Hyperstar sanity check

[Stinky_Pete]

Hi folks, can i just get a quick sanity check before i loose mine. First light with a hyperstar coupled with an asi294mc pro, im seeing v shaped stars at the edge of frame, collimation seems ok but i think i might have a back focus issue

The back focus is i think 44.3mm (although its a second hand older model so i could be wrong) and the back focus i need for the camera is 55mm so i just need a spacer of 10.7mm right?

[ollypenrice]

Sounds right but if you have a filter of x mm thickness in the train then add 1/3 x to the extension.

Olly

[AKB]

I have a C9.25 HYperstar and an ASI294MC-Pro camera.

You don't say what size scope you have, but looking here https://starizona.com/tutorial/hyperstar-specifications/ it seems like you have a 6" variety.  The spacer is specfic to the camera.

The camera itself should have a 6.5mm length between its thread and the chip.  It should also come with an 11mm spacer, which, perhaps not coincidentally, sounds like what you need.

The backfocus is very critical, but, again, on the Starizona site it says +/- 0.5mm (or perhaps I read it here: https://www.cloudynights.com/articles/cat/articles/the-amazing-hyperstar-a-guide-to-optimize-perf-r3013 which is also worth a look.)  It must be of that order because there would be an effective length change when using a filter, which the adapters are threaded to take, as @ollypenrice has indicated.

Edited October 24, 2020 by AKB

[AKB]

Further to this, I see that the imaging circle for a 6" Hyperstar is just 11mm, whereas the ASI294MC chip diagonal is 23mm, so not really suited to the scope?  (Unless, of course, I have it wrong and you have some other diameter, but in that case, your backfocus measurement is certainly wrong.)