Monthly Archives March 2013

Full Width Half Maximum: A Focusing Lesson While Imaging M81 and M82

The Bode’s Galaxy (M81) and the Cigar Galaxy (M82) are located in the Ursa Major constellation, which is right outside my backyard.  Taking advantage of clear night on 2/14, I decided to see if I could capture these two galaxies.  I got everything set up, ready, and started imaging.  Two and a half hours later, I started to process the images and boy was I disappointed.  M81 and M82 were severely out of focus.  I told myself no big deal and I could just wait for another clear night and image them again.  Here is the output from the night of 2/14:

M81/M82 out of focus

M81/M82 out of focus

Three days later , on 2/17, I tried it again.  Thinking I had a much better focus, I imaged the two galaxies for another 2.5 hours.  As I was processing the images from that session, the same thing happened!  I was out of focus again.  How could this happen two times in a row?  Taking a step back and do some research on focusing.  Turns out, there was still a great deal to learn.  The clouds rolled in after that night so this gave me some time to do some research into how to achieve a better focus.

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YouTube Direkt

Turning to youtube, I came across this fantastic tutorial by John Blackwell on understanding a Star’s Full Width Half Maximum (FWHM).  Huh?  What is FWHM?  Turns out this four letter F word (not the one you are thinking about) is very important for determining whether your image in focus or out of focus...

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