As a confirmed
telescope“junkie” and seasoned observer who has purchased
perhaps 150 telescopes over the past 50 years I’ve always come
back to the refractor as my very favorite type of instrument. Having
owned many sizes and configurations of the breed, I’ve been on
a seemingly endless quest for the ideal balance of capability,
portability, overall quality, and cost. Sadly, no such animal
exists. However, in restricting the parameters to the domain of the
rich-field telescope, several excellent choices do come to mind.
Looking in particular
at the 6-inch category of simple Fraunhofer achromats, I’ve had
the opportunity to try several designs from a variety of
manufacturers. Certainly, the Meade and Celestron 6-inch, f/8
achromats provide a range of capabilities running the gamut from
lunar, planetary, and double star observation to wide field and
deep-sky viewing. Unfortunately, their long tubes and significant
weight requires a fairly hefty mount that hinders easy portability.
The Celestron Omni XLT, a 150mm. f/5 scope, is much easier to mount
and transport, but its fine optical quality is compromised by a
severe dose of short f/ratio chromatic aberration.
Between these extremes
lies the Astro Telescopes 6-inch (152mm) f/5.9, an instrument
produced by Kunming United Optics in China and carried by its
exclusive US dealer, Hands on Optics in Damascus, Maryland.
Available as an optical
tube assembly with either a 2-inch or 3-inch rotating, dual-speed
Crayford focuser, this full 6-inch achromat is a hefty but relatively
short instrument that folds down to a compact 32 inches with its dew
cap retracted and focuser reeled in. A pair of nicely made cast tube
rings with Vixen-style mounting bar and a cylindrical carrying handle
(all in matte black) are complemented by a very attractive automotive
grade gloss white tube with a subtle “diamond dust”
effect. Because the scope has significant “in-focus” to
accommodate a binoviewer, a 2-inch extender with compression ring is
provided for normal visual use with a 2-inch star diagonal. A
thread-on metal dust cap with enough heft to double as a hat brim for
Goldfinger’s Oddjob is included, but no finder scope, star
diagonal, or carrying case is standard equipment.
The telescope arrived
in near-perfect condition, the only defect being a small scratch on
the bottom of the dew shield that I touched up with some auto scratch
remover. Collimation was absolutely dead-on, and the uniform
multicoatings on the objective lens made the dust-free glass just
about disappear. The tube’s flat black interior and trio of
glare baffles proved to be very effective in suppressing extraneous
light.
Mechanically, the 152
displays no glaring faults: the calibrated focuser is smooth and free
of image shift, but its rotation is noisy and stiff. Perhaps the
3-inch Crayford is better. Everything else, however, displays good
workmanship and the use of quality materials.
Optically, this beefy,
truncated refractor turned out to be a lot better than expected.
Yes, the obligatory chromatic aberration was there, but the
six-incher delivered amazingly sharp images with really fine
correction for both spherical aberration and astigmatism.
Ostensibly, Kunming U. O. hand-figures the optics, and the dealer
(Hands on Optics) personally checks each telescope for optical and
mechanical quality. I believe it, for the scope has proven to be
more than an RFT – it’s a very competent lunar,
planetary, and multiple star instrument as well.
To begin observational
testing, I mounted an Orion 40mm finder and bracket on a modified
Vixen-spec “shoe” attached to one of the 2, 5mm threaded
holes on the focuser. A 2-inch William Optics star diagonal was
added to the optical train, and tube assembly mounted on a Universal
Astronomics Unistar alt-azimuth mount mated to a Celestron tubular
steel tripod. This configuration turned out to be ideal: steady
enough for high-power observation but light enough to be carried
around as a single unit – authentic “grab-and-go.”
With an all-up weight of 26 pounds and fully extended length of 39
inches, the scope is also well accommodated by a CG-5-class
equatorial mount.
With an effective focal
length of 900mm and an f/ratio of approximately 5.92, the telescope
is capable of achieving a large range of magnifications with commonly
available eyepieces. Being unwilling to compromise tests of the
152’s optical qualities, I used premium TeleVue Nagler Type 6
oculars of various focal lengths, as their excellence is well
established.
A first view of the
waxing gibbous moon provided a delightful surprise. Just before
sunset, the deep blue sky background all but eliminated the expected
chromatic aberration. At 100x (9mm Nagler), the lunar surface was
rendered in strikingly sharp detail. The delicate Rima Birt near the
Straight Wall and the heavily cratered lunar south were impressively
defined with wonderful sharpness and contrast. This almost APO-like
picture faded as the sky became darker and the purple rim of c.a.
became noticeable. Detail, however, remained very sharp, and the
anticipated “wash” of violet over the vista was less
severe than expected. Stopping the scope to f/9 with a crudely
constructed 4-inch aperture stop removed a good portion the false
color and mimicked what one would expect from a very good long-focus
achromat.
Two days later, I tried
a variety of filters to further enhance the image. Both the Sirius
Optics MV-1 and MV-20 (the former being more effective) helped
considerably in reducing false color at full aperture, but the very
best image was obtained with a generic green “moon filter.”
This “long-shot” configuration almost totally eliminated
the c.a. and produced an image at 180x that was stunning in its
contrast and clarity. If the observer doesn’t mind a green
cheese moon, the scope and filter combination truly delivers.
Jupiter, always a
demanding target for any telescope, showed a bath of purple
surrounding its disc at 129x as one might reasonably expect. However,
a fair amount of detail could be discerned despite mediocre seeing
conditions. The recent dark spot near the GRS was easy to pick up,
as were details in the south polar areas. Here, employing the MV-1
filter helped greatly to improve contrast and eliminate a fair amount
of the false color; and the moon filter, although it dimmed the
image, produced a greenish orb but excellent contrast. I got the
impression, from the distribution of the chromatic aberration, that
the designers of the objective lens neatly balanced the red and
violet overspill so that neither appeared dominant – a good
compromise.
A quick look at the
rather distant (and low in the eastern sky) Mars confirmed that the
152 is capable of decent planetary performance. At 180x the red
planet displayed a cleanly defined gibbous disc, northern polar cap,
and Syrtis Major. For some reason, the false color was not much of a
factor, but the inclusion of a #23A (orange) filter enhanced the
image considerably.
Double star resolution
of the 6-incher was again a pleasant surprise. The tight,
well-defined Airy disc and symmetrical first diffraction ring
generated at medium and high powers allowed clean resolution of the
typical “test” multiple stars. Epsilon Lyrae at 129x
displayed 4 “bullet holes” with very little spillover but
the slightly greenish cast expected from achromats. Alnitak (in
Orion’s belt) was cleanly divided at 129 and 180x despite
rather poor seeing conditions, and the tricky unequal double Delta
Cygni revealed its tiny companion at 129x and above. Rigel showed a
prominent halo of purple around its bright primary, but the miniscule
companion showed clearly at powers of 70 and above. Overall, very
nice performance from a short-focus achromat.
Where the telescope
really shines, however, is in its rendition of deep-sky objects. The
tiny pinpoint stars produced by the instrument made a variety of open
clusters spectacular. With the chromatic aberration ceasing to be a
factor with stars fainter than roughly 4th magnitude, the
Auriga trio of M 37, M36 and M 38 generated the sought-after
“diamonds on black velvet” look that refractor
enthusiasts love. A 19 mm TeleVue Panoptic (47x) worked beautifully
here, but a sensational view was obtained with the new TMB 100 degree
16mm ocular(56x) that darkened the field of view and gave the strong
impression of floating in space near the clusters.
The Pleiades (though
c.a, was a factor) and Andromeda galaxy provided an equally memorable
observing experience as did the Double Cluster in Perseus. The Orion
Nebula was a glorious sight with tendrils of pale green and all six
stars of the Trapezium at higher magnification. When compared to an
excellent 125mm apochromat, the 152mm produced noticeably brighter
images with stellar rendition almost as tight as with the vaunted
apo. Clouds and haze interfered with forcing the scope to its
magnitude limit, but I do suspect that its image brightness and
contrast will enable it to perform very well when pushed to the
extreme.
Overall, my first
impressions of the Astro Telescope 152mm achro refractor are very
favorable. Its optical quality is significantly better than
expected, and it’s small and light enough to be portable. Yes,
the obligatory chromatic aberration is there, but the pinpoint
stellar images and higher power capabilities make it a versatile
instrument that is a great deal of fun to use. At $799, it’s
not exactly inexpensive, yet still a bargain given its quality. I
would like to see a slightly sturdier, smoother-rotating focuser –
but not much else really needs improvement. Of the several
short-focus 6-inch achromats I’ve owned, this one is my
favorite. I’ve sold the others, but this scope appears to be a
keeper.
Now, if they would just
produce this same telescope with ED glass for a bit more money, I’d
be the first in line.
p.s. I have no
affiliation with either Kunming United Optics or Hands on Optics
|
