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Benefits of 2" diagonal


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Can anyone offer me some advise on whether I should consider a 2" diagonal, please.

I have a 4" refractor with standard 1.25" diagonal but it will take a 2" diagonal, also have a 5.5" mak that may take a 2".

With the smaller size eyepiece on a larger diagonal, what will the benefit be, if any.

I assume the real benefit will be with 2" eyepieces, which I don't have (yet).

Ta for help.

Mand

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If you already have a good quality 1.25" diagonal, the benefits of putting a 1.25" eyepiece in a 2" diagonal will be absolutely zip! The only benefit is being able to use 2" eyepieces and accessories. Someone may try and tell you that 2" diagonals are easier to manufacture and thus have superior quality but that's just hog wash. Easier to manufacture...maybe. Superior quality...no! Both will be made to same optical standard.

Russ

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You never know when you're going to want a 2" eyepiece - Some might say "future-proofing"? :D

Upto ~32mm (50 Deg Plossl etc.) there is no real difference. (see above) But I think there is a useful gain in "robustness". With a 1.25" push-in diagonal, I always had a... neurosis that even my (heavier) 1.25" eyepieces might fall out, if the diagonal rotated! Accordingly, I now use a modest Sykwatcher 2" diagonal in my ST102 (mm) Achromat of same name. Your CAT can probably also benefit from a 2" system, despite the "vignetting theorists". With my Skymax MAK127, I use a Baader (nice. versatile!) 35mm diagonal, which can accommodate e.g. 1.25" nose-pieces, T2 Threads etc. as input, adapting to a 2" eyepiece holder, at the "business-end":

See e.g. http://www.telescope-service.com/baader/accessories/accessories.html#prisms

Finally, my favourite eyepeice (The Baader Hyperion), although having a 1.25" nose-piece, is (IMO) much more "mechanically" suited (see above) for use with a 2" diagonal...

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What Chris has said is absolutely correct, I should have expanded. Although what I said still holds, you won't see any gain at the eyepiece with your current 1.25" eyepieces. But it is always best to future proof and you never know when a 2" eyepiece will come along.

Chris, you're right, there's no issue using 2" accessories on the 127 Mak. Did it myself with no vignetting.

Russ

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Heheh. I didn't want to (accidently) come across as "adversarial" either! Always interested by people who manage to squeeze that little bit extra (TFOV) out of a CAT. Clearly a man after my own heart... :D

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Someone may try and tell you that 2" diagonals are easier to manufacture and thus have superior quality but that's just hog wash.

That'll be me :D :D

My understanding is that unless you already have a premium 1.25" diagonal (ie Televue) you will benefit from a 2" diagonal - even when using 1.25" eyepieces. The imperfections in the mirror lie at the boundary where the mirror is cut. The regular 1.25" diagonals supplied with Meade, Celestron, Skywatcher are 'adequate' and might achieve 1/4 wave accuracy whereas a good quality 2" with its bigger mirror can easily achieve 1/10th wave. If you also opt for a Dielectric diagonal (brighter) you will definitely see a difference.

Just my opinion.

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One issue that you do need to be aware of is that a 2 inch diagonal generally uses a cm or 2 more infocus than the 1.25 variety. In my case this means that I have a couple of eyepieces that won't come to focus with the 2 inch diagonal but will with a 1.25 inch. This probably won't be an issue with a Mak as they have a wide range of focus travel but it may be an issue with a refractor. I have these issues with my ED80 refractor.

If your current eyepieces all come to focus with at least a couple of cm inward focusser travel to spare then your probably OK.

With regard to quality I reckon that a budget 2 inch diagonal is going to be better quality than a budget 1.25 inch. I have an Antares 2inch mirror diagonal which cost very little but is much better quality than the standard Celestron 1.25 inch diagonal that came with my C8. By contrast I also have William Optics dielectric diagonals in 1.25 and 2inch fittings and there is no discernable difference between them in their build or image quality - both excellent. So as you go up market the differences seem to dissapear.

John

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