2" eye pieces are they all low power?

[Mick UK]

Im about to buy the Skywatcher 250pds and it comes with a 2" 28mm eye piece, i recieved one of these eye pieces when i purchased my Skywatcher 80ed pro. so Ive been looking to see if i can find a 2" 4mm or 8mm eye piece but i cant seem to find any?...is there a reason for this? and can anyone be kind enough to reccomend me any if they do exist.

[pvaz]

There aren't, I think. The reason you need 2" in the 1st place is so that the EP can grasp a large area of the sky. At high power you get a smaller area in the FOV so a 1.25" will have no constraints to show the best image possible.

[Bizibilder]

If they did exist in the sizes you mention the price would be... uh....Astronomical!!!

Just buy a 1.25" to 2" adapter and you are in business!

[Mick UK]

Thanks for the replies, i get it now;)

I ws under the impression that a 2" 4mm eye piece would obviously give the same magnification and a 1.25 eye piece, but i assumed you'd have a wider field of view, but as you say Bizibilder, its probably down to the price, which then begs the question, why is it cheaper to make a 2" 28mm eye piece than it is a 4mm eye piece, when theres more glass in a 28mm?

[Tantalus]

...which then begs the question, why is it cheaper to make a 2" 28mm eye piece than it is a 4mm eye piece, when theres more glass in a 28mm?

Good question Mick. I'd be interested in the answer to this one.

[Astro_Baby]

There are in fact some high power EPs in 2" fitting but theres little point in doing it. The FOV of a high powered EP would be no wider than the FOV of a 1.25"

I suppose stuff will come along in the mould of the Televue Ethos / Explore Scientific 100' stuff but the price will be steep I'd imagine (look at the cost of an Ethos in 9mm or the ES in 9mm)

Its not actually cheaper to make a 28mm in 2" its only cheaper to make a cheap 28mm in 2". Check out the price of serious orthoscopics in 1.25" and you'll see what I mean.

[pvaz]

From what I read it's physically possible to build an EP with 100 degree AFoV with a 1.25" fitting up to 13mm (the Ethos 13mm is 1.25" I think). Above that focal lenght you need 2" in order to get 100 degree AFoV.

As to price, well my Ethos 10mm costs more them my scope and it's a 1.25" EP so it's really up to how much you'd like to spend.

[PeterW]

It depends on the field stop diameter (the diameter of the black ring at the edge of the field of view). For long focal lengths it has to be bigger than you can get down a 1.25" tube... hence the NEED for 2" eyepieces. For short focal length eyepieces you don't need the extra diameter, hence no one makes 2" versions. As mentioned you just need an adapter to allow you to use the 1.25" in your 2" focusser. It is good to see manufacturers putting 2" focussers on scopes, allowing us to use long focal length eyepieces without the 1.25" limitation.

Hope this helps

PEterW

[Ben Ritchie]

There are in fact some high power EPs in 2" fitting but theres little point in doing it.

One useful aspect for substantial eyepieces like the XW and Ethos lines is that a 2" format is (much) more securely held in the focuser. That's particularly useful for an EQ Newtonian, where the eyepiece can end up at some rather odd angles at times.

So I find that i'm entirely 2", despite only having one eyepiece that actually needs that format.