Which guide scope?

[Bluemoonjim]

Hi
I have the Altair Astro Starwave 102ed-r, which is 102mm f/7 (715mm)
At the moment I am using a 50mm f4.1 (206mm) guide scope with a with ZWO Asi 120mm mini mono.
Guiding with PHD2 seems to be working ok... but I noticed that when using Sharpcap to focus the guide scope on the moon it is was not possible to achieve a sharp, detailed image. Presumably this has something to do with focal length and camera pixel size? I have been trying to get my head around info regarding this online but there seems to be (as always) differing ideas about what constitutes an ideal setup?
My question is: Is my combination of camera and guide scope going to give me best possible chance of accurate guiding?

Sorry for my non technical terminology but I'm still in the noob department.

[michael8554]

That's a common FL and camera combination that should work well.

Use the HFD readout in the PHD2 Star Profile window to improve focus - on a star of course.

Michael

[vlaiv]

47 minutes ago, Bluemoonjim said:

but I noticed that when using Sharpcap to focus the guide scope on the moon it is was not possible to achieve a sharp, detailed image.

This has nothing to do with actual focus. You are using very fast achromat lens - F/4.1 and it has a lot of chromatic aberration.

You are also using ASI120mm camera - which is mono and only has AR coated window - no UV/IR cut fitler. This will make image extremely blurred due to chromatic aberration.

This has zero effect on guiding as guiding is only concerned with star position and not how blurry the star is. In fact - guiding works if star is not in perfect focus, and sometimes if seeing is rather bad - it is better to slightly defocus star instead of having sharpest possible focus.

As far as best possible guiding - well that depends on mount you are using. Small guide scopes with short focal length don't have enough precision for very precise guiding when wanted RMS is less than 0.5" or there about. But that is only achievable on very expensive and mechanically excellent mounts. Most of regular mounts won't be able to go below say 0.8" RMS or so. If you want the best guide resolution - then use OAG, otherwise - your guide setup is quite OK.

[Bluemoonjim]

Hey Michael and Vlaiv

Thank you so much for the responses.
Explanation about chromatic aberration makes perfect sense of what I have been seeing in SharpCap.
Also appreciate the point about the quality of mount, at this stage my expectations are not particularly high with regard to accuracy, just needed to know I won't be making things worse.

So much to learn, but good to know I haven't got completely the wrong kit!

Cheers guys