TMB 100/800 vs Celestron C8 Review
I wrote the original version of
this article for another website almost a decade ago now, but I think it’s
worth updating it and posting it here as the results are interesting and as
valid now as they were then.
The idea of
comparing the iconic telescope from two generations – the 8 inch SCT and the 4
inch APO – has a special appeal for me. I’ll start this review by explaining
why. I want to take a little time over this, so make yourself a coffee, stick
another log on the fire (it’s snowing again here), put your feet up and read on
…
Thirty years
ago the Schmidt-Cassegrain was definitely the
‘fashionable’ telescope to own. I had previously owned a 4.5 inch Newtonian –
my first proper ‘scope, bought from the Christmas window display of a local
camera shop. That little white Tasco, with its
crinkle-black yoke mount, had done a superb job of getting me into astronomy.
With it I’d sketched Jupiter’s cloud belts and Saturn’s rings, photographed the
Moon’s craters and seen the polar caps on Mars. The scope was a wonderful
all-rounder, so with it I’d seen The Ring Nebula, Andromeda, Orion and many
other easy deep-sky objects too. I was out after school every clear night. Then
I got aperture fever and the hobby began to spoil: first a horrible 3 inch
refractor on a spindly GEM; then a superb-but-massive 8 inch Newt’ which had to
be taken to the garden in small pieces and which I almost never found the time
to use. Meanwhile, the Sky and Telescope magazines which dropped into my mailbox
straight from The States each month were filled with short orange tubes on
purposeful, rather space-age looking fork mounts. Like thousands of others at
that time, I decided a C8 was the telescope I’d been looking for.
The trouble
was, the C8 never really filled the hole left by that
4.5 inch starter-scope. It was more portable than the huge Newtonian, but still
had to be carried into an obstacle-filled suburban garden in big expensive
lumps. It still took time to set up and even more time to cool down. Once up
and running, the views were definitely inferior to the massive Newt (that scope
had been superbly engineered by Rob Miller of Astro
Systems and despite its unwieldiness I still regret selling it). I never used
it much and for most of the past 2 decades it’s sat in its trunk – perhaps not
a bad thing for this article, because on Astromart
you’d still describe it as ‘LNIB’.
When my
interest in astronomy re-kindled (it had never completely gone out in the first
place), I decided the most important thing for me, with a busy life living in
the cloudy UK, was portability and ease of use. I wanted a telescope I could
deploy at a moment’s notice, much like that 4.5 inch Newtonian. In the Noughties, the fashionable telescope to own was the
apochromatic refractor (or APO) and that’s what I decided to buy.
After much
angst, I decided on the TMB 100/800 for the following reasons:
- It’s
available, like this decade anyhow.
- The
lens is manufactured in Russia by LZOS, an ex-Zeiss contractor,
acknowledged to be amongst the very best.
- The
scope is short, light and very portable with a draw-tube and a retracting dewshield to help.
- The
focuser is the wonderful Feathertouch and
experience has shown me that any scope is really only as good as the
focuser.
- My
interest in the planets suggested the F8 would be a better choice than the
F6.5 version for me.
It’s a
choice I’ve been extremely pleased with and I’m happy to report I’m out
observing on every clear night, just as I used to thirty years back. So you can
imagine my interest in the idea of comparing the old with the new. Is the APO
really ‘better’ than the SCT or was the problem really just my laziness? Let’s
find out.
First, some history
Skip this
section if you’re familiar with APOs and SCTs.
In the
Sixties and early Seventies there was nowhere near the vast array of amateur astro’ equipment there is now. Small telescopes were
refractors, often of 60mm aperture. Bigger refractors were rare, expensive and
almost always achromats. If you wanted a bigger scope
it was likely to be a Newtonian reflector.
Now there’s
nothing wrong with Newtonians, but a long focus one above six inches is a big
and cumbersome thing that needs a big and cumbersome mount (equatorially, at
least). So imagine what happened when Celestron,
followed by Meade and others, popularised a design of telescope – the Schmidt Cassegrain Telescope (SCT) - which shrank an 8 inch into an
eighteen-inch long tube. People bought them by the truck-load.
Refractors
theoretically outperform reflectors of equivalent size because they have no
central obstruction. The problem historically was that to avoid too much
chromatic aberration (inability to bring all colours to the same focus), which
causes false colour and a less sharp image, refractors had to have very long
focal lengths, making them unwieldy and good only for planets. Then
improvements in optical design, glass technology and manufacturing techniques
made the apochromatic refractor a commercial reality. An ‘APO’ virtually
eliminates chromatic aberration by using exotic glasses or fluorite and perhaps
a triplet instead of a doublet lens as well. APOs typically have the following
advantages:
- Short
f-ratios are possible, meaning a more portable scope and a wider field of
view.
- No
false colour – you get a clean image, like a reflector.
- Crisp,
bright images with the maximum possible contrast.
Disadvantages
include high price and weight in larger apertures.
Design and
Build – The C8
The uncoated corrector and 35% obstruction of the C8
These
telescopes are pretty familiar, but I’ll start with a brief overview of the two
protaganists anyway. First the Celestron.
An SCT is a
hybrid design (technically known as a Catadioptric)
that uses a short focus primary mirror with a convex secondary mounted in a
corrector plate to produce a very compact telescope with a long(ish) focal length and good correction for a variety of
aberrations. It’s potentially an excellent multi-purpose design. The main
problems with SCTs are as follows:
- The
short focus primary mirror and complex-curve corrector are relatively hard
to manufacture to high tolerances. So often, but by no means always, the
optical quality is mediocre.
- The
secondary mirror is often large, producing a large central obstruction (as
much as 40%). This means lower contrast and more sensitivity to bad
seeing.
- The
large mass of air sandwiched between two large lumps of glass means they
typically take ages to cool down.
My version
of the C8, bought in the late Seventies, has none of the electronics of the
modern equivalent and it’s all made (in the USA) of metal, but the design is
basically identical. The ‘scope is an eight inch F10 squeezed into a short, (orange!)
metal tube.
Focusing is
achieved by the much-criticised method of moving the short-focus primary mirror
backwards and forwards. The stubby tube sits in a fork mount; there is manual
fine-adjustment (excuse my laughter) on both axes and a motor for the RA. The
fork sits on a metal tripod via a latitude wedge. Oh … and there’s a rather
nice, fully coated Japanese-made 6x30 finder. Perhaps the only other thing to
say is that on Seventies C8s the corrector appears to be uncoated.
After twenty
years of little use, the mirrors are still bright and dust-free. The only
maintenance required before the tests was a very careful clean of the corrector
plate.
Design and
Build – TMB 100/800
The TMB-designed, LZOS-made triplet.
Lens
Let’s get some acronyms straight at the start. TMB are the initials for
Tom M Back – the designer of the lens. The lens was actually made, however, by
LZOS in Russia. LZOS stands for “Lytkarino Zavod
Optychisovo Sticklo”, which
roughly translates to “Lytkarino Optical Glass Works”.
LZOS was set up in Soviet times to make high-end optics for military and
research purposes and has produced some of the world’s larger professional
telescopes. They used to make the lenses for Zeiss. LZOS manufactures (as you
might guess) its own glass – a key advantage when it comes to making APOs.
The TMB APO
is a 100mm F8 triplet made with a central element, not of FPL-53 like most, but
of OK4 – LZOS’ own brand of ED glass. The lens is mounted in a very finely made
cell and has excellent coatings. These lenses are one of the very few that come
with a test certificate that details the quality of the lens as tested on a Zygo interferometer and so of course the lens quality is
always good and usually superb.
When I
originally wrote this article, the TMB was a newcomer to the market, but the
100/800 is now widely recognised as one of the finest refractor lenses
available, with near-perfect design and fabrication.
Tube
The LZOS
lens has been integrated into the OTA by APM telescopes in Germany. It’s not
the more usual heavy CNC tube, but a short, light, Vixen-derived unit with a
draw-tube and retracting dew-shield. So it folds down to a much more compact
telescope than the 800mm F.L. might suggest, but you have to pull out the
draw-tube to achieve coarse focus. Unfortunately in my early version, that
drawtube was poorly fitting, but modern ones are much improved.
Focuser
At the far
end from the superb Russian lens is a Starlight Instruments 2” Feathertouch 2-speed Crayford focuser, mounted on that
draw-tube. The Feathertouch is widely regarded as the
best focuser available – it is beautifully engineered from hard stainless and
is supremely smooth and accurate. What’s more, it is equipped with a microfocuser, operated by the inner gold knob, which
provides extremely fine adjustment. However, the FT used in this TMB has quite
a short travel, so you do need to use the draw-tube for coarse focusing. A
design fault in this version is insufficient travel overall to bring some
eyepieces to focus.
Mounting
The TMB came
with Vixen tube rings and dovetail and for these tests I mounted it on a Vixen
GP German Equatorial mount. Incidentally, I chose the GP rather than the GP-DX
because it’s much lighter (and it really is – the mount and tripod can be
picked up with one hand). The TMB weighs in at about 4.5 Kg in this tube (CNC
version are heavier), so the GP easily holds it (as would any decent small
GEM).
In Use
Other Equipment
The
accessories I used for the comparisons are as follows:
- TV Plossls – 32mm, 20mm, 15mm.
- ‘Circle
T’ Orthoscopic 9 mm.
- Stellarvue 50mm
2-inch widefield.
- TV PowerMate 2.5x
- TV 1.25
Everbrite diagonal; Stellarvue 2 inch diagonal.
It happens that the focal length
of the C8 (2000 mm) is exactly 2.5 times that of the APO (800 mm), so for the
tests I was able to compare objects with exactly the same eyepiece giving
exactly the same magnification by using the PowerMate
in the APO.
Setting The ‘Scopes Up
The Celestron’s 8 inch aperture grabs more light, even without
coatings.
The Celestron isn’t really so difficult to set-up, easier than
I recalled. It is very compact for an 8 inch telescope. The tripod is light
enough, but awkward to move because folding it involves turning each leg
through 180 degrees. Consequently it stays unfolded in the garage and moving it
involves many near misses with car and bike paintwork. The frustration has
started. The tube/fork assembly is heavier than any component of the APO and it
attaches with three bolts which are fiddly to get in, despite the original hex-bolts
having been replaced with knurled knobs years ago (otherwise it would be a real
pain!). Surprisingly, once set-up, the Celestron as a
complete unit is lighter and easier to move around the garden than the TMB/GP –
one very obvious plus point for the SCT.
The APO is
that much easier to get going. The tripod legs fold inwards in a trice, just
like a big photo-tripod. The whole lot goes under my arm, legs ready-extended
with the counterweight attached. Slot in the dovetail plate with scope
attached, tighten a single screw and we’re done. A quick squint down the
polar-finder built-in to the GP mount and I’m aligned well enough for visual
use.
Cool Down
The biggest
problem with the Celestron is that it takes ages to
cool down. I have to leave it pointed vertically down with the focuser tube
open to the air for at least a couple of hours before I can use it. Any
observing session which lasts less than three hours is a non-starter with the Celestron, which is perhaps why it has been used so little.
Triplets take
longer to cool than doublet refractors, but after half an hour or so I can
start observing with the TMB. Meanwhile I can still feel warm air pouring out
of the Celestron’s focuser tube and the view is still
a complete mush of tube currents …
Mars
It’s just
two weeks past opposition, so the first object of study has to be The Red
Planet. I have it in the APO’s field in a few seconds. I don’t bother mounting
the finder for bright objects because the field of view is plenty big enough
with the 32mm TV Plossl - one advantage of the APO’s
(relatively) short focal length. As an aside, I notice how smoothly the GP
slews and tracks. The tripod maybe light-as-a-feather, but it damps vibration well and is extremely stable. It seems very
tolerant of different loads at the focuser too. I notice these things with
pleasure. One real annoyance, though, is the lack of outwards focus travel
available – with a 1.25 diagonal, long focus eyepieces like the 32mm Plossl take loads of fiddling to get to focus at all.
It’s
frustration time back at the Celestron as well. Mars
is centred in the almost-invisible cross-hairs of the finder, but it’s nowhere
to be seen, even with the 32mm Plossl. I spent ages
realigning the finder just recently, but somehow I’ve knocked it out of line
again. Five minutes of sweeping and I’ve finally got it. But as I’m sweeping I
almost push the whole scope over (my garden is on a slope and the tripod
doesn’t adjust). WHOOOAAAA!!!!
Back with
the TMB again, Mars focuses with a pleasing snap. How I love that Feathertouch! I step up to one of my favourite eyepiece
combinations - the 15mm plossl and 2.5x PowerMate give 133 times with a nice broad FOV. The seeing
is poor, as it often is here, but Mars still looks good. The image scale is
small, but Mars is a sharp disc with lovely colouring and not a hint of
chromatic aberration. The south polar cap is clear and I can see dark markings
on the orange disk.
The Celestron gives, on the face of it, a much brighter view.
The 15mm Plossl gives me the same magnification
without the PowerMate in the way. Trouble is the
image is all blurry. Actually, the much-derided Celestron
focuser is fine – smooth with little image shift - but I can’t seem to get a
good focus. Mars boils. It can’t be cool-down - the scope’s been out over three
hours by now. I assume the problem is the adverse effect of large aperture and
central obstruction in bad seeing. To cap it all, I notice how unpleasantly
jerky the Celestron fork feels after the GP mount.
I repeat the
experiment a few nights later when the C8’s had five full hours to cool. The
image is a little better, at least there’s a trace detail on the disk now, but
the TMB still wins easily on Mars.
One final go
with Mars well past its best almost six weeks after opposition. The seeing is
bad again, but occasionally it steadies for a moment to give me the best views
yet through the APO. Syrtis Major is clearly visible
and unmistakable. The South polar ice cap has shrunk to a dot in the Martian
summer, but it’s clear too. When the seeing steadies for a moment, I get a
mini-Hubble type view with lovely delicate shading on the surface and the dark
area around the north pole clearly visible.
The C8 still
delivers a big, bright orange ball that just hints at the details the APO shows
clearly.
Round one to the APO in these conditions.
The Moon
A few nights
later and there’s a gibbous moon. The sky is crystal clear, but as so often
here in Northern England, the seeing is mediocre at best. The telescopes are
side-by-side and I swap back and forth with the 15mm Plossl
and the 9mm ortho, using the PowerMate
to produce the same magnification in both scopes.
The moon is
thirteen days old and I track carefully down the terminator, concentrating on
the Gassendi region, Vallis
Schroteri, and Mons Rumker.
I’m expecting the aperture of the C8 to produce a clear win here, but it
doesn’t happen. The first thing I notice is that whilst the APO can handle 220x
with the 9mm plus Powermate, even in the poor seeing,
the C8 simply can’t – it’s a mush. So I settle for the 15mm Plossl
again, giving about 133x.
The next
thing I notice is that the image is so much crisper and more contrasty in the APO that it seems the magnification must
be lower, but it’s not. Despite the seeing, the APO delivers superbly. Shadows
are dense, textures are 3D and there is so much detail. There is no false
colour, none – a remarkable achievement by Mr Back and the Russian opticians at
LZOS.
In the C8
the image is much brighter, but there’s less detail. I keep swapping back and
forth over a period of several hours to check, but it’s true. A white smudge in
the C8 resolves down to a tiny crater in the APO. I see details in Gassendi – rilles, slumping, craterlets, that are just a smear in the C8. From the domes
of Mons Rumker to the hills and embayments
around Gassendi and the edges of lava flows on the
mare near Aristarchus, the APO shows more detail.
Surprisingly,
very surprisingly, the APO wins again.
The Double Cluster
The moon has
set, so I can try some star-fields and then some faint fuzzies.
Through the
APO, the double cluster is a beautiful sight. The stars are so crisp, the
colours so perfect. The core stars are so pin-sharp there’s almost a 3D effect again
and the wide field makes it a more pleasing view than with the Celestron. There’s no escaping it, the C8 goes deeper,
shows more, but the image is duller and less pleasing than with the APO. What’s
worse, the whole double cluster won’t fit in the field, even with the 50mm
eyepiece.
An artist,
at least, would prefer the APO; an astronomer would appreciate seeing fainter
stars. Call this a draw.
The Ring Nebula
To get at
the constellation of Lyra, home of M57, I have to lug
both telescopes over to the tarmac near my garage to avoid the trees which are
an awkward reality in my garden. I’m reminded that the Celestron
is in fact easier to move around.
It’s an
overall win for the C8, because the Nebula’s smoky ring is big, bright and
easily visible with direct vision; there’s even a hint of structure. The APO,
by comparison, just can’t keep up. With the TMB I really need averted vision to
see the fuzzy patch as a ring and the whole image is just dimmer and less convincing.
Later views
produce the same result with other small Messier objects – The Dumbbell, Crab
and Orion Nebula. Aperture wins here.
My wife
points out that what we need is ‘A telescope like that one [the APO] but with a
lens as big as that one [the C8]’. Who but my accountant could argue with
that?!
The Andromeda Galaxy
Given that
it’s another ‘faintish fuzzy’ you might expect the C8 should win here, but it doesn’t
because Andromeda is so large that the whole galaxy won’t fit in the C8’s field
of view. True, the C8 delivers a slightly brighter image at 40x
with the 50mm Plossl, but for some reason it’s just a
fuzz. By contrast (perhaps literally), the APO delivers just a hint of the dark
lanes in this spiral galaxy.
Overall,
call it a draw. One thing I do notice, though, is how much easier the C8 is to
use on objects near the zenith. The TMB is a typical refractor and the eyepiece
ends up in some very awkward positions.
The Pleiades
My wife
simply said ‘Wow!’ the first time she looked through the TMB at the Seven
Sisters. The whole cluster easily fits into the field of the 32mm Plossl (an eyepiece which works superbly with the TMB) and
it’s one of the most beautiful sights available with the scope. Jewels on
velvet etc – you’ve heard it all before. But, really,
the pin-point stars, the perfect colours …
The C8 looks
dull by comparison – the big obstruction reduces stellar intensity for visual
use - and once again the lack of field-width lets it down.
Another easy win for the APO on star-fields.
Double Stars
Both TMB and
C8 manage Polaris and the Double-Double. Rigel is a
bit easier in the APO, as the faint companion is easier to pull out in poor
seeing, whilst the bigger diffraction rings caused by the C8’s obstruction
throw off light and make the faint companion harder to see.
Conclusions
To be
honest, I had been kind of hoping to break down the APO myth a little here. I’d
really hoped and expected a Gentleman’s draw, with the APO producing the finer
images and wider fields, but the SCT trouncing it in reach and detail. It just
didn’t work out like that. Under the poor seeing conditions described, over a
number of nights of careful comparison, repeatedly swapping the same eyepiece
between the two, the APO won decisively. Particularly surprising to me was the
Moon, where I had anticipated an easy win for the C8.
Crucially, even hampered by the
poorer diagonal (I ended up using the TV Everbrite exclusively on the C8 to
give it the best chance) and all the extra glass in the PowerMate,
the APO still produced better images, with more detail on most of the objects
viewed.
The C8 only
beat the APO in situations where aperture counted above all else.
The APO is
undoubtedly a more pleasing telescope to use than the Celestron.
It’s quick to set up and cool down, giving gorgeous, crisp images, even in poor
conditions. It will give of its best over a wide range of objects, on days when
you only have an hour to spare. It’s a superb all-purpose instrument, capable
of a good showing on everything from star fields through planets. But none of
this explains why it beat an 8 inch reflector – in theory it simply shouldn’t
have.
At first I
thought maybe the mirror coatings on the C8 were the problem, but no, they are
absolutely pin-sharp, like new. Besides, image brightness isn’t the problem.
Then I checked collimation – spot on. After 3-5 hours of cooling, the Celestron settled and more time made no difference to the
image, so I don’t think the problem is cool-down. So we are left with just two
possibilities to explain the surprising result of this comparison:
- The
superb, uncompromising design of the TMB and near-perfect optical figure
made up for the aperture difference and then some.
- Under
medium-to-poor seeing, a large-aperture, large central obstruction scope
really does perform so badly that a much smaller refractor can easily
outperform it.
In reality I
think it may be a combination of these factors. I suspect that on a Nevada
mountaintop the results would have been quite different. Also, it may well be
that my C8 is a poor example.
So if you
own an SCT and are happy with it, don’t be offended and don’t give too much
significance to my comments here. This test was specific to my location and my
telescopes. What is more, a C8 OTA is a quarter the price of a TMB 100/800 and still
a nice, very portable telescope.
Both
telescopes are recommended, but if you live in an area with frequent poor
seeing, you might find a smaller APO makes a better compromise for you than a
larger SCT.
Addendum 2013
At the time I originally posted this review, a
number of people contacted me to say they thought it was the age of the C8,
that a newer one would have acquitted itself better. I thought so too. Then
later I had the chance to use a friend’s brand new Meade 8” SCT – same result.
I eventually sold the TMB to upgrade, but I now bitterly regret letting it go, as the 100/800 is now very expensive. The 100/800 remains the most perfectly CA-free refractor I have used and one of the sharpest.