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Old 30-05-2022, 11:00 AM
JA
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Join Date: Oct 2016
Location: Melbourne, Australia
Posts: 2,945
Quote:
Originally Posted by bojan View Post
Comparison between Meade (mine) and Celestron (courtesy of JA) FF-FR's at 85 mm back focus.

Centre is OK, but in corners of the APS frame Celestron is way better, similar to Damien's specimen.
Hi Bojan,

That's an interesting comparison. :

Some observations based on the Centre image:
1. I would say that the Meade is a little sharper or possibly(?) slightly better focused. The stars are definitely smaller, more than the ~5% difference (see below) in magnification/FOV would suggest.

2. Assuming that the images are shown at the same screen magnification, is that the Meade offers an approximately 5% wider field of view, based on my measurements between a couple of reference stars I chose (92mm on the Celestron Left Hand Side Image V 88mm on the Meade Right Hand Side), which equates to ~<5%.
Some observations based on the Corner image:
1. I agree with you on the issue of definetly better corner performance on the Celestron with the APS sensor, but that may simply be because 85mm is not the sweet spot for the Meade reducer, whereas it appears to be with the Celestron. Maybe you could continue to tune reducer/sensor distance to try to find the optimum distance for the Meade. This could be more easily achieved if the M42 spacer you made was adjustable like a helicoid focuser. There are infact M42x1 helicoid focusers on eBay in the $30-40 range which have either 12-17mm, 17-31mm or (32-55mm?) of adjustment range. You could somehow mount your Schmidt Cassegrain Female coupling ring to the helicoid focuser and have such an adjustment system.
I'm Always curious: One thing I notice is that there is a definite difference in contrast between the Meade and Celestron Images. Were the same exposure settings used for all images? It is possibly caused by more light scatter or perhaps some nearby cloud scattering light - or different lighting conditions ...possibly changing between the Meade and Celestron images ???.

Best
JA

Last edited by JA; 30-05-2022 at 11:26 AM.
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