Prism or star diagonal...

[Marc1964]

A used Skywatcher 90 mm Mak has come into my possession, with what appears to be the standard skywatcher 45° prism diagonal. 

Do I stick with a prism design or go mirror, and why? Any relatively inexpensive recommendations would be welcome! 

Thanks in advance... 

[Elp]

The 45s are usually for daytime spotting use, 90s for night and astronomy. A decent dielectric mirror one will do.

Edited February 11, 2023 by Elp

[vlaiv]

Yes, replace it for night time observation (but do leave it around as it is handy for daytime spotting with lower magnifications).

With 90mm Mak - it really does not matter if you get diagonal mirror or prism at 90 degrees. Prisms usually work better in slower systems than in faster - but since Maksutov is already slow system - prism diagonal will work fine. Mirror, as long as it is decent quality - works equally good in both slow and fast systems.

Affordable prism:

https://www.firstlightoptics.com/diagonals/celestron-90-prism-diagonal.html

Affordable mirror:

https://www.firstlightoptics.com/diagonals/stellamira-1-25-90-di-electric-diagonal.html

(there are cheaper mirrors - but are probably less reflective and not as good 91% vs 99%).

[Ricochet]

90° (non erecting) star prism. Less scatter so a sharper image than with a mirror. 

[vlaiv]

2 minutes ago, Ricochet said:

90° (non erecting) star prism. Less scatter so a sharper image than with a mirror. 

Always or does it depend on quality of the prism?

As far as I can tell - there aren't many "affordable" models out there. I would personally consider Baader T2 model (or maybe TS T2 model) as "starting" point.

I guess those are bound to be polished fine, but what about cheaper ones?

[Marc1964]

Hi, thanks for all the contributions. I must admit that I thought the consensus would be for the mirror. I will try the Celestron prism and see how that performs against the supplied SW one. 

[Agerskov]

On 17/02/2023 at 08:45, Marc1964 said:

Hi, thanks for all the contributions. I must admit that I thought the consensus would be for the mirror. I will try the Celestron prism and see how that performs against the supplied SW one. 

Have you bought it and tried it yet?

[Marc1964]

On 07/05/2023 at 12:52, Agerskov said:

Have you bought it and tried it yet?

Hi yes I have tried it briefly. Seems to be quite good, I have only tried it in my Mak 90 and suits well. Works better than the Svbony dielectric but need to spend more time to be certain of that.