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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.

Fine Focus and Bahtinov Masks

Q: How can I get focus easily and why don’t you write an autofocus routine?

Focus is something amateur astrophotographers worry about a lot. Some get so concerned about the challenge reaching focus that they push hard for having an autofocus setup, thinking this will make their lives a lot easier and that they can be assured of sharp images as a result.

Don’t get me wrong -- I like the concept of autofocus. But, there are two things to know before going that route. First, it’s going to cost you. To do autofocus well and to have it work smoothly, you really want not only a motor on the focuser but you want the ability to know just where the focuser is. You can do this with a micrometer style readout (like the Televue setup) or with encoders on the motor. If you go with encoders on the motor, you need to profile the system well to know just how much backlash there is as the encoders will turn without the focuser moving. If you don’t have encoders, life is more challenging and if your focuser has image shift, you’ll be looking at one direction of movement only. So, better have something with encoders and a solid enough setup that you don’t have shift.
Second, it’ll take time. It’ll either take time to run the full “V-curves” each time you’re out or it’ll take time to profile the focuser / scope’s V-curve so that you can hit a point on each side of focus and know, given the star’s size on those two points where focus is. Toss in parameters to derive and you’re not looking at something quick and easy that can work on any of the various bits of hardware people have out there.
Again, don’t get me wrong. For some setups, it’s essential. If you’re running a remote observatory, for example, auto-focus is going to be a huge win. But, for your typical user, if focus can be done quickly and easily without this, it may not be worth the hassle.

I’m here to suggest it can be done quickly and easily without any of this.

In one of my earlier tutorials, I posted a video of both rough focusing and fine focusing using an earlier version of Nebulosity’s fine-focus tool. Here, I’ll show a video of me using both the current version of the Fine Focus tool and augmenting this with the use of a Bahtinov mask.

What the heck is a Bahtinov mask you ask? It’s an elaboration on the idea of a Hartmann mask -- something you stick over the front end of your scope to induce a diffraction pattern that makes focusing easier. David Polivka over at has a great webpage on the mask and how to make one. I made one myself out of a piece of thin cardboard by printing out the pattern from David’s site, taping it onto the cardboard, and using a razor blade and straight edge to cut out the pattern. (I’ll post a picture up here soon.) With some creative folding of the cardboard, it snugly holds itself onto the front of the tube just fine.

The concept is that you adjust the focus until the middle spike is nicely centered. Here, I have a video of me going through the focus process. I’m working on an 8” f/5 Newt (I’ve done this at f/4 as well without issues) that has a heavy camera setup on a stock one-speed GSO Crayford. No nice Feathertouch here. No, I’m using a simple focuser on a mount pushed to its capacity. What we see is the image of the star in focus with the mask in the upper left, the profile in the upper right (orient the diffraction right and that profile could be very useful!), and the running log and current values for the max intensity and the half flux radius. Play the video and you’ll see (and hear) me go from this well-focused spot to taking it out of focus with the mask and bringing it back. I’ll then pull off the mask and show the star is in focus, nudge the focus out a bit and bring it back showing we get to the same focus spot. Note, the HFR will be different with the mask on and off, of course, but the point is that the focuser position is the same in each. When one is in focus, the other is as well. In the minute I’m actually doing anything here, you’ll see me hit focus with each method. So, that’s focusing the system twice in a minute.

Personally, I think this is pretty easy and straightforward. Watching this video should give you a good feel for using the Bahtinov mask with Nebulosity. Watching the other video should give you a good feel for using Nebulosity without this (and on a very unstable night). Either way, you can be confident that you’re hitting accurate focus on modest hardware with no investment and in a short time.