Samyang 135mm back focus

[Stefan73]

Hi all.

Been trying again to get my Samyang 135mm working properly now the nights are starting to draw in.

I'm using the M48 adapter from here https://www.firstlightoptics.com/adapters/astro-essentials-samyang-lens-to-m48-adapter.html and with that it claims I should need 44mm of back spacing.  I have tried this with ASI 2600 (with tilt adapter still in place), ZWO filter drawer (https://www.firstlightoptics.com/zwo-accessories/zwo-2-filter-drawer-m42-m48-v2.html) and 5.5mm of spacers to gave 17.5mm + 21mm + 5.5mm = 44mm.  This won't reach infinity focus (not even close at f2) and I have removed the little focus stop thing as per various youtube videos.  No filters involved either.

After some experimentation, I could reach infinity focus with 4.5mm of back spacing, and very near to where the focus stops (about 160 steps for the auto focuser) ie with a total back spacing of 43mm.  At that spacing I got the attached 60s sub at f2.  To me its got a slight rotational look to the star trails which I think implies the back focus should be a bit less even(?).  Any thoughts or advice appreciated!  Is the Samyang back focus that variable in practice?  Or is it always 44mm regardless and I'm doing something wrong? What are people using who are happy with their corner stars?

Also what's going on with the bright star in the bottom left quarter (Deneb I think)?  It looks like the sort of flaring with bites out of it that mirror clips on a newtonian cause.  I think Deneb was more or less straight up at the time so shouldn't have been any trees or anything in the way.  Other stars don't seem affected but obviously not as bright.

 

[michael8554]

I see elongation in the centre of the image, suggesting tracking and/or PA error.

Look at a much shorter exposure, stretched if necessary.

Michael

[Robert7980]

The Canon spec is 44mm, but with any optic it’s whatever spacing is required to flatten the feild. For terrestrial photography it’s not critical, for the extreme demands we place on optics for astrophotography even tiny differences can matter. With my Samyang 135 I’ve found adding about 0.7mm to the ZWO Canon filter drawer gives better corners with the 2600MC and factory infinity focus is much closer to dead on. I’m not sure what the exact spacing of the stack actually is though. This is without a filter, would be slightly less than the 0.7mm with filter, closer to 0.5mm. The lens needs to be stopped down some from f/2 to achieve perfect corners as well. F/2.8 is decent and f/4 is nearly perfect. This is best done with stop-down rings rather than with the lenses iris due to the diffraction the iris causes. I’ve only played with the one lens so I’m not sure how much they vary copy to copy, I doubt it’s much though. 

Edited August 15, 2023 by Robert7980