Smart Astronomy's 27mm EF Eyepiece
Have
you been looking for a quality wide field eyepiece for under $100?
Would you
like to find a binoviewer pair of good low power eyepieces that won't cost more
than your Denkmeiers? Smart Astronomy has two new eyepieces, the EF series. The
"EF" stands for "extra flat" field.
I thought I would give these eyepieces a
try, and the 27EF is the target of this review.
The
27mm and 19mm EF eyepieces were initially loaned to me by Smart Astronomy for
evaluation with the option to purchase them. First look is impressive. The
appearance, fit and finish is nothing short of excellent. The lettering is
large and easy to see. The eyepiece housing is ergonomically designed and easy
to grasp. You get top and bottom eyepiece caps, and even a small cleaning
cloth. Now that's a nice touch! There is a twist-up eye cup and I discovered
later that with the eye cup retracted, there is plenty of eye relief for eye
glass wearers. For me, that's another nice touch! I can easily see from the design
that the EF27 has potential as a binoviewer eyepiece. It is small and
lightweight for this focal length (see stats below) and the eye cup can be
adjusted so that you can properly align your eyes with the eyepieces.
Here
are the physical stats of the EF27mm eyepiece as I measured them:
Weight:
185gr (5-1/2 oz)
Field
stop diameter ~26.5mm (roughly same true field of view as a 32mm plossl)
Maximum
housing diameter 45mm (47mm at the eyecup)
Height
above the chrome barrel: 50mm with eyecup retracted
Twist-up
eye cup and ergonomic grip
Very
generous eye relief (great for eye glass wearers)
Cost:
currently under $100 USD
First
test was done in daylight. I compared the EF27 with a GSO 32mm plossl, a 28mm
XL Pentax (discontinued 1.25" version), and the Stellarvue 23mm FMC. I
targeted the door of my shed with my 80/F6 achromat to look for field
distortions. The 32 plossl and the 23FMC show some pincushion, more so with the
plossl. The EF27 though, showed no pincushion or field distortion that I could
detect. Sharpness, contrast, and detail of the 27EF were excellent on center.
However, the eyepiece looses definition just a bit at the edge. I would
consider this acceptable as it has about the same edge performance as other
eyepieces I own in this price range, like the Meade 5K plossl and the 23FMC
from Stellarvue. The image is not quite as good as the 28mm Pentax XL, and the
Pentax has better resolution at the edge. I have not tested the EF27 in a
telescope faster than F6, but just judging from what I see in two of my F6
scopes I would think that performance might not be as good in faster telescopes
due to the loss of edge detail, so the Pentax might be a better choice for fast
scopes.
In
all of the telescopes I tested the EF27 performed extremely well. Now, in my
80mm/F6 achromat, I actually like my 32 plossl on large open clusters or the
Sword of Orion. But, for about the same field of view the EF27 gives more
magnification with a darker background sky. To some, this fact might make it
more desirable than the 32mm. In my 8"/F6 Dobsonian, for example, I liked
the extra magnification and contrast that the EF provided, and I would prefer
this eyepiece over the plossl for hunting down deep space objects. The Pentax
edged out the EF in edge definition in my Dobsonian just as it did in the
achromat. The EF does have the advantage over the Pentax of being smaller and
lighter weight, so balance is not as much of a problem on my Dobsonian mount. I
found on-axis sharpness of the EF27 in my Dobsonian to be excellent. At 44x,
not only could I see the rings of Saturn well, on the evening of March 7th I
picked out the moons Tethys, Dione, and Rhea lined up on the western edge of
the rings, spanning only 22 arc seconds across at a maximum distance of only 1
arc minute from the center of Saturn. I would describe the contrast of this
eyepiece as excellent.
Lateral color is minimal, and I am impressed at just
how "clean" the image is, with the background sky being black right up to the
planet. With my 102mm/F7.75 doublet apochromat, bright stars show up as lovely
jewels. Orange, white, or bluish white, all bright stars had nice saturated
colors. Airy discs were perfectly round with no comet tails. I also saw an
improvement in edge definition. I would recommend considering this eyepiece over
a plossl or Erfle for long focal length telescopes, as the longer the focal
ratio of your telescope, the better it performs on-edge.
GSO
32mm Plossl, Pentax 28mm XL, Smart Astronomy EF28 (L-R). Note the purple tint to the EF28 versus the greenish
tint to the other two. I compared the EF19 (19mm version) to another popular
brand of comparable construction and identical price, and determined that the
EF19's coatings have noticeably more contrast and brighter image.
Lunar
performance is equally impressive. Looking at a waning gibbous moon through my
80mm/F6.9 doublet apo, the moon was tack sharp from top to bottom. There are no
ghosts or flares, even when used without any filtration. The Moon even flared
very little when I placed it out of the field of view. No doubt this eyepiece
is extremely well protected against unwanted light. Libration was not good for
the western limb at that time, but at 22x I could still see one mountain peak
poking out from the limb at Mare Orientale. Subtle albedo features between
maria and highlands showed up well. Off-axis lateral color was so slight I
would call that a non-issue.
Binoviewing:
If you have one of the ubiquitous Norin-made binoviewers (William Optics, Orion,
Stellarvue, etc) these eyepieces have more field of view than the binoviewers
are capable of giving, so there is considerable vignetting. Other than that,
the pair performed very well in my Stellarvue BV1's, giving a low power option
for lunar/solar viewing when the seeing is lousy. In premium brands, like
Denkmeier and Televue, there is no vignetting so you get the full potential of
field of view. I think the small physical size and weight of the EF27 make it a
very good choice for binoviewers.
Eyecup retracted
Eyecup fully
extended
Bottom
line: It is unrealistic to expect a $99 low power wide field eyepiece to match
the performance of a premium eyepiece like a Pentax. The Smart Astronomy EF27
however, met my expectations for a $99 eyepiece. With the possible exception
of
performance issues with telescopes faster than F6, in my opinion anyone buying
the 27EF would be happy with the views they get for their money. (I did in
fact, purchase the loaners sent to me.)
David
Elosser
Kernersville
NC
