FWHM keeps popping up in Astrophotography. But what does it mean, and do I want a low one or a high one?
What is FWHM?
Full Width Half Maximum is the measurement of a thing on a graph such that the FWHM number is the "width" of the graph when the left and right sides of the graph are at "half" the "maximum" value of the graph. This graphic might help, courtesy of Wikipedia.
In this image, you'll see that some thing is being measured. In astrophotography, this is generally a measurement of the stars in an image, indicating how "fuzzy" they are. If you imagine scanning a horizontal line (like a FAX machine) of a very zoomed in image of a star, the outside of the star is going to be fuzzy and the inside is going to be bright. The graph of that brightness can be treated like this image here, and then we can measure the FWHM of that brightness to determine how good the focus is of the star. Less fuzzy = better focus.
So do I want a High FWHM or Low?
Think to that measurement of the focus that the FWHM represents. If the fuzzy part extended way out into the image, then that FAX machine-style scanning of the star will make the curve look very wide. Lots of fuzzy; very little bright center. So that would be a high FWHM. The opposite of that - a low FWHM - means that there is very little fuzzy and lots of bright. In other words: a well-focus star. So, to make it short - you want as low of a FWHM as possible.
What's a typical value?
There are different ways to measure, but most Astrophotography software measures in either pixels or arcseconds. Either way, you want the number as low as possible. You can expect single digits on an average-focused star. You can expect 3-5 on a good star and two or less is considered great. Stacking software typically has tools built-in that will show you this information if you want to analyze it, but you don't have to obsess over it (unless you want to).
What Does This Mean for Astrophotography?
You want your FWHM to be a low as possible. Your auto-focus system (if you have one) will typically use this to make a series of measurements and figure out the best focus. If you don't have an auto-focus system, you can use these measurements yourself, then tweak your focus, measure again, and see if it's better or worse. Stacking software can also filter out sub-frames that have higher FWHM so you only use good, crisp, in-focus shots to create your final image. Lots of things happen under the hood here, but you can "pixel peep" if you want and get more involved.