1.25" v's 2" accessories

Tom Rauschenbach posted:

I still don't understand why my intuitive feeling that 1.25" eyepieces
would benefit from a 2" diagonal is wrong, but I'm grateful that someone
bothered to tell me that it is.

Well, the light getting in from the edges of a 2" star diagonal would
still not get into the 1.25" barrel of the 1.25" eyepiece, so there
would be no additional field provided by using the 2" star diagonal
(unless the system was strongly vignetted by a certain 1.25" star
diagonal, which isn't likely, but isn't exactly unheard of either). The
main reason to go with a 2" star diagonal is to have the future
capability to eventually use 2" eyepieces. Another reason would be if
all you have are 2" filters but still want to use filters with some
1.25" eyepieces, some 2" diagonals have either threads on the front for
2" filters or have 2" to 1.25" adapters which are threaded for 2"
filters. This would allow you to use 2" filters with 1.25" eyepieces,
which *would* be something of an advantage.

There may be a small "gotcha" with some 2" star diagonals and 1.25"
eyepieces. A few with moderate or higher profile 1.25" adapters may
cause a few 1.25" barrel eyepieces to not come to focus in some
refractors which have limited focuser travel. I ran into this when
trying to use a William's 2" star diagonal with its 1.25" adapter on my
5-8mm Speers Waler variable focal length eyepiece. With that adapter, I
just could not get the Speers Waler eyepiece to focus in my SkyView Pro
100mm f/6 refractor, although it focused just fine using the stock 1.25"
star diagonal which came with the scope. I eventually got a special
low-profile sleeve made for the Speers Waler which allowed it to come to
focus as a modified "2-inch eyepiece", but most of the time, I just
switch back to the 1.25" star diagonal when I want to use 1.25" barrel
eyepieces in that refractor. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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