Anyone using canon 200mm L f/2.8?

Been struggling for days trying to get sgpro autofocuser to run and get that “V” curve for this lens. I’m using a belt driven focuser that has the belt going around the focuser ring.

Been reading the instructions on sgpro autofocuser and apparently the step size is 2? Or atleast that works for now. I have the camera lens set at f/4 so I’m not sure if I should have smart focus working or not.

The lens is hooked up to the zwo asi1600mm and the backspacing is right to a T.

I have it set to bin 2x2 and have proper exposure times for each filter. I tried lowering the minimum star size to 3 and then back up to default 6 but no difference.

Basically I want to know if I’m not alone in this and if anyone has figured out their way of making the lens work with a belt driven focuser for nights of cycling RGB filters as the RGB filters are not parfocal with the 200mm.

I tend to toss out half my frames due to it not working the way I need it to.

Anyone with experience please help!

Yes, I use it quite successfully but mine is a very custom setup. A few principles:

  1. Lens requires a setup that has a motor with very small step sizes (I use the starlight system)
  2. Lens has tons of backlash so you need to set that very high
  3. The backlash can occur in both directions so initial setup of the lens when placing the belt so that you are on “one side” of the backlash is important
  4. I would not use the lens grip with the belt - too much slip possible. I cut away a section of the grip and glued a belt onto the lens focus barrel upside down so the notches engage each other - zero slip.

Here is picture of my system and then an example of an image taken with it.



FYI, these are all unguided even though the picture shows a guider - I have found it unnecessary with my Paramount ME.

Finally, I machined an f5 lens stop out of a metal lens cap. Otherwise you get the nasty spikes around the bright stars from the lens iris. Don’t even think about f2.8 with even a moderate sized chip unless you don’t care about round stars around the edges.


Hope you keep posting on your progress, I am in the process of setting up a similar system. My custom adapter is coming on Aug. 6th from PreciseParts. Then I’ll have to tackle the focus issue.