Focusing filters, focus every time or use offsets

Hi, I’m evaluating SGP so it might be a silly question. After reading the manual I’m still a bit confused about how to best use all the focusing options.

I understand at least once we need to focus every filter to set offsets ( profile setup), and the temp calibration, then what is the recommended procedure for every day focusing?:

  • focus with clear filter once a night
  • just use pre-set offsets for every filter change
  • let the temp calibration take care of the night adjustments

Or focus every filter every night?


Welcome to SGP!
SGP is very flexible in terms of focusing options. There are many
combinations of what people can do based on their setup. I think what
many people probably do is:

  1. Calculate filter offsets (once)
  2. In the Control Panel/Focus tab, choose “Use Auto Focus” and “Auto
    adjust focus per filter”
  3. In Auto Focus settings choose “Auto Focus with filter: LUM” and
    "Auto Focus every X degree change".

This means that between exposures if the temperature drops by whatever
degree you have set, SGP will switch to the LUM filter, run auto focus,
calculate the offset for whatever filter is for your next exposure and
move the focuser by that offset, switch to the imaging filter and start
the exposure.

I don’t have a temperature compensating focuser, but if I’m not
mistaken, SGP does not move the focuser during exposures but only in
between exposures. If you want to use temperature compensation so that
the focuser adjusts focus during exposures you will need to use the
software that came with the focuser independent of SGP. Someone please
correct me if I’m wrong about this part.

It will also help to know what equipment you are using.

Thanks Joel, that clarifies things for me.

My main AP gear is an SBIG STF-8300 camera, TSA102 TAK refractor with reducer so imaging at ~f/6. Atlas EQ-G. Mainly from home in narrowband.

I currently use a filter drawer, but now I see the advantage of using a motorized focuser, I guess I’ll be out looking for one now.

That is my understanding as well. I sometimes hear the claim that all you need to do is to use the temp. comp. that came with your focuser and focus at the start of the night. I have to say that in 20 plus years of imaging I have yet to see a system where that works very well. Joel’s method of having SGP focus every “X” change in temperature is perfect and I do it that way as well. All you need to know is your depth of focus (in focuser counts) and how many counts your system changes per degree temperature change. Then you can figure out what number to put into SGP to be sure you never get out of the focus band. It works great for me.

This is true. Some focus motors (probably most) are quite smooth and it would be just fine moving them during image integration. Others can introduce an unruly amount of vibration into the imaging train. Because of this we just choose to move in between frames.

Thanks for the inputs. The focuser in my system ( Feather Touch) does support temp compensation, so will try playing with it on top of Joel’s method.

I do long exposures and temperature drops can be significant during winter nights.

My only concern about that is if the focuser SW is doing continuous changes, then it might do so during a focus run and I would wonder what effect that might have on the accuracy of the focus. I know back in the pre-SGP days when I used Focusmax, it would temporarily disable the focuser’s compensation while the autofocus was running. This would seem a wise precaution. I don’t know if SGP does this.

CCDMan, thanks for the heads up.

We don’t but it seems like a pretty good idea (and I think it’s pretty easy to do).

I added code to the AF routine to temporarily disable any internal temp comp while running. If you had it on (because you are OK with focuser movement during integration), it will be disabled at AF start and re-enabled when the AF run is complete.

Of course, if your driver’s author does not expose control of this to the ASCOM interface, there is nothing SGPro can do to prevent temp comp from running during AF.

Ken is this functionality in the version released yesterday Oct 26?, v2.4.3.12 ?.


It is not yet released.