Sky & Telescope - November 2023 - 66

S&T Test Report by Alan Dyer
The Astro-Tech AT90CFT
This apochromatic refractor provides excellent image quality in an attractive carbon-fiber tube.
AT90CFT f/6 Triplet
APO Refractor
U.S. Price: $1,595
astronomics.com
What We Like
Sharp optics, excellent color
correction
Attractive finish and fittings
Solid and precise focuser
What We Don't Like
Minor off-axis aberrations
with 0.8x Reducer
Unnecessary locking screws
on visual back
Short dew shield
NORTH AMERICAN TELESCOPE dealer
Astronomics has introduced a new
apochromatic refractor to compete in a
crowded field of compact, photographically
fast astrographs. Its house-branded
Astro-Tech AT90CFT is a 90-mm f/6
triplet that has a stylish carbon-fiber
tube, which is why it bears the CFT designation.
That and the red anodized tube
rings, dovetail bar, and top handle help
the AT90CFT stand out from the pack, if
only for its striking visual appeal.
But does this attractive instrument
perform as good as it looks? I tested
an early production sample sent by
Astronomics (serial #00102) and put it
through its paces for visual and photographic
use, the latter with the optional
Astro-Tech 0.8× Reducer/Field Flattener.
Big Shoes to Fill
In the March 2009 issue of Sky &
Telescope, I tested an earlier 90-mmclass
apochromat from Astronomics:
the Signature Series TMB-92 with its
unusually fast f/5.5 triplet lens designed
by Thomas M. Back. The compact TMB66
NOVEMBER 2023 * SKY & TELESCOPE
The AT90CFT weighs 5.8 kilograms (12.8 lbs)
with rings, 0.8× Reducer/Field Flattener, and
Canon camera as shown, making the author's
Astro-Physics Mach1 mount overkill for an
imaging setup.
92 remained one of my favorite telescopes
for many years, with its premium
performance hampered only by the lack
of a dedicated field flattener for imaging.
The TMB Signature Series refractors
are long gone, and I sold mine a couple
of years ago. Astronomics promotes its
new AT90CFT as the successor to the
now classic and collectible TMB-92.
At an attractive price, too. The new
AT90CFT retails for $1,595, though the
optional Reducer/Field Flattener costs
an additional $199.95.
Unlike the TMB, the Astro-Tech 90
mm isn't quite as prestigious or unique.
The AT90CFT is similar to several other
90-mm apos currently on the market
and sold around the world under various
house brands. However, the lookalike
scopes all have differences in either
finish, tube materials, or the glass type
used in the objective lenses.
In Astro-Tech's version, one of
the elements in its 90-mm triplet
objective is made of FCD-100 ED, a
low-dispersion glass from Hoya in Japan
that's equivalent to Ohara's FPL-53, a
premium glass used for many top-class
apo refractors. Does the Hoya glass
perform as well?
Visual Performance
Yes, it does. There was no chromatic
aberration (in the form of a blue or
purple halo) visible around Venus or
bright stars, both in-focus and when I
racked through focus. Extra-focal star
images looked colorless.
A Strehl ratio of at least 0.95 is
promised for all units, with 1.0 being
perfect (possible only in theory), and
anything above 0.8 deemed ¼-wave
" diffraction limited. "
While some of the lookalike 90-mm
telescopes from other brands come
with individual test certificates measuring
that unit's optical performance,
the Astro-Tech model does not. This
avoids the potential for " Strehl-ratio
ALL IMAGES COURTESY OF THE AUTHOR

http://www.astronomics.com

Sky & Telescope - November 2023

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