By Chuck Hawks
Some entry level telescopes come with 1.25" only focusers, but intermediate and advanced level scopes are generally designed to accommodate 1.25" or 2" eyepieces. Each eyepiece diameter has its advantages. To wit:
1.25" Eyepiece Advantages
2" Eyepiece Advantages
The back of most telescopes are adaptable to both eyepiece diameters and most 2" star diagonals come with (or will accept) 1.25" adaptors. This being the case, in most circumstances the best approach is to take advantage of the benefits of oculars in both sizes. There are many amateur astronomers using 1.24" oculars who do not own any 2" oculars, but I will bet there aren't many using 2" oculars who don't also own some 1.25" oculars. I will guess that most of those who don't own any 2" oculars would like to, but are dissuaded by the increased cost, plus the hassle of switching back and forth.
Having a 2" star diagonal with a drop-in 1.25" adaptor makes switching eyepiece sizes relatively fast, minimizing the hassle. I find it much more convenient than changing from a 1.25" visual back and a 1.25" star diagonal to a 2" visual back and 2" diagonal. For that reason, I have standardized on using a Vixen 2", dielectric coated, star diagonal for almost all viewing, merely removing the (supplied) 1.25" adaptor when I switch to a 2" eyepiece.
Since 2" oculars are inherently more expensive to manufacture than 1.25" oculars (material costs are higher and there is more glass to grind and polish), 2" eyepieces tend to have sophisticated optical designs. There are a few inexpensive 2" oculars on the market, generally using three element (Kellner) optical designs, but the great majority use four or more lens elements. The four element Plossl design has an apparent field of view (AFOV) of about 50-degrees and is common in both 1.25" and 2" eyepieces. Five to eight element designs, available in both 1.25" and 2" oculars, can yield an AFOV from 60-degrees to as much as 110-degrees.
The AFOV is important, because the true field of view of the telescope system is calculated by dividing the AFOV by the system magnification. For most purposes, I find an AFOV in the 50-degree to 70-degree range comfortable and entirely practical for most viewing. Note that a 2" diameter mounting barrel does not necessarily confer a greater AFOV. That is a function of the optical design of the eyepiece. However, at focal lengths over 32mm the 1.25" diameter mounting barrel vignettes the 50-degree apparent field of view of a Plossl ocular, making wider AFOV designs pointless. Because of this physical obstruction, oculars with a 1.25" mounting barrel in focal lengths exceeding 32mm have a smaller AFOV than shorter focal length oculars of the same design. The practical result is that a 1.25", 40mm Plossl has about the same true field of view as a 32mm Plossl, negating the primary advantage of the longer focal length ocular. That is why you seldom see 1.25" oculars in focal lengths longer than 40mm.
Making the eyepiece mounting barrel 2" in diameter allows an unobstructed field lens, eliminating vignetting. For example, a 1.25" 40mm Plossl has a vignetted AFOV of 43-degrees, while a 2" 40mm Plossl has an AFOV of 50-degrees. In a telescope with a prime focal length of 800mm, the true field of view using the 1.25" eyepiece is 2.15-degrees, while the true field of view with the 2" eyepiece is 2.5-degrees. Even better, the 2" 40mm eyepiece could be a five element (Erfle) design with an AFOV of 60-degrees (impossible in a 1.25" barrel) and a true field of view of 3.0-degrees. This clearly illustrates the advantage of the 2" mounting barrel for long focal length oculars.
2" oculars are commonly available in focal lengths as long as 56mm, while 1.25" oculars are typically limited to 40mm. Drop a Meade Series 4000 56mm Plossl 2" eyepiece (52-degree AFOV) in our 800mm telescope and the true field of view is a whopping 3.6-degrees!
Short focal length, high magnification eyepieces are more common in 1.25" diameter. For example, a quick check for this article showed that Oceanside Photo and Telescope (OPT) stocks 23 eyepieces in focal lengths between 2mm and 4.9mm. Only three of those are adaptable to 2" mountings and those three have combination 1.25" and 2" mounting barrels. There were no unique, short focal length, 2" oculars. If you do high magnification observing using short focal length oculars, 1.25" is the way to go.
If you follow my example and use a 2" star diagonal (with a 1.25" adaptor as required) for general viewing, you can conveniently use a selection of 1.25" and 2" eyepieces. For wide field views, the 32mm Plossl represents the tipping point. Oculars from 33mm to 56mm should be 2" diameter and it is definitely worth owning a couple of eyepieces in this focal length range (for example, 40mm and 55mm). Shorter oculars can be either diameter, but the more expensive 2" eyepieces have no dramatic advantage over their 1.25" equivalents. To minimize switching from one size to the other, many of the premium oculars in the shorter focal lengths come with dual size mounting barrels. (Note the Meade Ultra Wide Angle oculars at the top right in the photo at the top of this page.) For example, the 32mm and 22mm Celestron Ultima LX eyepieces come with 2" (only) mounting barrels, but the 17mm, 13mm, 8mm and 5mm focal lengths have dual diameter 1.25"/2" mounting barrels.
This is a considerable convenience. If, for example, you use a 2" 50mm eyepiece to locate an object, you can switch to any of the shorter focal length Ultimas for a closer view without inserting an adaptor in your star diagonal or focuser. On the other hand, if you are using your short focal length 1.25mm oculars (4mm, 6mm, 7mm, 9mm, 10mm, 12mm, etc.) for high magnification viewing, the 5mm, 8mm, 13mm and 17mm Ultimas fit right into the progression without removing your 1.25" adaptor.
For example, my personal eyepiece selection, accumulated over a period of years and periodically refined, includes these focal lengths:
1.25" DIAMETER MOUNTING BARRELS
1.25"/2" DUAL DIAMETER MOUNTING BARRELS
2" DIAMETER MOUNTING BARRELS
This gives a reasonable assortment of both 1.25" and 2" mount diameter oculars and minimizes the necessity to switch eyepiece mounting adaptors or star diagonals. To me, this seems to be the best of both worlds!
Copyright 2011 by Chuck Hawks. All rights reserved.