Re: Hi ! Question for MarkS - Focal Reducer Problem

[Rick]

Hello friends of Astrophotography & Astronomy!

While searching the internet I found another person that had the same problem with me.
Coma when using a focal reducer...

I have the Meade f/6.3 Focal Reducer which I've started using lately for astrophotography.
Recently I took a photo of the Globular Cluster M5 with the following setup :

Celestron C9.25" --> Meade f6.3 --> Baader 20mm SCT Adapter --> SBIG AO-8 unit --> SBIG ST-2000XCM ccd.
The whole setup distance from the reducer to the CCD chip is approximately 77mm

While everything was perfect, guiding @ 24hz, collimation with an artificial star and sharp focus,
star coma is apparent especially in the upper part of the image.  Please have a look :



Even the flat image has an off center vignetting :


I am posting because I've read that the member MarkS had the same problem.
Hello MarkS, did you finally found a solution?  

From my internet research, nobody seems to know exactly the focal distance of the reducers !!!
How strange.  I measured the focal distance of my Meade focal reducer by holding it
near a wall and changing its distance until the image of the room lamp comes to focus on the wall.
I found that the distance is 210mm.

In the post you say that given a 285mm focal length you need an 105mm distance
from FR to CCD.  How do you calculate that distance?

Just by analogy my 210mm focal reducer needs a 77mm spacing from FR to CCD.
If this is correct, then I am using the correct spacing, just by accident!

Why then I have the star coma?  Either the focal reducer spacing is wrong or I might
have a faulty reducer.  I am confused...  

I hope MarkS or any other member can help.
Best Regards,
George.

Hi George,

Welcome to the OAS forum. You should now be able to post in the other sections.

...and the rest of this thread has gone where it belongs.