The "best" star is the brightest one closest to zenith. Sirius for me comes very close to zenith. What is most important though is seeing conditions. They must be as good as possible because you need to rip high magnification for this exercise. So if seeing is good, then chose the brightest star you can that is say at least 45deg above the horizon. The higher the better.
You will also need to be dark adapted to give your eyes every chance. The outer rings will be very faint, and even require averted vision to see the outer most.
Don't beat yourself up over this. This is starting to get into the realm of nerds within nerds that is astro. Until I started to see for myself that mass production SCT's are not created equal I would not have bothered investigating the why and then having the veil lifted off my eyes to what exceptional optics can actually do...
When star testing, no matter what scope you are using, keep the star in the centre.
PS: You MUST have the collimation of your scope really tight to the point that it is squealing. Mirror flop in some scopes makes this a near impossible task. If your scope shows mirror flop, you must use a very disciplined method to focus your scope by winding out a little in just ONE direction, and then slowing wind in to achieve focus. If you go too far then wind out again and then in again to focus. Do not try to focus by winding back out if you have gone too far. This exact same pattern you must do with collimation. Wind the focuser out to just past where you want the donut's size to be, and then wind in that small touch. NOW is when you collimate. Because with this discipline when you are focusing your scope this action will set the mirror back into its optimal position with collimation too. Not follow this regime of focus and collimation, and mirror flop will always put the optics out of whack.
Also, if your scope shows mirror flop, do not attempt to collimate with the scope set horizontal. It MUST be pointed up to at least 45/60deg elevation as this is the working position of the mirror. Having the scope horizontal is not putting the same load on the primary mirror and it does not move in the same way.
Edited by maroubra_boy, 04 March 2021 - 05:55 PM.