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Smart Astronomy's 27mm EF Eyepiece

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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


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