------------------------------------------------------- Garrett Optical 30mm 82° 2.0" Ultra-Wide Angle Eyepiece A deep-sky delight with Sky-Watcher 20cm f/6 Dobsonian ------------------------------------------------------- As an enthusiastic beginner in the science and art of deep sky observing -- the viewing of objects beyond our solar system such as open or globular star clusters, nebulae, and galaxies -- I'm pleased to report that the Garrett Optical 30mm 82° 2.0" Ultra-Wide Angle Eyepiece gives new meaning to the concept of a "rich-field telescope." With my Sky-Watcher 20cm (7.9") Newtonian reflector at f/6 on a Dobsonian mount, seeing all of a spectacular object like M24, the Small Sagittarius Star Cloud, in a single 2° field is indeed a joy to behold; or framing a glorious open cluster like M25 in an engaging vista of surrounding star patterns and asterisms. Even in the light-polluted urban skies seen through an open window of my apartment observatory, the wide field makes these objects a special delight, along with simply browsing through random star fields. An advantage of the 82° apparent field of view (AFOV), which in my scope means a true field of view (TFOV) of about 2°03' at 40X, is that it not only enhances the enjoyment of known deep sky objects (DSO's), but helps in the finding and identification of those that are new to me. For example, one fine evening I saw for the first time what looked like a globular cluster, a "faint fuzzy" a bit brighter than the background, and framed by two asterisms maybe half a degree on either side. Sketching the globular and asterisms, and comparing this sketch with a 2° field around M22 as shown by the fchart software program for Linux, helped me make a positive identication. Seeing the "big picture" in a single convenient field of view facilitated this process. Interestingly, this Garrett Optical eyepiece matches the AFOV of the famous Nagler design, also at 82°. Obviously, of course, this more economical design does not claim the quality and edge sharpness of the Nagler; but for many amateur astronomers including myself, a good test might be a principle associated with the UNIX family of operating systems for computers, a vital element in the building of the electronic highway that we call the Internet. The UNIX principle holds that if you can get 90% of the benefits for 10% of the costs, then it's probably an attractive solution. With my 20cm f/6 Dobsonian, at least, the Garrett Optical 30mm eyepiece may approximate this adage, although I haven't done an actual comparison with a Nagler. What I can say is that the eyepiece gives me a very satisfying and "immersive" viewing experience, and has more than lived up to my expectations at the time I ordered it. One concern was that at f/6, my scope might be near the lower recommended limit for this eyepiece, close to the zone where the optics might be too fast for comfort. Would everything in the outer third of the field look weirdly distorted, as some colorful reviews of other economy ultra-wide angle eyepieces might suggest? Well, I told myself, if so, I would enjoy it as a kind of "space warp" effect . The actual experience was different -- and very pleasantly so! The whole field is useable, and a positive asset, whether taking in the untrammeled reaches of M24 (a portion of the Milky Way about 10,000 to 16,000 light years from us in the direction of the galactic center) or simply being able to spot two bright stars at opposite ends of the field. Thus the first question one might ask about such an eyepiece, "Does the added periphery enhance or detract from the overview impression?" could be answered, "It indeed enhances." A closer examination -- from my novice's point of view, of course -- showed that there seemed to be some degree of field curvature. That is, if I focused stars in the central portion of the field as sharply as possible, then those near the edges would be a bit out of focus, although I could sometimes move my eye slightly to compensate and get these stars more in focus. If my interpretation is correct, then this might be evidence that the edge sharpness isn't quite equal to a Nagler, but it hardly seems a fatal flaw or even a serious annoyance; more like a routine compromise of the kind that is commonplace in amateur astronomy and much else in life. As with other ultra-wide angle pieces including the Naglers, accurate eye placement can be important in getting the full view; a bit of experience rapidly got me attuned to finding the right position. The eyecup facilitates this process for me, as does adjusting my StarDust observing chair for a comfortable viewing angle. Humorously, I might add that this eyepiece faithfully emulates another attribute of a Nagler: it cannot show regions of the sky where opaque terrestrial obstructions intervene! I learned this when investigating an apparent "viewing problem" near the horizon, which happened to be my window sill, as further observing and a daytime test confirmed. From a technical perspective, one quirk of my Sky-Watcher 20cm Dob might be worth mentioning: the rather small diameter for the optical tube assembly (OTA), evidently only around 225mm. In one Internet discussion forum, a participant suggested that ideally the OTA should have a diameter equal at least to the sum of the aperture (here 200mm) plus the field step of the eyepiece with the widest AFOV (here 43mm for the Garrett Optical 30mm) -- or, for this scope, 243mm. Whatever theory might say about this narrow OTA design, in practice it doesn't seem to be a problem as I relish those 2° fields. If indeed this telescope design might be taken as an optical imperfection, then maybe we could conclude that the Garrett Optical 30mm eyepiece is yielding very nice results even under less than ideal conditions. Having conveyed that this eyepiece is for me a big winner, and indeed my favorite choice, I should prudently offer a few cautions and disclaimers, following the example of Garrett Optical itself, and conclude by amplifying the "rich field telescope" or RFT theme a bit. First, as Garrett Optical wisely cautions, f/6 may be around the lower recommended limit for satisfactory results with this eyepiece. While f/6 is moderately fast, it's not so dramatically fast with today's optics often reaching to f/4.5-f/5 for Dobsonian and other telescopes. For example, 10" Dobsonians at around f/4.7, 12" models at around f/5, and larger apertures with even faster ratios are common. Here the watchword should be "try before you buy," for example at a star party. Secondly, for the sake of completeness, I should note that observing with a telescope from indoors through an open or closed window is hardly an ideal arrangement, and it has properly been deemed especially suspect as the venue for any optical test. However, my purpose here is not to offer any kind of scientific or even artistic comparison with competing products, only to share the impact of the Garrett eyepiece on my own night-to-night observing lifestyle, however ideal or otherwise its parameters. My main reasons for relying on an apartment observatory are to avoid the problems of getting a rather large telescope up and down two flights of stairs in a building without an elevator, and also some safety concerns about being alone outdoors with my scope at night in an urban neighborhood, albeit a rather placid one. Of course, the indoor situation has its special comforts and charms, and I must admit basking in them. Also, I necessarily speak at this point from inexperience. However, much more seasoned amateur astronomers such as "Uncle Rod" Mollise have reached the same general conclusion that economic ultra-wide angle eyepieces can be a very cost-effective choice. Now for an explanation of the RFT theme. In 1936, S. L. Walkden published an article, "The Richest-Field Telescope," describing an instrument with an aperture of about 3" which should in theory show more stars in a single field than either a smaller or larger instrument when viewing the denser precincts of the Milky Way. Perhaps it is especially appropriate to mention this germinal article of seven decades ago when reviewing a Garrett Optical product, given the renown of Garrett Optical binoculars, including some giant models which might precisely answer to Walkden's original RFT concept. Somewhat more loosely defined, a "richest-field telescope" should have an exit pupil roughly matching the eye dilation of the observer, plus a reasonably wide field. Since maximum pupil dilation can vary from person to person with age as one significant factor, what is considered an optimal RFT might depend in part on the observer. As it happens, observing at my not-so-young age, a dilation of 5-6mm might typically be expected. With the Garrett Optical 30mm eyepiece and my 20cm f/6 Dob, we have a 5mm exit pupil at 40X, and as noted above a field of 2°03', wide enough for some majestic sweeps of starry regions. This is a considerably larger aperture, and rather narrower field, than Walkden's RFT model, but possibly a near-optimal variant for light-polluted conditions where some extra light-gathering power and magnification can help to cut through the murk and reveal some of the stellar riches beyond. Margo Schulter mschulter@calweb.com Sacramento, California, USA 10 October 2007