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.  In its simplest form, the FWHM is a measure of the size of a star.  In the program, MaximDL, you can view the FWHM’s of the stars and plot the data on a graph.  The brightness of a star is calculated as a number between 0 and 65,000, the maximum value a CCD camera can record.  Here is a simplified version from John’s youtube tutorial:

Full Width Half Maximum (FWHM) of a Star where B =Brightness and FWHM = the diameter of the Star at Half of its maximum value

Full Width Half Maximum (FWHM) of a Star where B =Brightness and FWHM = the diameter of the Star at Half of its maximum value

Let’s take a look at two identical stars from my imaging session:

Is the third time really a charm?  Maybe?  The third time imaging M81 and M82 turned out great because I took the time to learn how to focus better and also spent extra time making sure the focus was great by looking at the first exposures.  I highly recommend John Blackwell’s tutorial as it helped me understand a critical part of focusing my images.  Here is the result of my third imaging session of M81 and M82:

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