Bahtinov Focusing Mask Review

By

Mike Borman

Anyone who has done astrophotography for awhile knows that it can take quite a while to make sure you have achieved the best focus before taking an image. There are many ways to accomplish this task, but I think I have finally found one that not only achieves an accurate focus, but also allows it do be done rather quickly.

One day while scanning the messages on the Cloudy Nights Website, I saw several mentions of something called a Bahtinov Mask. It is a focusing mask that fits over the front of the telescope. When you point your telescope at a moderately bright star, the mask will produce diffraction spikes around the star image. When the middle diffraction spike is centered in the outer spikes, you're in focus. Very little focusing travel is required to move the inner diffraction spike, which means the focus is extremely accurate.

Here is an image I took of a star that I was focusing on with my Orion Starshoot Pro CCD camera and Televue 102iis refractor:

It took about five minutes to achieve good focus using this method. And from what I have read, it is immune to bad seeing.

There are several people now selling these masks. I bought mine from All-Pro Software at http://www.spike-a.com/ . The masks they make are made of aluminum and use nylon set screws to attach the mask to the front of the telescope.

It is also possible to make your own mask using something like foam board. Here is a website that has a Bahtinov Mask pattern generator:

http://astrojargon.net/MaskGen.aspx

My first images to use the mask for focusing, are the M33 and IC-405 images seen elsewhere on this website:

http://www.geocities.com/mfb_eas/m33_112608.htm and

http://www.geocities.com/mfb_eas/ic405_112708.htm

I was very pleased with the small sharp stars in these images.