Telescope-Service 102mm f/6.5 refractor
Introduction
I've always been
a fan of refractor scopes for over a decade. When properly made, these
instruments serve up ultra-sharp optics and contrast rich views of the Moon,
the planets and deep sky objects. In the past, I have owned several achromatic
and apochromatic refractors in the 60-125mm range. Much of the time, my
instrument of choice for observing the Moon, bright planets and double stars
was a good quality 4-inch f/10 achromat. But for wide field sweeps of the Milky
Way and bright deep sky objects, I have resorted to using short-focal length
apochromats up to 80mm in aperture. Ultimately though, I missed the extra light
grasp afforded by larger achromats and apochromats; and that got me wondering.
Was it possible to obtain a good quality, yet relatively inexpensive refractor
that could do both well? There are of course superb telescopes manufactured
by a number of companies that already deliver on all fronts, but they do
so at a
very steep cost premium. Undeterred, I stumbled across a German-based company
called Teleskop-Service (TS) which offered a 102mm air-spaced doublet achromat
with a focal length of 660mm (f/6.5); a compromise between the ubiquitous short
tube f/5 and the longer and somewhat more awkward f/10 instruments. The company
advertised these scopes as being solid performers on both solar system objects
and the deep sky. Luckily, one such instrument came up on the second-hand market
and over the last few weeks I've spent countless hours evaluating its optical
and mechanical performance in the field.
Fit and finish
When the scope
finally arrived, I was delighted to see how sturdy it was. The white, aluminium
optical tube assembly (OTA) weighs in at 5.0 kg - that's significantly heavier
than I had imagined. If there's one word to describe this instrument it's got
to be 'overbuilt'. Even though the objective lens was only 4-inches, it came
with a massive dew shield nearly six inches in diameter. The objective lens
had nice smooth multi-coatings which give a deep green daylight tint.
Three baffles
help to dampen any stray light and the interior was painted an even matte
black. Turning my attention to the 'business end' of the scope, I was delighted
to find a very high quality rack-and-pinion focuser, which moved very smoothly
with a fair amount of tension. It could also accommodate a two inch diagonal
and had provision for the attachment of a DSLR. The OTA is also graced by a high
quality 8 x 50mm finder, which was mounted securely on the main scope. Placing
the OTA (which came with tube rings and a dovetail plate) on my LXD75 mount, I
was immediately thrilled by how good-looking the whole set up was.
Optics
So, how well does
it perform- Overall; very well indeed! During daylight hours, I set the scope
up to look at the leaves of some distant trees. Inserting a good quality 26mm
plossl eyepiece, the TS102 snapped into focus serving up very bright, crisp
views with little distortion - even at the edge of the field. The slower f
ratio (f/6.5) compared to the ultra-fast F/5 short tubes was a definite plus
here. Then I began cranking up the power. With a 9mm eyepiece yielding 74x, the
image remained very sharp, with only the merest hint of blue fringing caused by
chromatic aberration. It's only when I inserted eyepieces yielding powers over
100x that the chromatic aberration became prominent enough to notice easily,
but in no way did it otherwise compromise the quality of the image. So far so
good. But how would it perform under the stars- As it happens the sky was very
clear and dark the same night as the scope arrived at my home in rural central
Scotland, and I was able top put it through its paces.
First inserting a 26mm plossl eyepiece
yielding 25x, I examined a whole host of objects including star clusters,
galaxies and nebulae. First stop was the Perseus double cluster. The view was
superb, with mounds of stars remaining sharp pinpoints nearly all the way to
the edge of the field. The Andromeda galaxy (M31) was simply breathtaking, with
its spiral arms filling the field of view either side of its bright, condensed
nucleus. This is where the telescope really excelled.
Next up; a star
test. Looking at Vega in focus using my 4mm orthoscopic eyepiece yielding 165
x, the TS102 showed a nice tight stellar image in focus. It was surrounded by a
faint 'purplish' halo which was quite unobtrusive. Racking the image inside and
outside focus revealed a nice concentric airy disc in moments of good seeing.
There were no obvious signs of spherical aberration. Since there was no bright
planets or the Moon in the sky I resorted to splitting some closely separated
doubles. Both Epsilon Bootis and the Epsilon Lyrae (the famous double double)
were easily resolved with this scope with magnifications of 200x and 73x,
respectively.
A few weeks
later I finally had a chance to look at the Moon at low and high
magnifications. Inserting my 26mm plossl and aiming the scope at a first
quarter Moon served up a wonderful amount of detail.
Yes, the lunar edge had a
blue fringe but one could easily have forgotten about it given the amount of
detail that was on offer. A 1.25 inch Baader semi-apo filter inserted ahead of
the diagonal effectively removed the glare and much of the blue fringing
without significantly shifting the colour of the lunar surface. Cranking the
magnification up to 205x yielded very good images but no finer details could be
seen beyond about 165x.
Those interested
in doing a bit of photography will find lots about this scope that's good. For
one thing, its solid, over-built construction easily handled the additional
weight of a DSLR camera body and off-axis guider. Short exposures of 30 seconds
or so confirmed what I noticed visually: stars remained pinpoints of light
almost to the edge of the field. You can improve the situation still further by
attaching a focal reducer to reduce the focal ratio and flatten the field some more.
Bright stars show faint purple halos but these can be effectively cut down by
using a number of filters. The Baader semi-apo filter I tested was very good in
this respect.
Conclusions
All in all, I
was very impressed with the optical and mechanical quality of the TS 102mm
short tube achromat. The company sells these scopes as optical tube assemblies
complete with a 50mm finder and tube rings. Alternatively, you can choose to
purchase the instrument with a mount at additional cost. If you're after
extreme portability, you might find this scope too heavy to handle. On the other
hand, if you're after a budget-priced telescope with decent light grasp that
can do a good (or even very good) job on most celestial targets, then this
could well be the scope for you.
Postsript: The author has no affiliation, commercial or otherwise, with Teleskop-service.
