Diagonal's. 2'' Vs 1.25''

[Damo636]

Hi everyone. Is there any advantage in using a 2'' diagonal other than the obvious acceptance of 2'' eyepiece's. Will the larger mirror show more detail? I often wondered when i bought my William Optics diagonal should i have went for the 2'' version.

[kniclander]

you need a 2" diagonal if you want to use a 2" e/p. other than that, people say that the central bit of the mirror is likely to be better than the edges and if you have a 2" diagonal and a 1.25" e/p you would be using that "better" part. I suspect the difference is very very marginal and a good 1.25" diagonal would (marginally) beat a cheap 22 diagonal.

[russ]

you need a 2" diagonal if you want to use a 2" e/p. other than that, people say that the central bit of the mirror is likely to be better than the edges and if you have a 2" diagonal and a 1.25" e/p you would be using that "better" part. I suspect the difference is very very marginal and a good 1.25" diagonal would (marginally) beat a cheap 22 diagonal.

I agree totally. Doubt you'll see any improvement with a 2" diagonal over a decent 1.25" when using 1.25" eyepieces. Plus the better 1.25" diagonals use oversized mirrors, so the used part of the mirror surface should be good.

[MjrTom]

Dont 1.25 inch diagonals have a shorter light path length?

This could cause an issue with having enough infocus when using some 1.25 inch EP's in a 2 inch diagonal if I am correct in my assumtion (?)

Mark

[Merlin66]

Yes, that could certainly be a factor you need to consider......

[michael.h.f.wilkinson]

In my SCT (so no infocus problems) the WO 2" quartz dielectric diagonal performed way better than the 1.25" stock Celestron. The difference in light throughput and quality is clearly noticeable. I have heard people say that a Revelation 2" dielectric beat the WO 1.25" by quite a margin (do not know whether this was a dielectric). Dielectric coated mirrors are not just brighter, they last longer and are harder.

[Damo636]

The William Optics one i have is indeed dielectric. I have to confess other than it looks really good and has a compression ring fitting i personally didn't notice any real difference to the views that the original supplied Tal diagonal produced. I read the thread yesterday where a member found the 2''Revelation diagonal knocked the spots off the 1.25'' WO part and it got me wondering would a 2'' wring out a little more performance from the 100RS.

[russ]

I have to confess other than it looks really good and has a compression ring fitting i personally didn't notice any real difference to the views that the original supplied Tal diagonal produced.

Really not surprised by that.

Of all the accessory upgrades available, i have found the diagonal to be the one that shows the least gains. I got sucked into the 'must have' dielectric and 2" mindset a few years back. Finally, after many upgrades, arriving at a Televue 2" Everbright. Really nicely made diagonal but could i hand on heart say it was better than the 1.25" Stellervue Dielectric......nope! I'm sure the optical experts will say there is a huge difference but i have to say that difference wasn't apparent to me. The TV was sold asap and i settled on the Revelation 2" Quartz because it was cheap, still nicely made and worked well.

[michael.h.f.wilkinson]

Of all the accessory upgrades available, i have found the diagonal to be the one that shows the least gains. I got sucked into the 'must have' dielectric and 2" mindset a few years back. Finally, after many upgrades, arriving at a Televue 2" Everbright. Really nicely made diagonal but could i hand on heart say it was better than the 1.25" Stellervue Dielectric......nope! I'm sure the optical experts will say there is a huge difference but i have to say that difference wasn't apparent to me.

A really top notch 1.25" could well perform similarly to a top notch 2". My comparison was between a run-of-the-mill 1.25" and a very good 2". That was a lot more of an improvement than I had expected. The TAL diagonals may be very good (would not surprise me in th eleast), in which case the main gain is access to 2" EPs.

[russ]

A really top notch 1.25" could well perform similarly to a top notch 2". My comparison was between a run-of-the-mill 1.25" and a very good 2". That was a lot more of an improvement than I had expected. The TAL diagonals may be very good (would not surprise me in th eleast), in which case the main gain is access to 2" EPs.

You're right Michael. Although the Stellarvue only cost £58 at the time and the TV was £150 secondhand. Such a large jump in cost for so little gain...other than being able to use 2" accessories.

Funny thing is i wasn't even convinced the Stellarvue was any better than the old (ancient) Intes 1.25" it replaced. And the reason i went looking for a replacement in the first place was due to the Intes not appearing to be any better than the bog standard Celestron diagonal that came with my C6. Which all means i wasn't convinced the TV 2" was giving a huge improvement over the standard Celestron.

But i have since compared the Celestron with a decent diagonal and there is good improvement. So it was a case of the conditions perhaps not allowing the better diagonals to perform before i ditched them in disappointment.

[michael.h.f.wilkinson]

You're right Michael. Although the Stellarvue only cost £58 at the time and the TV was £150 secondhand. Such a large jump in cost for so little gain...other than being able to use 2" accessories.

Funny thing is i wasn't even convinced the Stellarvue was any better than the old (ancient) Intes 1.25" it replaced. And the reason i went looking for a replacement in the first place was due to the Intes not appearing to be any better than the bog standard Celestron diagonal that came with my C6.

My Celestron one was old (11-12 years at least) when I got my WO 2" (which was on offer for 99 euros ). Maybe a clean, new Celestron would have been a better match for the WO. I already had a 2" EP (the Paragon) so really needed a 2" anyway.

[russ]

My Celestron one was old (11-12 years at least) when I got my WO 2" (which was on offer for 99 euros ). Maybe a clean, new Celestron would have been a better match for the WO. I already had a 2" EP (the Paragon) so really needed a 2" anyway.

Sorry Michael, i added a bit to my post while you were posting. There is a clear difference from the bottom end cheap models. I have seen the difference now. But like all things, once you reach a certain level of quality (Stellarvue/WO) the gains from that point on are small and those small (minimal) gains cost a lot of money.

[michael.h.f.wilkinson]

Sorry Michael, i added a bit to my post while you were posting. There is a clear difference from the bottom end cheap models. I have seen the difference now. But like all things, once you reach a certain level of quality (Stellarvue/WO) the gains from that point on are small and those small (minimal) gains cost a lot of money.

Absolutely. The difference between a Plossl and Vixen LV was big, and clearly noticeable. The difference between Vixen LV and Radian more subtle. I tend only to replace kit when the limitations become noticeable, or start to annoy, no sooner.

[russ]

Absolutely. The difference between a Plossl and Vixen LV was big, and clearly noticeable. The difference between Vixen LV and Radian more subtle. I tend only to replace kit when the limitations become noticeable, or start to annoy, no sooner.

I had a similar experience with the WO UWAN 16mm and the TV Nagler T6 16mm. Quite a difference in price but the differences in performance were very subtle indeed.

[Damo636]

Thanks guys for putting my mind at ease. I won't go jumping in now and buying a 2'' diagonal. At least not until i actually buy some 2'' ep's!

[michael.h.f.wilkinson]

I had a similar experience with the WO UWAN 16mm and the TV Nagler T6 16mm. Quite a difference in price but the differences in performance were very subtle indeed.

So will you hit 10,000 posts today? Looks like it

[russ]

So will you hit 10,000 posts today? Looks like it

oh i hadn't noticed that. Looks like it will very soon indeed.

[michael.h.f.wilkinson]

oh i hadn't noticed that. Looks like it will very soon indeed.

Glad to be of service:D

[Merlin66]

The ultimate test of any diagonal is to use the highest power eyepiece in your box and look at a 3rd mag star as high in the sky as possible (reduce atmospherics)

Look at the star image just inside and outside focus. There should be NO signs of astigmatism (elelongation of the image)

I've found on some of the cheaper diagonals they would never pass this test.

I now use the UHTC Meade 2" and 1.25" and 2" TV diagonals.

[cs1cjc]

Some 1.25" diagonals are restricted internally. In particular, the WO Carbon Fibre is restricted to 25mm as against the 27mm field stop of the widest 1.25" eyepieces.