Sky and Telescope - June 2017 - 59

Photographic Performance

p The telescope weighs 5.7 kilograms (12.5 pounds) with the supplied tube rings and star
diagonal in place. It measures 54.6 centimeters (21½ inches) long with the dewcap retracted,
and without the 2-inch diagonal and none of the three extension tubes, one of which is on the
telescope here. The optional field flattener is also shown.

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-theline performance for a mid-range price.

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 ﬂat-

p Left: The air-spaced triplet lens is very well coated with Explore Scientific's "EMD" multi-coatings. Its internally blackened tube has one baffle to
block stray light and off-axis reflections. Right: The Crayford-style 2½-inch rotatable focuser includes a 10:1 fine-focus control. It extends a modest
44 mm and is supplied with three screw-on tubes to extend the unit's focus range. The single short tube is shown here attached to the 2-inch adapter.
s k y a n d t e l e s c o p e .c o m

* JUNE 2017

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Sky and Telescope - June 2017

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