Feature Request : offset after Auto Focus

On my 10" f4 Newton I use to perform AF routine every 40 minutes since the focus is continuously drifting over the night.
The average drift between two consecutive AF routines is around 50 steps (more than the CFZ of the scope). It is not really correlated to the temperature drop, probably due to differential thermal dilatation between the carbon tube, the mirror cell and the focuser.

To account for this I could perform AF every 20 minutes or so… at the expense of 3 minutes for each AF.
It could be better, in this case, to decide that the AF result must be offset by a certain amount (positive or negative). In this way, the focus could be set optimum for the image in the middle of the interval between two AF, instead of being optimal for the first one only, the others being more out of focus.

This is an option I would love to see. As I take 30 min subs, I often find the focus has moved too much during that time and having a box to put a step figure in to move the focus (inwards in my case) say four steps will keep a better averaged focus during the exposure.

I realize that as the night draws on, the need for this offset will reduce, so this feature is only really going to suit those who monitor their imaging throughout the session.


Temperature compensation does this. You set 2 steps/degree or whatever you’re seeing, and SGP will adjust the focus as it runs.

Thank you, I will give that a try. Mike

As I just said, on many setups the focus drift is NOT exactly correlated to the temperature drop… I even saw instances where the temperature drops continuously during the night, but the focus goes first inwards, then reverses outwards 3 hours later !

I don’t see how applying an offset to the focus position can be done when you don’t know how much or in what direction the drift is in.


Most of the time, the behaviour on my setup has a fairly good reproducibility :

  • 40 to 50 steps of inward drift during the first hour ;
  • 20 to 30 during the subsequent hours.
    This behaviour not being correlated to the temperature drop, except by chance.

Rarely, strange behaviours like reversing drift direction are observable.

Therefore, for the general case, applying an offset to the AF position, in the Control Panel, would be useful.


IIRC SGP Temp compensation adjust the focus in between lights right? Or does it continuously while exposing a light frame?



Correct. SGPro will never move the focuser during integration.

Well there goes my reason for having an offset to a mid point where the focus is likely to be half way through the integration. The only other way round this is to manually adjust the focus during the integration. I know this works ok for my system as it is mechanically sound and no vibration occurs during the focus. But one has to remember and be around to make the adjustment.

If your ASCOM focuser driver has temperature compensation then this is intended to be used during the exposure.

It would need to be enabled by SGP after focusing because you can’t move the focuser while it’s temperature compensating and vice versa.


This code already exists in SGPro. Just before AF, SGPro will turn your focuser’s temp comp off (if it can) and then after complete will turn it back on (if it was on before AF).