2 inch vs 1.25 eyepieces?

[Steve-hobart (Steve)]

I have a new 10" dob. If i decide to get another eyepiece is there any advantage to 2" eyepieces over continuing with 1.25" ?

[Damienandwendy (Damien)]

From my research over the last week or so trying to work out a few new EP's for my two new scopes, I think it has more to do with low magnification and wider AFOV. Once upon a time you could only get a 40mm with say 45d AFOV and that would be fine in a 1.25" EP. Now with 100d AFOV I think you can get a 2" EP at say 18mm to give a similar TFOV.

Hope that reads how I thunk it

cheers

[Steffen]

Yes, larger diameter eyepiece barrels are required for larger true fields of view. The TFOV only depends on the scope's focal length and the eyepiece's field stop diameter. Since the field stop can't be larger than the barrel you hit the limit at about 27mm with 1.25" eyepieces, and at 46mm with 2" eyepieces.

Cheers
Steffen.

[brian nordstrom (As avatar)]

Grab a 2 inch GSO super view 30mm from Andrews communinations and at $49 ( don't know how they do it ? at that price , but they do ) , a great way to test the waters , so to speak , its a great eyepiece that will give you an idea how nice a good 2 inch eyepiece is in a 10 inch scope , cruzing the milky way on a dark night ,,, nice stuff , and wont break the bank .
These are awesome 68 degree eyepieces and you wont be disappionted I guarantee that .
I have looked thru the 15mm and 40mm and both compare favourably to a 19mm TV Panoptic and a 31mm Celestron Axiom LX , both sweet eyepieces worth $100's more .
I think the 30mm SV will be just right for your scope as a starter 2 inch , but be aware ,,,,,, this can get addictive and expensive real fast , that's why I suggest grabbing one of these nice eyepieces . .
But most of all , enjoy .

Brian.

[omegacrux (David)]

Yup a 2in 30mm ep in a Dob gives great views
Still see faint fuzzies , one of my most used ep's
In my 10in Dob , oh and its a gso ep I use
I don't care if there not pin sharp to the edge .

David

[Wavytone]

Steve,

The point of 2" eyepieces is that on a long focal ratio scope such as a refractor, an f/10 SCT or f/15 Mak it takes a very long focal length eyepiece to come close to the maximum exit pupil. For example, an f/7 refractor takes a 40mm eyepiece to produce a 6mm exit pupil. On an f/10 SCT, 60mm. On an f/15 Mak, 90mm.

The point here is that the longest useful eyepieces in a 2" barrel are around 40-50mm focal length. These are big, heavy lumps of glass.

2" eyepieces are simply not really relevant to fast dobs - for the simpe reason that you can reach the maximum exit pupil around 6mm using 1.25" eyepieces around 20-30mm focal length depending on your scope. If you put one of these big long focal length eyepieces into a fast dob, the result is an exit pupil larger than the iris in your eye. The result is that a significant portion of the light from your scope does not enter your eye, and you may as well use a rather smaller aperture.

I see a lot of angst on cloudynights where people try to stuff big 2" eyepieces into a big dob in the hope of ever wider fields, a rather better solution is to use a smaller refractor.

[MattT]

Wavy is correct. I use a 70mm f7 refractor as my finder on my only working scope ATM and thats a 150mm f8 refractor. The 70mm gives a very nice 2.5º sky with a 1.25" 20mm 70º eyepiece. I was after an exit pupil under 3mm to keep the sky nice and dark...could go wider than 2.5º but not much use for my preferences in the city. The biggest 2" eyepiece I have is a 100º ES 20mm that is very nice in the f8 refractor but only about half the same TFOV as the 70mm frac.
If you want wide get a small fast refractor. The f7 will eventualy end up on the 10" Newt as a super finder/ wide field scope.
Matt

[noeyedeer (Matt)]

I'm testing out a 2" 32mm ep in my 10" dob and I agree with David. I have a GSO 2" 30mm ep which came with it to test ... but this unbranded 32mm is producing nice views.

swapping from the 9x50 finder to the ep looks about 2-2.5 times as magnified. (I'm not good with numbers after a few beers).

its more like the view I get with my 1.25" 40mm, but a lot clearer.

I say go for it Steve, you won't be disappointed at all

matt

[Steve-hobart (Steve)]

Ended up getting a 30mm GSO 2" ep. Tried it out the other night and quite liked it. Very easy to use with or without glasses and quite panoramic. Initial impression is that I like it more than the 25mm 1.25" plossl that came with my scope.

Next step is going to be to get a high magnification ep for planets and the moon. Will be looking at 1.25" for that however.

[bigjoe (JOSEPH)]

Originally Posted by Steve-hobart

Ended up getting a 30mm GSO 2" ep. Tried it out the other night and quite liked it. Very easy to use with or without glasses and quite panoramic. Initial impression is that I like it more than the 25mm 1.25" plossl that came with my scope.

Next step is going to be to get a high magnification ep for planets and the moon. Will be looking at 1.25" for that however.

You have done well Steve 30/42 gso, all very good.

Orion expanses or edge on planetaries, will serve you well also, if you buy those.
.
I have naglers and deloses, but the Orions are not that far behind at slow F ratios
Bigjoe

[noeyedeer (Matt)]

Originally Posted by Steve-hobart

Ended up getting a 30mm GSO 2" ep. Tried it out the other night and quite liked it. Very easy to use with or without glasses and quite panoramic. Initial impression is that I like it more than the 25mm 1.25" plossl that came with my scope.

Next step is going to be to get a high magnification ep for planets and the moon. Will be looking at 1.25" for that however.

Good choice that 30mm, can't go wrong for the price either.
I have a 6 and 5mm TMB Planetary II (thanks doug mc) that I would highly recommend. I've only used them on Saturn, but the views are remarkable compared to a barlowed plossl. they give me 208x and 250x in my 10" dob.

things move quick through them thou.

matt