009, 08:03 PM
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Quote:
Originally Posted by Grahame View Post
Wow Peter! what camera was this image taken with?
The same one as you've got. Well, same model etc.

I've seen a lot of this weird star shapes around 2/3 out with these GSO RCs, and I'm not sure if it's the focuser, camera isn't perpendicular to the light cone or the absence of a flattener, or out of collimation, or all of the above.

Dave
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  #18  
Old 13-09-2009, 08:03 PM
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Quote:
Originally Posted by Grahame View Post
Hi Marc and anyone else thinking of getting one of these scopes,
dont let this thread diswade you from getting one of these babies! It is a great scope and defiantly serves its purpose. Just get it from someone like Peter as opposed to (AC) for arguments sake.

Grahame.
AC? You mean Andrews? My experiences so far are that their pre sales support is very good, post sales support is so-so.

Dave
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  #19  
Old 13-09-2009, 08:11 PM
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It seems odd as Petes Omega pic is really very good wo a flattener
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Old 13-09-2009, 08:15 PM
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Pre Sales is great - its just post sales that lacks a great deal. However this post is not about the dealer - just the scope in question. It seams a few mods are needed to convert the original RC GSO 8" scope into an excellent usable scope. Its necessary but these few problems greatly outweigh the faults of an Awesome RC scope for its price... and I for one will vouch for this one!

Grahame.

PS, Yeah David - tell me about it!
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