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1.25" or 2" diagonal?

michaelmorris

By michaelmorris
in Discussions - Eyepieces

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michaelmorris

michaelmorris

Posted
Posted

I've got an old 8" LX200 Classic with it's original 1.25" diagonal. I think it's about time I upgraded this to a modern di-electirc model - but what size?

The rear opening in the old 8" LX200 is smaller than the opening in the larger models, so you can't take full advantage of the extra aperture if using a wide-angled 2" eyepiece.

In these circumstances, are then any advantages to buying a 2" model?

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russ

russ

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Just agonised over this myself. In the end i decided a 1.25" was all i needed for the LX50. I have the dob for widefield and low power views. The SCT i use for planets, the moon and the odd glob, all of which would not benefit one single bit from a 2" diagonal. So i saved some cash and bought the WO 1.25" Dielectric. And a very nice little thing it is too.

Russ

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John

John

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Mark

The WO carbon dielectric sounds tempting, but I got a finite budget and my main shopping priority is going to be an off-axis guider. The Skywatcher di-electric 1.25" or the WO 1.25" look like good deals..

I don't know if you use ultra-wide eyepieces Michael but just be aware that the WO (or at least the couple that I've used) has quite a thick retaining ring at the bottom of the eyepiece tube that can cause vignetting at the edges of the FoV in eyepieces with wide aperture field stops. It's excellent in all other respects though.

You can just see the retaining ring in this photo:

post-12764-133877424772_thumb.jpg

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AndyH

AndyH

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Posted

Ooooh.

Good call. I had forgotten about that.

I have a similar type of diagonal to the W.O. one shown, and it too has this lip. Don't know why they made it so wide. Really unecessary, to be honest.

Andy.

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Moonshane

Moonshane

Posted
Posted
I don't know if you use ultra-wide eyepieces Michael but just be aware that the WO (or at least the couple that I've used) has quite a thick retaining ring at the bottom of the eyepiece tube that can cause vignetting at the edges of the FoV in eyepieces with wide aperture field stops. It's excellent in all other respects though.

You can just see the retaining ring in this photo:

Good point John. Never noticed this on the one I have (W/O - but that's because I am banned from using it yet as it's my birthday present!).

I was considering a 35mm Skywatcher Aero lens eventually for wide field work (FOV 68 deg.). do you think this would be affected by this? any idea what the limit is (i.e. when vignetting starts to show?).

Sorry for the slight hijak but this may be of interest to others also as it appears a consideration?

cheers

Shane

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michael.h.f.wilkinson

michael.h.f.wilkinson

Posted
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I don't know if you use ultra-wide eyepieces Michael but just be aware that the WO (or at least the couple that I've used) has quite a thick retaining ring at the bottom of the eyepiece tube that can cause vignetting at the edges of the FoV in eyepieces with wide aperture field stops. It's excellent in all other respects though.

You can just see the retaining ring in this photo:

Have no problems with the Paragon 40mm (45.7mm field stop). I have not got my WO 2" diagonal at hand at the moment, but as I said, no vignetting visible. The fact that I have an F/10 light cone entering would mitigate the problems compared to say an F/5 Apo. The 35mm Skywatcher Aero lens should not be affected if the wider FOV Paragon has no problems (same AFOV, longer focal length).

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John

John

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Have no problems with the Paragon 40mm (45.7mm field stop). I have not got my WO 2" diagonal at hand at the moment, but as I said, no vignetting visible. The fact that I have an F/10 light cone entering would mitigate the problems compared to say an F/5 Apo. The 35mm Skywatcher Aero lens should not be affected if the wider FOV Paragon has no problems (same AFOV, longer focal length).

I'm referring to the 1.25" WO diagonal Michael. It's not an issue with the 2" ones.

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LukeSkywatcher

LukeSkywatcher

Posted
Posted

Newbie question/comment here.

I really do not understand the difference of 1.25" and 2" EPs. Ok I KNOW 2" EP's offer a wider FOV.....................BUT SURELY if a scope is sold as using 1.25" EP's then surely those EPs are the best to use for that scope and likewise for scopes sold as using 2" EP's?

?????

Can ANYONE point me in the direction of a link that shows the FOV difference on say a 130mm scope (or any apeture scope) that was sold with 1.25" EP's compared to the same scope using 2" EP's.

Many thanks.

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AndyH

AndyH

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Posted

2" ep's have the advantage only really in the lower power wide field area.

It's all to do with the limiting size of the eyepieces barrel in a 1.25" size.

eg: I have a 1.25" 32mm U.O. Konig. It has a 52 degree AFOV. This is the widest you can go in the smaller barrel size.

Not so with the bigger size. I also have a 2" 32mm U.O. MK80. It has a 82 degree AFOV.

So, in my Burgess apo, my 1.25" eyepiece gives 2.38 degrees of sky.

The 2" one gives 3.75 degrees.

Andy.

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LukeSkywatcher

LukeSkywatcher

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So essentially 2" EP's give the same magnification of a given object while offering a wider FOV of the same object. I knew this. I'd just really like to see a comparison image of both.

I'm really just wondering if its worth buying a converter to change all my 1.25" EPs into 2". I do prefer widefield views so it probably would be a good thing for me to do.

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LukeSkywatcher

LukeSkywatcher

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Thanks Andy. That is convincing enough. So i need to be on the hunt for an adapter that will convert all my 1.25" EPs into 2" EP's.

Where do i start? whats the technical term for such a contraption? and how much does it cost?.

Much obliged.

I'm adding all these parts to my birthday wish list for this year (end of March). So far i have a:TAL 2x barlow,one of these EP convertors/adapters.

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AndyH

AndyH

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Number 1. you're focuser must be able to accept the 2" size eyepieces. If it's just a 1.25" one, then you're stuck with using eyepieces, that size. The only option would be to change the focuser to the bigger size.

Sorry if I'm on the wrong track

Andy.

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LukeSkywatcher

LukeSkywatcher

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Posted
Number 1. you're focuser must be able to accept the 2" size eyepieces. If it's just a 1.25" one, then you're stuck with using eyepieces, that size. The only option would be to change the focuser to the bigger size.

Sorry if I'm on the wrong track

Andy.

Gotcha. I had a feeling i had things mixed up AGAIN.

Good lesson though cuz now i know if and when i may buy a new scope that it will be one that takes 2" EP's.

OK so the TAL 2x barlow is still on my wish list.

Sorry for being so moronic......but we all have to learn somehow/somewhere.

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