It is hard, sometimes, to discern if your images are really as sharp as they might be. While your eyes are great to spot the difference between a blob and a sharp star, it is preferable to have some real data to actually analyze.
Stellar Sharpness Measurement ( One image )
As soon as you download an image from your camera, you can very easily see if your seeing and/or focus are up to par. Just right click on your images and choose Stellar Sharpness Calculations then the size of the zone to be measured. If you choose 512x512 for a fast turnaround. If not, the entire image will be used.
Here is how you measure a single image
Measure Multiple Images
Another great tool in PRISM is the "Quality Check (Stellar Images)" under the analysis menu. This tool allows to analysis a whole set of images ( same size and pixel type that is ) in order to understand how the imaging session evolved of the course of a night for example. Here is a quick rundown.
1. Load the tool, check an area of 1/8(for faster analysis) the frame and press OK
2.The output is as follows:
Mean sky background
Median sky background
Elongation of the stars
And finally, the actual output of the tool with all the data