Quote:
Originally Posted by peter_4059
Interesting test David. Do you think the improved focus at larger spacing is a function of the scope being f6?
I've got my paracorr at 55mm spacing with the qhy8 on my f5 Newt. I focus on a star using a Bahtinov mask then slew to the object I'm after.
Here's how the centre and corners of my image are turning out.
Peter.
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Yes, I definitely believe that the optimal focus position depends on the f-ratio and perhaps the aperture. Televue have not published any data on this that I can find, but ASA have published data on their 3" Keller corrector for Newtonians which shows this. Of course they may be different optical systems, but it seems likely that there will also be some relationship for the Paracorr. In the Keller design, the longer the f-ratio the larger the optimal spacing; also for a given f-ratio, the larger the aperture the larger the spacing.
Looking at your images it is a little hard to tell, as the top left / bottom right stars look elongated, but the others look closer to round. I suspect that a larger spacing will be optimal for you based on my experience, but you probably first need to find out why the focus is not symmetrical in the field. Maybe collimation or image tilt - I think someone suggested before that you take some test shots and rotate the camera to see how that affects the image.