At the risk of going off topic, star testing can be challenging but it'll click with time. Correction, astig, coma, zones, turned edge and snap test and magnification (in focus) all exhibit some behavior we can recognize.
Whats hard is quantifying it, just a rough guess of excellent, good, or poor. We've had long discussions of magnification, it depends on the object. I think Mel means a star. Even stars can take 100x per inch in any scope, really. Jupiter can't...in any scope.
More to topic, I think we start out star testing at very specific defocus. We don't really have to except for the shadow break out. You wanna know where that happens, so you need to know how much defocus we are seeing. And to compare test images, it helps to match the defocus shown.
Other than that, general scrolling through focus is a good technique if we're familiar with the behavior each or a combination of aberrations exhibit.
Edited by Asbytec, 27 August 2018 - 12:29 AM.