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Fine Focus in Nebulosity

We’ve covered focusing here a few times before, but I thought it would be worthwhile hitting it one more time with one more video. Previously, I’ve talked about fine-focusing in Nebulosity with a Bahtinov Mask and there is also a movie showing and older version of the tool up in the Tutorials section. So, the question is:

Q: How do I get critical focus in Nebulosity?
If you’ve not read the section on fine-focusing in Nebulosity with a Bahtinov Mask you may want to head on over there for a longer treatment, but the upshot is that I firmly believe you don’t need an auto-focus system to easily reach critical focus. Auto-focus is great if you’re running unattended (be it a remote observatory or having the camera change filters, etc. while you’re sleeping). But, you can hit crisp focus without it and without tearing your hair out. The Bahtinov mask is one way, but without this you can get quick, clear, numeric and graphical feedback on your focus.

I run routinely at f/4 and I don’t even have a motor on my focuser these days. I’ve done this on an f/4 Newt (where the motor really did help) and I currently do this on my Borg 101 f/4 completely manually. It only takes a minute and it’s not something I fret about.

How? Rough focus is obtained with the Frame / Focus command. Click on it and you’ll loop through images. Don’t obsess here and just get the stars to be fairly small. Then, click on Abort, Fine Focus, and then some star in the field. The video below will show the Fine Focus in action. Personally, I pay attention to the HFR (Half Flux Radius) and make small adjustments while watching the graph (allowing for the scope to settle between adjustments). Keep in mind, with a 1s exposure, you’ll always have a bit of variation from frame to frame. As you go towards focus, the HFR will get smaller (graph goes down). Once you go past it, the graph will go up. You can then back off, knowing the sharpest focus you obtained, and using that value as your target. Despite being a fast touch-typist, it took me longer to write this paragraph than it often takes to focus.