Focus question


These days when it can be 60* in the evening and 20* at night are killing me. It is not uncommon for me to see a 2* or more drop during a single narrowband exposure, and that puts focus out of the CFZ during the latter part of the exposure. What I would like to do is use temp compensation during the exposure and then have the AF routine run as usual. Unfortunately, best I can tell, my Rigel USB nStep won’t let me do this. I have also build an Arduino-based focuser that came with an open-source ASCOM driver. It is not clear to me whether this driver can support what I want to do, but I can at least modify it if I need to.

I recall a thread that I can’t find now where I think Ken explained that if an ASCOM focuser has an internal temp compensation capability, SGP will disable it when the AF routine runs. So, what I’m looking for is the ASCOM property that is used for this. I think it would require toggling the TempComp property off for AF and then toggling it back on again when the AF routine is done. Is this correct?




If TempComp is enabled the focuser should throw an exception if we attempt to move it. From the ASCOM Docs:

If the TempCompAvailable property is True, then setting TempComp to True puts the focuser into temperature tracking mode. While in temperature tracking mode, Move commands will be rejected by the focuser. Set to False to turn off temperature tracking.
If temperature compensation is not available, this property must always return False.

So we check if this is enabled prior to Auto Focus and if so we disable it, run auto focus and then re-enable it.

Hope that helps,


Tim, I am using myFocuser, an Arduino-based focuser that includes a temperature sensor and that works with SGP refocusing without my intervention with a temperature change (0,7 to 1°, in my case).

Probably yours can be modified too. I am not too familiar with Arduinos or Ascom drivers but a friend of mine helped me to complete this project and if you have any questions I can ask him for help.




Thanks, Jared. That’s what I thought but wanted to be sure.



Thanks, Renan. If I run into any hitches I’ll let you know.



This is correct… that said I did spot a bug in this logic that will prevent it from working. When this logic was implemented, the old logic to just turn it off on focuser connect was not removed (meaning every run will think the original state of temp comp was off). The fix will be available in


Hi Ken:

Has this been fixed? It’s not in the release notes.





Yep. From the release notes:

Fixed a bug that would automatically turn ASCOM focuser temp compensation off when it connects.


Shoot, how did I miss that. Thanks a bunch, Ken.