*Review: Astrosystems 30mm WIDE SCAN III Eyepiece

Product Review
by David Knisely

30mm WIDE SCAN Type III Eyepiece

Available from Astrosystems
$225

The popularity of wide-field eyepieces has surged in recent years, and,
to a certain extent, so has their cost and complexity. Systems containing
more than the usual handful of elements are required to achieve good
correction of aberrations over wider fields especially in the shorter f/ratio
telescopes we see today. It was with some trepidation that I heard of the new
WIDE SCAN TYPE III eyepiece available from Astrosystems. I tried one and
decided that, for its cost, it might have a place in my eyepiece box.

The 30mm WIDE SCAN TYPE III is a 5-element 2 inch barrel diameter
eyepiece similar in size to a larger Erfle. The unit is 11 cm (4.33 inches)
long, about 5.7 cm (2.24) inches in diameter, and weighs in at around 455
grams (1 lb). The barrel is a chrome surface which is about 4 cm in length
and the standard 2 inches (5.08 cm) in diameter, with a safety groove for set
screws in its upper third. The 2 inch barrel is threaded for standard 2 inch
filter threads and the interior is grooved to reduced scattered light. The
main section of the eyepiece is a hard black surface with a rubberized knurled
base for better gripping in the hand with an eye lens end that has a standoff
or fixed "eyecup" to help position the eye.
Optically, the lenses are said to be multi-coated, and from the looks of
things, this is true. The coatings had no visible flaws and yielded an
overall greenish cast to light reflecting off the lenses. The eye lens is
about 3.7 cm across and was slightly concave on its outer surface. The field
stop on this eyepiece measures out to 43mm, and is slightly smaller than the
field lens itself, which appeared to be slightly convex. The nature of the
outer surfaces of this eyepiece might indicate that it could be a variation on
the Erfle design, which also has 5 elements. The apparent field of view is
stated to be 84 degrees, and an optical bench measurement showed this to be
accurate. The eye relief is difficult to measure (probably in the 11 to 13mm
range), but the fixed eyecup above the eye lens prevented me from seeing quite
the entire field of view with my glasses on.
I put the eyepiece to the test using my 10 inch f/5.6 Newtonian, my
100mm f/6 refractor, an 8 inch Celestron Schmidt-Cassegrain and a 14 inch
Celestron Schmidt-Cass. In larger focal ratios such as the f/10 SCTs, the
eyepiece performed fairly well, with fairly sharp star image over much of the
field of view. For a more stringent test, in the C14, I slewed Antares from
one edge of the field to the other and the eyepiece did start to show a little
astigmatism as the star got into the outer portions of the field. However, it
was nice to be able to fit the entire moon in the field of view with that
scope, even if the edges weren't perfectly sharp! The view was fairly high in
contrast, so the coatings looked to be doing their job. However, in shorter
f/ratios, the performance did decline in the outer third of the field. The
eyepiece yielded a whopping 4.1 degree true field in my 100mm f/6 refractor,
but at the cost of some noticable astigmatism. This became most apparent from
about 70% of the field radius out to the edge, where star images became short
arcs rather than points. It wasn't quite as bad as the astigmatism I had seen
in the 24mm Speers-Waler, but it was present. However, the views of the star
fields of the Milky Way were quite stunning even with the aberration present.
There is some pin-cushion distortion with this eyepiece which is
noticable if looking at straight lines near the edge of the field. However,
it isn't excessive, and in fact, is comparable to that found in some of my
Plossl eyepieces with their much smaller apparent fields of view. There was
little chromatic aberration present in this eyepiece (except perhaps towards
the edges when the eye is moved out of its normal centered observing
position), so it is fairly well corrected in that regard. Still, it looks
like this design could be a somewhat "pushed" Erfle, and that the design was
stretched perhaps a little too much really shows when used at focal ratios
below f/10. Use of a Barlow improved things somewhat, but did not eliminate
the astigmatism. I noted no severe kidney beaning or blackout, although you
do want to keep your eye well-centered on the eyepiece and right up to the
hard eyecup "guide" if you want to see the entire field. Compared to things
like the Meade 14mm Ultrawide or the TeleVue 31mm Nagler, this eyepiece's
performance wasn't exactly stellar, but it didn't cost nearly as much as those
two eyepieces either. If users want tack-sharp star images across an 80+
degree apparent field, they should be ready to pay the big bucks for a more
complex eyepiece design that delivers that kind of quality.

In summary, the Astrosystems 30mm WIDE SCAN III eyepiece is not in the
same league as the more expensive wide-field designs, but it does offer some
interesting wide-field performance which some people might find useful,
especially in an eyepiece that costs less than $250.

--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

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* Attend the 10th Annual NEBRASKA STAR PARTY *
* July 27-Aug. 1st, 2003, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
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