Experiments - Samyang 135mm and Ekos Auto-focusing and correct spacing

[paul]

If anyone is planning to do something similar, I can report that an 11:1 reduction is enough resolution for accurate Bahtinov focusing. But more important (to me 😀) when using EKOS autofocus steps = "1" and backlash = "0", Ekos is able to autofocus accurately for Narrow and wide band filters.  This is for a mono camera with 3.8 micrometre pixels.

There are not many steps to play with, for my current spacing Lum and Oii  are focusing at step 14, Ha and Sii are at step 18 but it is a very measurable difference per step. There is some mechanical backlash for somewhere but the routine copes very well.

 

These focus points will change as I trim spacing for better star shapes (I'm too far out at the moment). The good news is Samyang EOS to M48 adapter from FLO has eliminated the image tilt I had previously, the distortion is very symmetric.

 

The stack up is from the FLO Samyang M48 flange: Baader M48 0.8 spacer, ZWO 16.5 mm, M42 0.7 spacer, EFW (20mm),  ASI1600 (6.5mm) (Total 44.6mm). I think I need around 44.5 to get good shapes in the corners - this will also push the focusing position out a bit which is a good thing.