Is it that size really does matter?

[Patbloke]

Hi Fellow Gazers,

Our small but growing society twitter feed @WMFS_AstroSoc has just bought two scopes a 200p dob and a Skywatcher 200p on an eq5...

We are just about to buy two sets of eps and the question has been asked should we buy 2" rather than 1 1/4" Is it all about size?

I'm at a loss on this one and usually do hours of research... but, (sympathy required) I am just recovering from a horrendous bout of 'Norovirus' and I need to make a decision soon...

In you I trust...

[nightfisher]

if scopes are used maily for lunar/ planetary then go 1.25, if maily dso then go 2"

[John]

Many folks end up with a mix of both sizes, 1.25" for high and medium magnifications and a 2" or two for lower power, wide field views. It's the wide fields that the 2" size brings to the party rather than any additional optical quality.

[umadog]

I wouldn't worry about the eyepiece barrel size too much. Look for, price, magnification, and apparent field of view and choose accordingly. The more expensive, longer focal length, wider-field, eyepieces are more likely to come in 2" flavours. The reason is that the eyepiece's true field of view depends upon the field stop diameter and so getting a larger field means a bigger field stop. That's why wide-field, low-power, eyepieces tend to come in 2" barrel sizes. There are even specialist manufacturers who make 3" eyepieces to provide wider views in very long focal length telescopes.

2" eyepieces can be more costly and some can be rather heavy. You will need to swap the 2" to 1.25" adapter in and out if switching between sizes. You will need to buy 2" filters, which are more expensive. You don't need to buy 1.25" filters, though, since you can threaded 2" -> 1.25" adapters. Personally, I buy eyepieces in whatever size they come, subject to cost, focal length, and field of view constraints.

[Patbloke]

Three great responses so far... thank you for the advice, maybe we can get a set of each and swap them between our two scopes? Whatever we get I will eventually leave reviews for

[Mr Spock]

The 2" size is to allow a larger field lens. What this does is allow the same focal length eyepiece as 1.25" to have a wider field of view.

A 32mm 1.25" is at it's limit at 50°, whereas in 2" you can get a 31mm 82° - a big advantage for wide field observing.

At shorter focal lengths the field lens is smaller, so these are 1.25".

[Patbloke]

Mr Spock,

Once you have eliminated the impossible, whatever remains, however improbable, must be the truth.

--SPOCK, Star Trek (2009)

Thanks for that simplicity...

[ChrisThePuma]

and there was me thinking that the difference between the 1.25 and 2 inch was all about the size of the hole they go into - i.e some scopes only take a 1.25 and some only a 2 inch.

Still a noob!

[cantab]

Well yes, a scope with a 1.25 inch focuser takes 1.25 inch EPs, a scope with a 2 inch focuser takes 2 inch EPs. You simply get adapters to use the one in the other. An adapter to use 1.25 inch EPs in a 2 inch focuser is common, the other way round less so.

[Capricorn]

I would get a selection of decent 1.25" eyepieces.

They come in a selection of angles, 52 to 80, there is a wider choice and they are generally less in cost.

As it is for a society I would have thought that a good selection was of more use.

[ChrisThePuma]

Thanks cantab - corrected again :-)

[damnut]

H Patbloke,

Have a play with these, just plug in your scope & ep etc... :--

http://www.12dstring.me.uk/fov.htm

N.A.A. Telescope Math Calculator

Might keep your mind off the norovirus!!

Cheers

[Patbloke]

Great links very interesting

[damnut]

I have a mix of 2" eps - 40mm,32mm,30mm,25mm,15mm --- all my other eps are 1.25" as there is no real advantage to them once you start significantly decreasing the focal length.

[Moonshane]

I have 10 eyepieces of which just one is a 2" and that's a 26mm. The remainder are 1.25", so I agree that a mix is a good thing.

[Stu]

Agreed. I have 41mm, 31mm, 28mm, 21 mm and 17mm in 2", below that they are 1.25". I must confess there is something wonderful about a nice widefield view through a 2" ep, but there is no need once you get down to the shorter focal lengths where it is likely to be more about magnifying smaller objects well rather than fov

Stu

[ollypenrice]

Mr Spock,

Once you have eliminated the impossible, whatever remains, however improbable, must be the truth.

--SPOCK, Star Trek (2009)

And Conan Doyle a hundred years earlier, if I'm not mistaken?

No need for 2 inch at short focal lengths. They offer no advantage whatever. As Mr Spock says, if you want a really great widefield view then a 2 inch wide angle at long EP focal lengths is a real treat.

Olly