I tried different techniques to bring in focus my Canon XTi,
Even by taken series of short exposures and using the preview / zoom functionality of the DSLR camera I found issues and results were not completely satisfactory.
I found mention of the Bahtinov mask on different blog post – and decided to give it a try.
The Bahtinov mask was built in-house for my orion 102ED using Bahtinov’s mask free templates posted on this web site
The materials used for building the mask can be found in stores such as Office depot,
I used a black plastic portfolio. Use a portfolio not too thick, that can be easily cut using a paper cutter – but rigid enough to be used as a mask. Using adhesive, the template (printed from my computer on Letter format) was glued on the portfolio plastic sheet. I used a DVD plastic box as a cover to secure the mask on the 102ED optical tube.
The way to bring the camera in focus is simple. The basic principle is the same as without the mask – doing an approximate in-focus first using the view finder – then tweak the in-focus by using short exposure and the preview mode of the camera – with the Bahtinov mask in front of the optical tube.
Below are pictures from Betelgeuse. The left picture is a 3 sec. exposure and shows Betelgeuse out of focus. The right picture is in focus. When in focus, the diffraction spikes are symmetrical with a easy to recognize pattern.
Betelgeuse – 3 sec. exposure – out of focus (left) and in focus (right)
Here are some pictures taken using the mask for focusing. The picture was shot in San Bruno, CA in a very light polluted sky – using the Orion SkyGlow anti-pollution filter and a Canon Digital Rebel XTi DSR Camera. Focus on M1 was made using approximately 6 shots on 3 sec. each on Betelgeuse. In this astrophoto session on 3/27, Focus was not changed during 2h where a total of 5 deep sky objects were shot using multiple short time exposures.
Picture of M1 – 3/27/2009
Astrophotography in Urban Environment 