Bahtinov Focusing Mask
Have you ever used a Hartmann Mask to help focus a telescope? Yeah, me too. It didn't help a lot. They separate an image into two or three images which merge into one when the telescope is properly focused. The problem is, it's pretty hard to tell EXACTLY when the telescope is perfectly focused.
I ran across the answer on a forum. The idea was originally posted on a Russian astronomy forum. People here embraced it and decided to name it the Bahtinov Mask, after it's inventor, Pavel Bahtinov.
The idea is simple. When the telescope is aimed at a bright star, it creates two diffraction spikes arranged in an elongated X. There is a third diffraction spike that moves as the focus changes. When it intersects directly through the center of the X, you are in perfect focus.
I bought a sheet of foam at Hobby Lobby. I printed out the pattern on two pieces of paper taped together. I placed it on top of two sheets of foam. Using an Xacto knife I cut out two at the same time. I added a wooden hoop to mine, one made for embroidery which I had to cut down a little. I gave the second one away. Here is a program which generates the pattern for any size telescope. Here is a guy that sells masks made out of hard plastic.
This is a photo of mine.
This is a drawing I made to demonstrate how it works.
This can easily and quickly focus you telescope whether it has an eyepiece or a camera attached. Try one, they are well worth the effort.
Here is a photo - left image out of focus, right image in focus
"Amateur" of The Netherlands wrote a program to be used with a web cam that makes perfect focusing effortless.