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Maintaining focus on a refractor


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This might be a stupid question but....

If I intend to keep my refractor (SW Evostar 72ED) permanently on the mount

and I've previously focussed it using a bahtinov mask

and I only want to photograph DSOs

Can I mark (in some way) the focus point on the focussing tube and rely on that for future sessions?

Or is the recommendation to do a full re focus at the start of every session?

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It is recommended that you refocus at start of each session - but also on any significant temperature change.

If temperature drops more than 1-2C, you should refocus (actual temperature depends on length of tube, tube material and F/ratio or focus critical zone).

Metal in tube contracts significantly to change focus position (or rather focus position remains the same but scope shortens) when it gets colder (and vice verse when it gets hotter - but it never gets hotter during the night) that it can throw off your focus.

You can either monitor FWHM values in your subs or watch the temperature (or if you feel it getting colder - it might be worth refocusing) and refocus as needed.

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To all intents and purposes, you sort of can. I've marked my 80ED focus tube, but it's really only to get me in the ballpark - I fine tune using a bahtinov every time, run a sequence for an hours worth of exposure and check the focus is still spot on. I wouldn't rely on a marking alone without using the tools we have at our disposal like masks. 

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I tend to image a lot of targets before they hit the meridian and invariably the star count goes up on the image as it climbs in the sky (expected) while the FWHM values usually get worse. This may be due to temperature but the kit is in a dome so it’s not due to cooling to ambient, or possibly changes in the mechanical configuration of the focuser. Either way, there is no way I could keep the focus unchanged over a 5-6 hr imaging session, let alone from night to night.

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