What affects focus that I can’t control?

You can’t control the twinkling of stars which causes wander and blurring of the star in the eyepiece.  Wander is the star image drifting/jumping around in the eyepiece by about 3-5 arcseconds.  In a long exposure image wander blurs the airy disk out to 3-5 arcsecond diameter in the image. Twinkling also causes the airy disk to blur out and sharpens up -- the “seeing” changes as the atmosphere twinkles.  At moments of crisp seeing the airy disk is visible, at instances of bad seeing the star is a cottonball. 

video of Betelgeuse twinkling

You can’t control twinkling or seeing, waundering or blurring but you can minimize the effects of the atmosphere by observing stars nearer overhead, where there’s less atmosphere to look through.

What affects focus that I can control?

One effect you can do something about is collimation -- making sure all the mirrors (primary, secondary, diagonal) are perfectly centered and aligned with each other. our Aline collimation tool will help you with collimation. Fortunately collimation is something you only have to check occasionally.