How often do you focus?

[Benjamin]

I did a run of HaLRGB subs tonight on my ED80 requesting Sequence Generator Pro rotate through the events and to autofocus on each filter change. This meant autofocusing on every shot. The file name had the time and position of the stepper motor recorded in it so I could see what was happening with focus and could estimate filter offsets. What I wasn’t expecting was the rate of change of the focuser position - 300 steps every 25 minutes. When autofocusing there is a zone around the actual focus point of about 150 steps either side where it’s all pretty much okay but 300 steps is still quite significant. How often do others focus during an imaging session? It seems I should be focusing every 15-20 minutes?

[Atmos]

With my refractor I refocus every 1°C of change with a DSLR and with my Honders I refocus every 3°C and have filter offsets.

[multiweb]

Every 10s. :lol:

[Benjamin]

Ha! I never seem to be able to focus at all after a night of imaging. I’ll try to work out my Rigel temp sensor and work through similar steps with the autofocus and see how temp and focuser position relate and maybe work out an offset? Was really surprised to see how much focus changes over just a two hour period.

[RobF]

More temp shift with some designs than others Ben. If your ED80 has metal tube then you'll need to focus more frequently during times of the night temp is changing the most.

With my refractor the need to focus can reduce a lot if the temp stabilises through the night. Depending on length of your subs and temp fluctuations you might want to vary between focus on filter and focus on time options in SGP. All the better if you have a reliable temp readout to trigger refocusing.

[Benjamin]

I looked up local temp forecasts and the temperature variation was pretty minimal last night (a degree at most). Can’t imagine the temp would be vastly different in my backyard a few kms away from the weather station? Wonder whether the shift in focus had more to do some kind of backlash in the system caused everytime autofocus runs? The weight of the camera and filter wheel might have put some strain on it all although V curves were consistent. The difference was always 250-300 inwards although I reverse the auto focus direction in SGP for my refractor. Not sure what this means for the focuser position readings. If IN is in fact IN then backlash on the gears and/or motors would make sense as the focuser would find it more difficult to drag the imaging train in (after heading out first for the autofocus run) resulting in an estimated focuser position that seems further IN (focus actually is in the same position but the motor think it’s futher in). I had a nominal 10 steps for backlash. Might up that to 250! It’s a single speed Skywatcher focuser so that might also have an impact.

[RobF]

Is it a rack an pinion focuser? Could it have been slipping a bit?

[Benjamin]

Nup, it’s a Crayford focuser. Had issues with the focuser before and the focusing system was slipping. It’s seems solved now (racks in and out well with the weight) but the extra pull on it would likely still be a factor. Focus seemed to be good in images (will see if I can post some).