1.25" star diagonal upgrade

[LondonNeil]

I'm thinking of upgrading the basic diagonal on my mak127.  Does anyone have any experience with:

https://www.firstlightoptics.com/diagonals/stellamira-1-25-90-di-electric-diagonal.html?_gl=1*qpwke4*_up*MQ..*_ga*MTU0MDQyNjA1NC4xNzEyNzkwMDgx*_ga_F6VGWQ0MW9*MTcxMjc5MDA4MS4xLjEuMTcxMjc5MDA5My4wLjAuMA

Or with:

https://www.firstlightoptics.com/diagonals/stellalyra_diag_90_d_125.html?_gl=1*t0q9dj*_up*MQ..*_ga*MTU0MDQyNjA1NC4xNzEyNzkwMDgx*_ga_F6VGWQ0MW9*MTcxMjc5MDA4MS4xLjEuMTcxMjc5MDA5My4wLjAuMA

Both 99% reflectivity and £69.... is one better than the other? Or for that matter I'll take recommendations on any other options.

 

 

[Cosmic Geoff]

Be aware that upmarket diagonals offer better build quality and longer life rather than any noticeable improvement in the view. I upgraded the diagonal on my 127mm Mak and could not see any difference.

[Mr Spock]

I can't see into the black one but the red one looks to have a restricted aperture. It might be an issue with some wide field eyepieces.

[LondonNeil]

Wide field and mak don't really go, but still a useful observation!

[Mr Spock]

You can get 24mm 68° eyepieces for it 

[LondonNeil]

Just like the Hyperion in my box,  yes,  yes you can. 

[F15Rules]

I'd look for a used Takahashi 1.25" prism or Baader T2 prism diagonal, both come up fairly often for c.£70 used. 

Both optically excellent, but the latter is more robustly built.

HTH,

Dave

[jock1958]

24 minutes ago, F15Rules said:

I'd look for a used Takahashi 1.25" prism or Baader T2 prism diagonal, both come up fairly often for c.£70 used. 

Both optically excellent, but the latter is more robustly built.

HTH,

Dave

Never tried the Tak diagonals but for me the Baader T2 diagonals are the way to go with lots of interchangeability options ie different nosepieces and eyepiece holders also the sheer quality.  

[wookie1965]

Is the light path shorter in a prism diagonal than a mirror diagonal. My Svbony 3-8mm wont focus in my Vixen 102m.

[John]

12 minutes ago, wookie1965 said:

Is the light path shorter in a prism diagonal than a mirror diagonal. My Svbony 3-8mm wont focus in my Vixen 102m.

Yes. I recently picked up a cheap Celestron branded 1.25 inch prism diagonal for use with my 75mm F/16 refractor and gained around 10mm-12mm of focuser travel. The optical performance seems quite good too.

 

Edited April 11 by John

[LondonNeil]

I've read good comments on the baader diagonal before,  but wouldn't a  mirror be better than a prism diagonal? 

[wookie1965]

20 minutes ago, John said:

Yes. I recently picked up a cheap Celestron branded 1.25 inch prism diagonal for use with my 75mm F/16 refractor and gained around 10mm-12mm of focuser travel. The optical performance seems quite good too.

 

Hi John is that inward travel.

[John]

8 minutes ago, wookie1965 said:

Hi John is that inward travel.

Yes - the prism diagonal uses 10-12mm less of the light path of the scope so eyepieces focus that much further outwards / you gain inwards travel.

 

[Mr Spock]

Best for image quality:

2" - Baader T2 prism

1.25" - Takahashi prism

Both have better planetary performance than mirror diagonals - and I have the excellent Baader 2" dielectric.

[Highburymark]

2 hours ago, LondonNeil said:

I've read good comments on the baader diagonal before,  but wouldn't a  mirror be better than a prism diagonal? 

Prisms are not advised for fast scopes below F/7 or thereabouts, but they’re great for slow scopes. 

[LondonNeil]

Thanks.  Doing some Google research it gets very confusing... some say the higher reflectivity on mirrors makes a big difference in light polluted skies (I'm bortle 7-9) , others say its not noticeable.   I guess it comes down to other factors,  scope, eps,  viewing experience,  personal foibles,  target type etc etc. 

OK..... I'm in no rush..... I'll keep options open and pick up something from the classifieds. 

[Highburymark]

The biggest difference I saw between diagonals was when I first traded up from the basic Celestron that came with the C6 to a WO dielectric. Noticeably brighter and sharper - in my experience at least. The difference between more expensive diagonals - be they mirror or prism - is pretty small tbh. They’re a bit like binoviewers - very close in quality - trying to distinguish them optically can be a challenge. But they’re all excellent, as long as they’re well collimated.

[LondonNeil]

Good,  this is reinforcing the,  ' pick up whatever decent one comes along in the classifieds' view. Were I able to fit a 2" I'd be spoilt for choice,  but I've searched and 1.25" are currently thin on the ground.   I can wait though. 

[John]

I found quite a few positive reports on the Celestron branded (China made of course) prism diagonal - the one with the part number of 94115-A.

They chimed with good experiences I had quite a few years back with the same diagonal which came with a C5 scope that I owned back then.

Given that the scope that I wanted it for was F/16 I felt that the risk of 20 quid for the thing was worth taking. It's not in the same league build-wise as my Baader T2 Zeiss prism of course but that cost more than 10x as much !

I probably won't be comparing these two, in case the 20 quid prism performs just as well as the 200 quid one 😉

So far though the cheapie prism has done fine in the undemanding long refractor. I have replaced the chromed set screws with nylon ones to give my eyepiece barrels a chance though 😬

 

[Cosmic Geoff]

44 minutes ago, John said:

Celestron branded (China made of course) prism diagonal - the one with the part number of 94115-A.

I have two of these. They work well on my SCTs. I have no problem with them.. 

[Zermelo]

1 hour ago, LondonNeil said:

Were I able to fit a 2" I'd be spoilt for choice

Is your Mak a Skywatcher? You can actually replace the visual back if you wanted to use a 2" diagonal. There is some vignetting towards the edge, though I've not noticed any myself. I've used a 1.25" Tak prism and a 2" GSO dielectric and I can't see any difference. I prefer the 2" as it feels more secure with heavier eyepieces, but I rarely use 2" EPs in it.

https://stargazerslounge.com/topic/336529-sky-watcher-mak-127-visual-back/#comment-3662339

https://stargazerslounge.com/topic/356717-sw-127-mak-ota-with-2-focuser/

 

[Don Pensack]

6 hours ago, LondonNeil said:

I've read good comments on the baader diagonal before,  but wouldn't a  mirror be better than a prism diagonal? 

That depends on the f/ratio of the scope and the price of the prism.

 

Generally, f/8 and longer scopes are OK with prisms or mirrors.

Shorter than f.8, a prism tends to add lateral chromatic aberration, so a mirror is likely better.

Likewise, an inexpensive prism might be better made than an inexpensive mirror type, but once you head up in price, the mirrors become the equal of prisms and may be better.

One think is for sure--there are a lot more mirror diagonals to choose from.

And it might be worth pointing out that AstroPhysics and Tele Vue, both makers of high-end refractors, both sell only mirror diagonals.

[LondonNeil]

Yes zermelo, skymax 127. From a bit I've read there seems no real gain going to a 2" visual back ... unless you feel the need to fit a crayford focuser.  No I intend to stick at 1.25" I think. 

Dan, It's a slow scope,  f11 ish, so tolerant to CA.

Seems like it's not a big upgrade and I definitely shouldn't rush....  'I'll keep a close eye on tnt classifieds etc and see what comes along. 

[Louis D]

5 hours ago, LondonNeil said:

skymax 127. From a bit I've read there seems no real gain going to a 2" visual back

I've found it immensely helpful in getting targets centered.  The field of view does vignette somewhat as seen below, but the extra true field opened up is quite useful.

As you can see, it opens up roughly 80% more linear true field of view.  Light falloff is around 35% center to edge which is surprisingly hard to detect visually.

The longer path through the 2" diagonal does necessitate moving the mirror quite a bit to reach focus, increasing the focal length by at least 200mm.  Thus, if you're never going to use 2" eyepieces, you'd be unnecessarily decreasing your true field of view with each and every eyepiece by making them work at a higher power.

Edited April 12 by Louis D

[LondonNeil]

That's interesting Louis, thank you.   Hmm. I've also got an st120 though which I think does wide field views better (it is fitted with a bresser 2" diagonal).  I've also got the two sides of my gtiX fairly well aligned so in effect the st120 can become a high power finder scope for the mak127 if there are tricky targets I really want to mak-view.