Bahtinov Mask
The Bahtinov mask is an invaluable focusing aid for amateur telescopes of all sizes. It was designed in 2005 by Pavel Bahtinov a Russian amateur astrophotographer whilst he was teaching at a children's astronomy school. It was here he adapted the Hartmann mask which is not that easy to use and produced an early version of his now famous mask.
There is according to Bahtinov's account no real relationship between the gaps and the bars he only did what looked right at the time or in his own words translated from the Russian by Google it was intuition and guesswork.
There are a number of Bahtinov mask generators in the Internet the one I used for my mask was from the Deep Sky Watch website. I simply selected the right option and downloaded the PDF file.
This site also shows how to make a mask using a plastic ring binder. The one I used for my project was a Rexel budget ring binder which I had lying around after clearing out a load of paperwork.
One Rexel budget ring binder ready to be repurposed.
I have a small cutter on a pair of compasses to cut the disk out.
Setting the correct diameter. I was making a mask for a sky watcher explorer 200p.
The plastic disc cut from the ring binder. It was at this point I belatedy decided to find a design to work with and found the Deep Sky Watch website. This suggest leaving taping the template to the binder and leaving the cutting of the disk to the last. This will work much better.
The template taped down over the disc ready to start cutting out and scoring the plastic. Use firm pressure here to make the score marks on the plastic
The first half of the template cut out, scoring the plastic disc in the process.
The pattern scored on the plastic
Cutting out the plastic, you may find it easier to use a heavier craft knife for this process. Once the slots have been cut go over the cuts to ensure they are as clean a possible with no stray pieces of plastic to catch the light.
The finished product.
Even with all this work you are still reliant on your eyes to determine if something is in focus or not and the vanes holding the spider of a Newtonian reflector also act as a focusing aid. Especially when used with a camera using looping exposures where small adjustments can be easily made with a laptop at the telescope. This mask though provides a quick and easy way to achieve focus with a surprising degree of accuracy.
Masks this size retail from £20 upwards. So not a bad saving for a little effort. Some of you may argue about how long it took me and yes in that way it was no cheaper but at least I had the pleasure of making something for myself.