Focusing a telescope is not a trivial task to be overlooked when imaging deep sky objects (DSO). There are many factors that go into focusing, especially when you do not have an eyepiece and you have to rely on a computer screen to determine if what you are trying to image is in focus or not. Most photos of DSO’s take a couple hours to image and it is pretty disappointing to process out of focus images after you have spent a lot of time capturing them. Relying on your own eyes would seem to be a good solution but your sense of sight can fool you into thinking that an object is in focus when it is completely out of focus. This is where looking at Histograms and Full-Width Half Maximum (FWHM) comes in. In the Astrophotography program, MaxIm DL 5, there is a focusing tool that gives you a readout of of the object’s brightness and plots the brightness on a histogram, like the one in the image below.
Ideally, if image is in focus, the histogram should have a sharp peak and the FWHM should be less than 2. Not knowing this handy piece of information when I first imaged the Orion Nebula on January 29, 2013, I was left with exposures that were out of focus. Thanks to clear skies tonight, I was able to re-image the Orion Nebula to see if I could get a better focus, which you can see the comparison photos below.