REVIEW: Garrett Optical 30mm 82-o UWA eyepiece
- From: Margo Schulter <mschulter@xxxxxxxxxxxxxxx>
- Date: 11 Oct 2007 07:50:43 GMT
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Garrett Optical 30mm 82-o 2.0" Ultra-Wide Angle Eyepiece
A deep-sky delight with Sky-Watcher 20cm f/6 Dobsonian
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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-o 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-o 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-o apparent field of view (AFOV), which in my
scope means a true field of view (TFOV) of about 2deg03' 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-o 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-o. 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 <grin>.
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 Dobmight 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-o 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.
<http://adsabs.harvard.edu/abs/1936PA.....44..146W>
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 2deg03', 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.
Most appreciatively,
Margo Schulter
mschulter@xxxxxxxxxx
Lat. 38.566 Long. -121.430
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