SCT - C925 help with odd shaped stars?

[SimonfromSussex]

I've started imaging with a C925 because I want a larger scale on galaxies. However i'm getting odd "pacman" shaped stars on the outskirts of the image. I guess I can crop them out but just wondered what was causing them?

I'm using a modded canon 1000d, a meade 0.6 focal reducer and guiding using phd and a finderguider.

Here are a couple of examples:

M63 - 5 min subs:

M13 - 3 min subs:

Many thanks for any advice.

[todd8137]

can not help i have the same scope ect and never seen this ,are they there before you put them in you PS?

[Cornelius Varley]

It looks like coma. The elongated stars radiate from the centre of the image, the stars in the centre of the image are more rounded.

Peter

[Rustysplit]

Just a thought Si, Have you tried it without the reducer, just to check that it is not causing the odd shapes? I know you really need it for imaging, but worth a look.

[cgarry]

Do you have the correct distance between the focal reducer and camera sensor?

Cheers,

Chris

[Merlin66]

Is this an HD version of the C925??

[SimonfromSussex]

Thanks everyone for the replies.

They are on every sub before any processing.

I will try without the reducer and see if that stops it.

I'm not sure what the correct distance should be Chris, do you know? The reducer screws onto the end of the sct, then there is a 2" bracker that screws onto that. Into the brackets goes the 2" t adapter and then the camera - I'd say just over 5 inches between reducer and camera chip?

Sadly just the standard 925, not HD! - Mabe one day, lol!

[Coco]

Thanks everyone for the replies.

They are on every sub before any processing.

I will try without the reducer and see if that stops it.

I'm not sure what the correct distance should be Chris, do you know? The reducer screws onto the end of the sct, then there is a 2" bracker that screws onto that. Into the brackets goes the 2" t adapter and then the camera - I'd say just over 5 inches between reducer and camera chip?

Sadly just the standard 925, not HD! - Mabe one day, lol!

Its supposed to be 55mm from rear element to chip surface..

On my CPC I had the f/6.3 - SCT2" Adapter - 2"nose to EOS - EOS Camera

[SimonfromSussex]

Thanks Guy, I'm double that length easily.

Because my t-adapter is a push fit I have to use an sct adapter too and this is 55mm in itself. Might try and get a screw on t-adapter that will fit direct to the focal reducer.

[Lars]

Isn't 55mm the distance between the front of the camera to the sensor for a Canon DSLR?

I think Celestron recommends about 110mm from the rear element of Celestron reducer to sensor surface though on my 9.25 I get better results with about 90mm.

Or is there such a difference between the Meade and Celestron reducer?

/Lars

[ollypenrice]

Isn't 55mm the distance between the front of the camera to the sensor for a Canon DSLR?

I think Celestron recommends about 110mm from the rear element of Celestron reducer to sensor surface though on my 9.25 I get better results with about 90mm.

Or is there such a difference between the Meade and Celestron reducer?

/Lars

They are the same. I'd try the distance Lars suggests.

Olly

[Merlin66]

BTW, Over on the SCT yahoo forum there has been some strong discussion that the latest Meade x0.63 reducer is different from the original and now has a shorter focal length and hence a shorter backfocus distance.

The claim is that the backfocus is now just 45mm which means it won't work with a DSLR....

[SimonfromSussex]

Thanks to everyone for the help.

Just a quick update in case anyone else has this problem.

I purchased a new camera adapter from RVO: http://www.rothervalleyoptics.co.uk/ant ... d2696.html

This has shrunk (over half) the distance the DSLR camera is away from the reducer and I think it has solved the problem.

Here is my latest pic and this frame has not been cropped at all.

[Coco]

Much better Simon, DSLR's and SCT's are not very well suited, you have done well there..

[Lars]

I'm glad you got it sorted out. I think my adapter gives about F/7.5 and that gives much less field curvature and nicer stars.

Btw, the link doesn't seem to work.

/Lars

[SimonfromSussex]

Thanks Guys,

Sorry here is the proper link: http://www.rothervalleyoptics.co.uk/antares-sct-t-thread-long-adaptor_d2696.html

[Merlin66]

Just to close the loop on this issue.....

What distance did you end up with between the x0.63 reducer and the T2 adaptor on the Canon??

(The link you posted shows a 60mm spacer, add the 55mm for the T2 and camera = 115mm - very close to the original design)

[SimonfromSussex]

The gap with the new adapter between DSLR chip and reducer is now roughly 50mm.

The spacer isn't 60mm? As I say it's nearer 50mm. This has sorted the odd shape stars.