PolarAlign

Nutshell

Here is a fast easy way to accurately focus a telescope with a custom mask.

The cost can be virtually free if you make your own.

There are sites that sell ready made masks if you would rather not fool with making one. Prices vary from $20-80.

My suggestion is to make one from card stock, try it, then either make a permanent one, send a vector file to a fabricator, or buy one.

The mask seems to have been seen on a Russian site by one of Cloudy Night's Russian speaking users.  Bahtinov has donated his design to the public domain asking only that his name remain associated with the mask.

Background

The Bahtinov mask is basically it's a slotted mask designed to produce spikes.  The result is an "X" with an additional set of spikes in the middle.  The middle set act as an indicator and travel from side to side with focus changes.  Not only can you tell which way to turn the focus knob, you get an indication of how close you are to ideal focus.  When the 3rd set of spikes is centered, you're focused.

It's much quicker than watching changes in mean star diameters and maximum pixel values. I find it quicker than doing a FocusMax run.

I made one, used it for the first time last night, and I'm sold.

There's an extensive thread on Cloudy Nights in the CCD Astrophotography forum.  The sizes of the slots are determined by the diameter and the focal length of the telescope.

There's a website with a calculator that'll give you a file specific for your telescope.  It'll be in the format of a vector file.  If you don't have a vector file drawing program, you can use a free one.  With that you can print the pattern and cut your own, or you can send the file off to have a professional mask cut. 

Links you need follow:

Information site explaining much more

Bahtinov Focusing Mask Generator page

Inkscape open source vector drawing program

Here's a link to a very reasonable fabricator.  One fellow had two custom made in 1/8" black acrylic for $80.

Pololu

 First, here's a picture of a mask. The angled slots are 40 degrees from each other (each are 20 degrees from the horizontal bars).  The black triangle is not necessary, some masks have this area slotted.  The circle in the middle is designed for a telescope with a central obstruction, a refractor would not need this circle.

In this image You can see how the middle spike can be adjusted with the focuser.  When the spike is centered, you're focused.

(Image courtesy of Jerry Hailey and www.spike-a.com.) I've got one of his masks, it's excellent.

You can read the messages on Cloudy Nights in the CCD Imaging & Processing thread.

It's a very busy thread, there are 23 pages and over 450 messages dealing with the Bahtinov mask. I have not yet seen these masks described elsewhere.

Having used one, I can tell you it's easy to make and every bit as easy to use as it seems.