Bumbling Through Eyepiece Selection: A Former Newbie Confesses All

By Matthew Buynoski

I'm an engineer, and as such I tend to work with and generally believe in mathematical models of things. So, when I first started out to buy a tele- scope, I got out optics books, studied formulas, read books on beginning in amateur astronomy (all you newbies, read "The Backyard Astronomers' Guide"!), and generally stuffed myself with a lot of book learning on the subject. I did not (But should have!) go to enough star parties and look through enough telescopes/eyepieces in advance. Partly that was due to the weather (it was late fall and cloudly a lot so there were lots of non-happenings) but mostly because I was consumed by throbbing of the want gland. There is no one quite so unable to resist high-tech hobby stuff as an engineer; you might as well be dealing with a four-year-old the week before Christmas. Now! Now! I want it now! In the books, I'd read about wide-field eyepieces and their captivating views, so when I got to the telescope store I elected to look through some of them. Yes, there was a definite difference between them and the 50-degree units, so I elected to go with as many as I could afford. The books also told me how to compute the diffraction limit to a magnification range, which I did for the scope I was buying (203mm aperture, so about 400X limit). Fortunately, the book also mentioned seeing limits, and I elected to stop at 300X on that advice. I happen to work with microscopes a lot, so I thought I understood how much change in magnification you can stand when switching from one to another optical element. However, not having ever really used a telescope, I underestimated the amount of jiggle in them relative to a very stably mounted flat-field metallurgical microscope. In the latter, you can comfortably change magnification by 2:1 anywhere in the range from 50X up to 800X. I also do some close-up photography, and one thing that taught me was that adapters are a royal pain in the ***, so I tried whenever possible to avoid using them. Since I knew I wanted to use 2" eyepieces, but that they don't exist for high magnifications, I chose to use where possible double skirted ones (i.e. the eyepiece will mechanically fit either a 1.25" or 2" star diagonal without an adapter). For the one 1.25"-only eyepiece, I bought an adapter, but since there was only 1, it could be dedicated and thus not really a separate item. So I went in, and selected the following:

                Focal Length   AFOV      Magnification     Size
                    55mm        50 degr.     37X            2"
                    40          65           51X            2"
                    22          68           92X         1.25"/2"
                    13          82          158X         1.25"/2"
                     7          65          290X          1.25"
Note the following failures:

(1) the 55mm and 40mm have virtually identical Tfov. Being overly confident of my knowledge of optics, I did not realize I didn't understand Tfov all that well.

(2) There is an almost 2:1 gap in magnification between 13mm and 7mm. I didn't yet understand that this was too much of a jump for high magni- fications on a telescope (being blinded by my experience with microscopes).

(3) The sequence does not reflect the fact that seeing often allows more than 158X, but not so much as 290X.

Getting out into the field, I discovered (after the obligatory two months of rain to satisfy the New Telescope Curse) the following:

               a. The 55mm and 40mm showed exactly the same view
               b. I lost the object I was hunting down a lot when switching
                   from the 13mm to the 7mm.
               c. There were many times other people slapped on 200X or 220X,
                   but my 290X was too much and 158X not enough.
Well, back to the telescope store. I swapped out the 55mm for a 27mm, 68 AFOV, 2", and added a 9mm, 82 AFOV, 1.25"/2". Not a bad set after that. I became enamored of a neat little diffraction-grating spectroscope and bought a small 17mm 1.25" eyepiece to use with it (the new toy required an eyepeice that was physically small at the viewing end, which did not describe my other eyepieces. I had planned to use it on the 22mm, but it didn't fit).

As the scope and eyepieces got used more and more, I eventually found one more thing I didn't like. When I put the 13mm on, occasionally the scope would "take off" on its own when I released the clutches on the mount. This was due to a substantial difference in weight between it and the other eyepieces. After being bonked in the eye more than once by this, I retired the 13mm (with some regret, it gave great views) and replaced it with a 14mm, 65 AFOV unit. The 13mm went to a new home and a 5mm 52 AFOV, high constrast eyepiece came in the trade. I had also, by this time, discovered that the 27mm unit didn't get a whole lot of use. At the low end of the magnification range, the 2:1 ratio rule held up pretty well in actual field use. It is mainly used when I want to observe an celestial object that doesn't quite fit in my 22mm, and objects of such a size are relatively few in number. Curiously, the new 5mm turned out to be parfocal with the 7mm, even though they are different brands. This turned out to be a great convenience at the top end of the mag range. The final set:

              Focal Length   AFOV      Magnification     Size
                  40mm        65 degr.      51X           2"
                  27          68            75X           2"
                  22          68            92X        1.25"/2"
                  17          50           120X         1.25"
                  14          65           145X         1.25"
                   9          82           226X        1.25"/2"
                   7          65           290X         1.25"
                   5          52           406X         1.25"
Couple of notes:

(A) Except for the 17mm (with the spectroscope) and the small 5mm, all of these weigh within a couple of ounces of each other. I can live with that because the two exceptions are fairly rarely used compared to the "main sequence". (Bad astronomical joke, that).

(B) Each of the 1.25"-only eyepieces has its own private 2" adapter attached. I still hate fussing with separate adapters. That's a large part of why I have no Barlow, either.

(C) The fact that the 5mm, 7mm and 14mm are all parfocal with each other means less need to use the 9mm as a magnification "bridge". It's easier to switch between the parfocal 7mm and 14mm now than it used to be between 7mm and 13mm. However, the 9mm still finds plenty of use when seeing makes the view thorough the 7mm too wobbly.

This set works well enough in practice that I am happy with it. Given the chance to do it all over again from scratch, I'd probably go with an almost all parfocal set from one manufacturer. Replacing the 9mm with the 10.5mm cousin of the 7mm and 14mm would make the whole top end of the magnification range parfocal.

When the now-non-newbie went out and got a telescope for the wife, what did we do for eyepieces? First, the small refractor she wanted (for ease of handling, and light weight) didn't take any 2" eyepieces. It also came with a high quality 20mm Plossl. We elected to add 8mm, 15mm, and 32mm from the same series, all parfocal with each other. Unfortunately, the store was out of 32mm in that series, so we got another 32mm which turned out to be not very far from parfocal when we tested it. At the top end of the magnification range, we took a 3.8mm which gave a magnification of 126X or close to the 140X suggested by diffraction limit on the 70mm aperture scope. It turned out in practice that this telescope was of sufficient optical quality to push that limit a little, and I surprised her (eyepieces and other astronomical gadgets are a never ending source of gift ideas) with a 2.5mm eyepiece (192X). Other than that addition, this all 1.25" set has been fine in practice:

              Focal Length   AFOV      Magnification    Mag.Ratio
                  32mm        50 degr.      15X            ----
                  20          50            24X            1.60
                  15          50            32X            1.25
                   8          50            60X            1.88
                   3.8        50           126X            2.10
                   2.5        45           192X            1.52

We found that you can get away with somewhat wider ratios of magnification when the telescope mount is motorized (hers is, mine isn't) so it tracks an object. This gives you time to take out one eyepiece and put in another at mags. over 100X without the object "taking off" on you.