Alphonsus is the circled crater of the moon. According to Wikipeda, this crater has a diameter of 73 miles.
The moon in my image has a height of 2366.5 pixels. The moon's polar diameter is 2,117.9 miles, or .895 miles per pixel for my image. I found the diameters by using Pythagorean Theorem on the X-axis and Y-axis location of the pixels.
Alphonus has a diameter of 88.2 pixels. At a scale of .895 miles per pixel, Alphonsus has a diameter of 79 miles in this image.
My calculation is 6 miles too large, or an error of 8%. One source of error is trying to locate the edge of the craters bottom when it is heavily shadowed. Also, a second error is due to the edges of the moon are a little fuzzy. They're fuzzy because the telescope has a wide angle view and the eyepiece doesn't properly focusing on the extreme edges.
Nonetheless, this is a more accurate diameter than if I had just guessed.
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