Amateur Astronomy and Imaging.

Reviews > Miscellaneous > Bahtinov Mask


Bahtinov mask

Since the first time I looked through a telescope there has been one thing which has always frustrated me. I have never been completely confident that the telescope is correctly focused. When observing visually this is not quite so important as the human eye is quite capable of compensating for small errors. When it comes to astrophotography there is really no margin for error. The effects of seeing can also make focusing more difficult as you may appear to be in focus one moment only to find the seeing has changed and you are no longer seeing pinpoint stars. For years I would spend up to half an hour trying to reach optimum focus only to eventually settle on what I felt was best, and rarely was I confident it was accurate. When I first started using Nebulosity, a software package used to acquire images from the camera during astrophotography sessions, I started to make use of its built in focusing tool. Unfortunately I didn't get on very well with this, and although it helped, I was still not satisfied I was in proper focus.

I started reading various articles around the Internet to see how other people solve this problem. Initially I had planned to use a motor focuser and to drive this remotely in the hope that this would help. Part of the reason for this was because I was also looking at remotely controlling as many of the telescope operations from the comfort of my conservatory while the scope works away out in the cold. A motor focuser costs several hundred pounds and I wasn't convinced it would solve the problem anyway. I kept reading.

Then I came across the Bahtinov Mask. It is simply a piece of paper, card or plastic (or any other flat material that can be cut to the required shape) in which a number of holes are cut according to a template. There are some websites that sell them and you just need to select one designed for the aperture of your scope. Alternatively you can simply download a template, print it out and use it to craft your own out of your preferred material. The website I used to download the template is here.

Once you have your Bahtinov mask you simply place it over the aperture of your scope (can be directly over the objective or over the dew shield) and point your scope to a bright star and capture an image. The Bahtinov mask causes a pattern to appear instead of a point. This pattern takes the form of 3 lines. 2 of the lines always remain in the same position and form a cross. The third line moves depending upon your focus position. The aim is to make sure the third line intersects the point where the other 2 lines intersect. If you move the focuser in one position you'll see the third line move either slightly left or slightly right. You'll immediately see if you are moving the wrong direction. The beauty of this focusing method is that you can be absolutely certain you are in optimum focus and it does not take longer than a few minutes. To be quite honest I feel this is just about the best improvement I've made to my setup since I bought the Losmandy GM-8. The difference being that this was a no cost solution! Now I rarely spend more than 2 minutes achieving focus. I recommend that anyone else struggling with focussing makes one of these. If you'd like to see how this works then click here.