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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 Astrojargon.net 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.