Imaging Equipment - Bahtinov Masks
Bahtinov mask is a very clever tool for focusing. It is a full aperture mask that has narrow openings cut out at different angles producing 6 diffraction spikes on each star, three on each side. Two spikes are at around 30º angle, the third spike cuts this angle in half (see Antares image below). In focus the middle spike is exactly between the the other two, while out of focus it moves up or down. It is very easy to tell when the focus is achieved and, if out of focus, easily tell in which direction it is.
Bahtinov mask is an ideal do-it-yourself project. I made two - one for 10" Newtonian and the other for C14. There is a Bahtinov mask calculator online, it can be used to compute cutout pattern for any given telescope. For an SCTs there is an option to produce a secondary mirror cutout, which I ignored: the easiest way of using the mask is to attach it on the end of a dew shield, it works with Hyperstar this way, and for that one does not need secondary cutout. Once the pattern is computed it must be printed, unfortunately often it is larger than letter sized paper ... I used a trial version of poster printing program ProPoster, it prints big posters on a set of letter sized paper sheets, so they can be assembled together to form complete document. Trial version prints "This is a trial" across each page, but since we cut out white parts it doesn't matter at all.
I glued the pages together on a .25in foam core poster board and cut the mask out with Exacta knife. Finally, I attached small rectangular pieces of poster board on four opposite ends of masks perpendicular to the mask -they slide on the outside of the telescope (or dew shield) to keep the mask in place. Below is the picture of my C14 Bahtinov mask, and the mask on the scope for focusing. The rightmost image shows Antares as seen through the mask in good focus.
It is also possible to buy commercial Bahtinov masks ...
I think it is the best focusing tool I've encountered so far! Fast to use and very straightforward. The accuracy is similar to one can achieve with Nebulosity focus assist tool, but I think Bahtinov mask is faster.