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The 102-mm FCD100 triplet APO refractor: explore scientific's new line of FCD apochromatic refractors promise premium optics at an affordable price.

I'LL DISCLOSE AT THE OUTSET that I'm a big fan of this class of telescope. I own several 4-inch refactors and have tested many others. But this is my first experience testing an apochromatic refractor from Explore Scientific. Overall, I was impressed.

The unit in question is the 102-mm model from their new line of premium apochromatic refractors that all feature a triplet lens with an element made of the new Hoya FCD100 glass. Other telescopes in the series offer 80- and 127-mm apertures, the latter with the option of a carbon-fiber tube.

FCD100 low-dispersion glass has optical characteristics nearly identical to the Ohara FPL53 glass widely used in other top-of-the-line "apo" refractors. The difference is price. Explore Scientific's 102-mm refractor in the FCD100 series sells for $1,500, hundreds of dollars less than most competing telescopes with this grade of triplet optics.

Is this indeed a price breakthrough product? I would say yes. Its image quality was as good as any apo refractor I have used.

The Optics

Some lower-cost apo refractors I've used have sharp optics, but images are inevitably tinged with false color. With the 102-mm FCD I saw none--not a trace of false color even on demanding targets such as Vega, and on crater rims and the limb of the Moon. Even racking through focus showed no fringes of cyan or magenta in the defocused Airy disks of stars. And in focus, there were no blue halos surrounding bright objects, even at high power with a 3-mm Radian eyepiece.

Even more critical was the lack of any significant astigmatism or spherical aberration that would detract from the sharpness of the image. Diffraction patterns of stars were nearly textbook perfect both in focus and when defocused.

The tight components of Epsilon Lyrae, the Double-Double, were cleanly split with dark sky between each of the two pairs, with the component stars all exhibiting tight, well-defined Airy disks. This was true even in temperatures just below freezing when contracting lens cells can sometimes pinch or distort optics. Not here.

The tube is well blackened inside with one baffle halfway down the tube. Many other top-class refractors have two or three internal baffles, causing me some concern that the FCD scope would be subject to lens flare.

Not so. In direct comparison with another 4-inch triplet apo costing thousands more, with FPL53 glass and several tube baffles, I saw little difference in performance. Both showed a similar level of modest glare from the Moon when it was just outside the field of a 17-mm Ethos eyepiece. The FCD's tube design was working just fine.

In addition, color correction and sharpness were also on par with the premium apo. The FCD100 glass was doing its job well, providing top-of-the-line performance for a mid-range price.

Photographic Performance

Our test telescope on loan from the manufacturer came with an optional ($150) field flattener that retains the telescope's f/7 focal ratio and 714-mm focal length. Measurements of the photographic field size showed the telescope was indeed delivering just over 700 mm of focal length.

While f/7 is a little slower than I prefer for deep-sky imaging with DSLRs, it's certainly possible to get well-exposed images at modest ISO speeds in exposures of 4 minutes or more.

With the optional field flattener, images with a full-frame DSLR exhibited excellent sharpness across most of the frame, with elongation of star images increasing toward the corners. I'd judge the flattener as good, but it fell short of fully correcting a full-frame field. However, vignetting at the corners and along the bottom edge of the frame from DSLR mirror-box shadowing was minimal, as I would expect at f/7. The flattener is best-suited for APS-format or cropped-sensor cameras.

The focuser can be rotated for framing fields. I found doing so did not affect focus. It can also be locked down to prevent any focus shift from the weight of the camera.

One minor issue I found was that care was needed to ensure the field flattener's 2-inch tube was squarely inserted into the focuser. Despite the focuser's 2-inch collar having a compression ring, I found I could tighten the collar's three thumbscrews and still have the flattener tilted, which would result in unevenly distorted stars across the field.

Mechanical Quality

The telescope is quite light for its aperture, weighing just 4.1 kilograms (9 pounds) without rings or other accessories. (By comparison, the premium apo I compared it to weighs 5.8 kg, even though it has a carbon fiber tube.) The FCD 102-mm will work well with a light "grab-and-go" mount, such as Explore's own Twilight alt-azimuth model, at least for visual use. I used the scope on a similar Astro-Tech Voyager mount, a combination that worked well. Of course, long-exposure photography requires a heavier equatorial mount.

The FCD100 series comes with Explore Scientific's standard HEX focuser, a Crayford-style unit with both coarse and 10:1 fine focus adjustments. It has a relatively short 44 mm of travel. So to allow for the varied back-focus demands of eyepieces and cameras, the FCD100 comes with three extension tubes, one short and two longer ones, plus a final screw-on collar that accepts 2-inch accessories.

For visual use the one short extension tube sufficed to enable all the eyepieces I tried, from low-power 2-inch to high-power lV4-inch eyepieces, to reach focus with the excellent 2-inch mirror diagonal supplied as standard equipment. For photographic use with a DSLR, one additional longer extension tube was required. Attaching a CCD camera might require the third tube, depending on the spacing requirements of the camera.

The extension tubes screw on with fine threads that might be easy to cross-thread. Also, if they weren't on tight, I found they could loosen and turn under the weight of a star diagonal and heavy eyepiece if they were tilted towards the left side of the focuser, causing them to unscrew and flop down unnervingly. There are no lock screws to secure the extension tubes.

On the plus side, by not using any extension tubes I could get my Baader binoviewer and its own low-profile diagonal to reach focus without requiring any Barlow lens. That produced a lovely 2[degrees] field with a pair of 24-mm Tele Vue Panoptic eyepieces.

However, the much more affordable William Optics binoviewer, typical of most units, did not reach focus, even when used with a l1 1/4-inch diagonal. It still needed its add-on Barlow lens. So in my judgment the extra back focus supplied by the focuser wouldn't be sufficient for most binoviewers without their accessory Barlow lenses.

At first, the focuser exhibited a fair degree of image shift, but this was largely eliminated by tightening the two tiny hex screws on the focuser barrel that serve to add tension. Even so, a slight image shift of about 15 arc-seconds remained, not serious but a little annoying when trying to achieve fine focus while imaging.

More important was what I would describe as a "general mushiness" in the fine-focus adjustment. I could turn the knob and nothing would move at first, then it would grab, even with the focuser locks off. On occasion it would outright slip and not move at all, though not necessarily under a heavy load. This behavior made it more difficult to nail precise focus. When locked down, however, the focuser did stay put with a DSLR camera in place.

The HEX is a nice focuser; it just isn't top class, at odds with the scope's premium optics. I would have preferred to see a small Feather Touch focuser, the brand supplied with Explore Scientific's flagship 152-mm refractor. I would have also preferred a focuser with more travel, to reduce the need for swapping among the suite of extension tubes. Of course, all of that would add additional weight and cost to the telescope, partly negating two of the telescope's top virtues. And users can always upgrade the focuser to a third-party model at a later date.

As it is, if the 102-mm's performance is a gauge, Explore Scientific's new FCD 100 series delivers superb optical quality for affordable prices, with the 102-mm model hitting the sweet spot in cost vs. aperture vs. portability.

Contributing Editor ALAN DYER, a member of The World at Night, authored the e-book How to Photograph & Process NightScapes and TimeLapses.

102-mm FCD100 Air-Spaced

Triplet ED APO Refractor

U.S. Price: $1,499.00

What We Like

Superb optical quality Compact size, light weight

What We Didn't Like

Average focuser Field flattener not fully effective

Caption: The Explore Scientific FCD 102-mm aperture apochromatic refractor features a new type of low-dispersion Hoya FCD100 glass, meant to produce apochromatic performance at significantly lower cost than competing models.

Caption: With the optional field flattener, stars appeared as pinpoints across most of the field, only exhibiting aberrations at the corners. This image of NGC" 7000, the North" America Nebula, is a stack of five 6-minute exposures at ISO 1'600 with a modified Canon EOS 5D Mkll camera.
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Title Annotation:S&T Test Report
Author:Dyer, Alan
Publication:Sky & Telescope
Article Type:Product/service evaluation
Date:Jun 1, 2017
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