Limited focussing with 2 inch diagonal

[simple_legate]

I have a Skywatcher 102/500 scope which Have mostly used with 1.25 inch eyepieces. I have mostly used 32 and 12.4 mm Meade series 4000 eyepieces (sometimes with a 2x barlow) although I do have a range of plossls from 20 to 4 mm.

Over Easter a friend at work lent me a 2 inch star diagonal from Williams and a 2 inch GSO 30mm eyepiece. That was a fantastic experience - so much so that my 30 minute test run turned into 3 hours outside.

One thing I noticed, though, was that I could not bring my 1.25" eyepieces into focus with the 2 inch diagonal. In fact, I noticed I only had about a centimetre of play when I was focused using the 2 inch EP.

Is this a normal sate of affairs for this (or other) scopes? I had thought enough of the view to consider getting a 2 inch set, but worried I might get bored with the capabilities of just one eyepiece. Then I thought that a 2x Barlow might give me a solution to making the scope a little more flexible, but I am worried about shelling out for one to find it, too, doesn't come into focus. Unfortunately my friend doesn't have a Barlow that I could use to test it.

Is there any way of finding out it will work before I buy, and would it be worthwhile to use a Barlow this way in any event? I could live with it being a rich field scope only, I suppose, because I can't use binoculars, but I would like maximum flexibility on my scope.

Any help would be greatly appreciated.

Regards,

Matthew

[astro744]

A 2" diagonal will have a longer path length than a 1.25" diagonal thereby leaving you with less back focus.

A 2" eyepiece set is not necessary or typically available and most sets will have a combination of 2" and 1.25" or if they are only 2" they will be in longer focal lengths only. The barrel diameter simply limits the maximum true field possible. A short focal length eyepiece even with enormous apparent field will still only require a 1.25" barrel and most manufacturers will only make them as such. There are a few hybrid dual barrel eyepieces available in some focal lengths, eg. 12mm Type 4 Nagler.

Note not all diagonals have the same path length and sometimes you can gain valuable focus travel by choosing the right diagonal provided the information is available to make the choice and most suppliers won't publish this.