SGP autofocus on Quadruplet issue
Posted 16 April 2018 - 12:57 PM
Everytime I try to autofocus on my TS65Q I am defocused! SGP creates a very nice U curve in the procedure, but my images are still defocused.
My scope is quad, so during autofocus stars become elongaded. Is this causing the defocus or somwthing else?
Posted 16 April 2018 - 01:40 PM
My best guess is that this is backlash. Have you set a reasonable amount for backlash within SGP? But first can you talk us through how you set up autofocus? I use the method whereby you get good focus with a Bahtinov (or similar), then measure HFR and note focuser position. Then keep racking the focuser out until you get an HFR that is 3-5 times larger than the original, in focus HFR. Note focus position again. Calculate the difference between focuser positions and divide by 4 (for a 9 step focus run). That is your step size.
The backlash setting may be different from one focuser to the next. I have one set at 100 and the other set at 200 steps. I’ve probably set them too high, but better too high than insufficient.
Posted 16 April 2018 - 02:58 PM
Thank you for your reply. I did the same procedure as you describe but with no backlash. How can I now how much backlash do I have?
Posted 16 April 2018 - 03:19 PM
From another post concerning this focuser:
-So far I did not encounter slippage, but PL’s advice if you do experience slippage, is to set all the speed steps to 1. It will make it a bit slower, but nothing to worry about. Personally I found the default speed settings extremely fast so I set them slower anyway.
Dan in NY
Posted 16 April 2018 - 05:15 PM
If the star images used for autofocus are elongated - that's a problem. It may mean the focuser is moving or something is settling during the exposure. For some reason sgp refused to allow a settling time during autofocus - and in addition there was a bug that allowed it to expose while the focus was actually changing.
So I would make sure things are working properly by sitting next to the scope during autofocus and watching how the focuser moves.
I don't think there is any reason a quad should have particular problems - but if something is moving during the exposures I can imagine you could get a decent parabolic focus curve that didn't actually lead to good focus.
One thing to do is to manually take an exposure of the same duration and binning as the autofocus exposures. If the stars are small and round in a manual exposure but elongated during autofocus - there's a problem.