Talk about a timely set of posts. I too am struggling with Hyperstar backfocus.
After 10 years of limiting my imaging to astrometry I'm once again doing "aesthetic imaging." Got a new ASI2600MC-P for use with a 2004 Meade 14" XL200GPS. And resurrected my2007 Hyperstar M14 for wide field applications. Been working on proper mounting and the backfocus issue since Jan 2nd and will fabricate a custom adapter as soon as the backfocus issue is addressed.
Starizona literature emphasizes that backfocus is critical to ±0.5mm. And Scott at Starizona tells me that the M14 backfocus should be either 67.4mm or 70.5mm. He also explained "If the spacing is wrong you get spherical aberration which shows up as what we call "space bubbles," sort of halos around bright stars. Even though you can makes the stars small, they have these artifacts at best focus.
I've now run with both settings and don't know how to evaluate the results. Focusing is weird as normal criterion such as brightest level or minimal FWHM don't work. I get initial focus on a bright alignment star and then must perform a refined focus on a dim star within the target FOV, doing a visual adjustment at 400X. At the 70.5 distance I've got a pinpoint star with a small halo filled with outburst rays. At the 67.4 distance the star is about 2/3 the size of the 70.5mm halo but it's solid and has no halo.
There are a lot of factors involved and it's difficult to provide accurate comparative images. Has anyone got any observations regarding which backfocus would be optimum? At the moment I'm considering playing with some intermediate distance just to see if there's a happy medium.