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130mm frac struggling to acclimate in hot summer weather?

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I had not noticed this in winter, but now that I am doing shorter narrowband imaging stints during twilight months I have seen my focus slipping through the night.

Due to the good weather I have kept my scope outside, and I suspect as it is a carbon-reinforced plastic construction of some thickness, it is absorbing a great deal of heat from the sun during the day. At night, I will autofocus at the start of my session, however I have noticed that my focus is quite poor by night's end. Last night I set it up to refocus every 45 minutes (3 focuses total, as I forgot to ask it to focus at the start, oops!)

This resulted in a total change of 55 ZWO EAF steps between first focus and last focus. A rather considerable change. And since this is on a R&P focuser I don't believe the focuser itself is moving between focuses. Could it just be that the tube is getting very hot in the day and slowly cooling off at night, leading to dramatic thermal contraction?

Curious if others experience this issue, or if there is a remedy to this that doesn't involve air conditioning the scope during the day to match the forecast night's temps haha.

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  • 2 weeks later...

This is normal behavior. I also image with a refractor and I need to refocus during the night. The change in temperature induces a change in focus position.

You will have to refocus.

In my case, I got an Optec Leo focuser with an external temperature probe. The probe is fixed to the scope. Then I simply made a liniar regression model between temperature and focus position. For my setup the correlation coefficient R^2=0.92. So I have a 92% correlation between focus position and scope temperature.

Then I introduce the slope coefficient in my software and I can get away with not focusing during the night.

Sometimes I simply use temperature compensation sometimes I refocus

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I have 150 mm F7 refractor and have to refocus roughly every 45 minutes during a summer night, less frequently in the Winter. The scope is inside a dome and gets very warm after a sunny day.

I tried temperature compensation but never got satisfactory results and as a refocus takes about 2 minutes in NINA and is 100% automated, I just stay with that.

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CF is more thermally stable than aluminium or steel but it still contracts to some degree with temperature drop.

I don't own a CF frac but I have a 10" CF newt and I've owned the aluminium tubed equivalent.  With the VX10 I had to refocus every 1°C temp change, with the CF10 it's every 4 or 5°C. 

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