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8" Celestron SCT vs TMB 4" f/8 APO


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The idea of comparing the iconic telescope from two generations – the 8 inch SCT and the 4 inch APO – has a special and poignant appeal for me. I’ll start this review by explaining why. I want to take a little time over this, so print it out, make yourself a coffee, stick another log on the fire (the leaves outside my window are turning and falling), put your feet up and read on …

A quarter century ago, when I bought my Celestron 8, the Schmidt-Cassegrain was definitely the ‘fashionable’ telescope to own. I had started my interest in astronomy as a boy with a long-focus 4.5 inch Newtonian. That little white Tasco starter-scope with it’s 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 and with it I’d also seen The Ring Nebula, Andromeda, Orion and many other easy deep-sky objects. 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 thousand of others at that time, I decided a C8 was the telescope answer 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 and despite it’s unwieldiness I still regret selling it). I never used it much and for most of the past 2 decades it’s sat in it’s trunk – perhaps not a bad thing for this article, because on Astromart you’d still describe its as ‘LNIB’.

When my interest in astronomy re-kindled last year (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. This decade the fashionable telescope to own, for good reason, is the apochromatic refractor (or APO) and that’s what I decided to buy. Having read just about every telescope review on the web, I finally decided on a TMB 100/8 in the lightweight tube, mounted on a Vixen GP (the natural match to its Vixen rings and mounting plate). It’s been 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. So imagine what happened when Celestron, followed by Meade and others, popularised a design of telescope which shrank an 8 inch F10 into an eighteen-inch long tube – people bought them by the truck-load. The scopes in question are Schmidt Cassegrain Telescopes (or SCTs for short). An SCT 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. The two main problems with SCTs are as follows:

  1. The short focus primary mirror and complex-curve corrector are relatively hard to manufacture to high tolerances.
  2. 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.

Refractors theoretically outperform reflectors 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:

  1. Short f-ratios are possible, meaning a more portable scope and a wider field of view.
  2. No false colour.
  3. Crisp images with the maximum possible contrast.

Disadvantages include high price and weight in larger apertures.


The uncoated corrector and 35% obstruction of the C8

In The Left Corner…

These telescopes are pretty familiar, but I’ll start with a brief overview of the two protaganists anyway. First the Celestron.

My version of the C8, bought in the late Seventies, has none of the electronics of the modern equivalent and it’s all made of metal, but the design is basically identical. The ‘scope is an eight inch F10 squeezed into a short, unbaffled (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. 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.


The TMB triplet.

And in the right corner …

The TMB APO is a 100mm F8 exotic-glass triplet apochromat mounted, not in the more usual heavy CNC tube, but in a short, light, Vixen-derived unit with a draw-tube and retracting dew-shield to help with portability. At the far end from the superb Russian lens is a Starlight Instruments 2-speed Crayford focuser. The scope came with Vixen tube rings and dovetail plate and a very nice Antares 7x50 finder which I’ve never fitted (I’ll explain why later).

The TMB is mounted on a Vixen GP German Equatorial mount, to which the dovetail plate attaches with just a single screw. 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).

After much angst, I decided on the TMB 100/F8, rather than any other 4 inch APO for the following reasons:


And at the other end … the Feathertouch focuser

Other Equipment

The accessories I used for the shoot-out are as follows:

The PowerMate was a piece of luck in more ways than one. I bought it as part of a deal, intending to dispose of it – I’d always disliked barlows. Now it’s one of my favourite and most-used bits of kit. Now it just 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.


The TMB is the bigger of the two when set-up.

Setting The ‘Scopes Up

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 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 us 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.

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 already. A quick squint down the polar-finder built-in to the GP mount and I’m aligned well enough for visual use; half an hour later I can start observing. Meanwhile I can still feel warm air pouring out of the Celestron’s focuser tube...

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 back focus 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-invisble 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, maybe 3-4, but Mars is still really nice. 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 time; the scope’s been out over three hours by now. I assume the problem is the adverse affect 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.

My wife agrees – 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, 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 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 3-d effect 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.

The Ring Nebula

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.

It’s an overall win for the C8 here, 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. Aperture wins.

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 bank manager could argue with that?!

One deep sky, aperture wins.

The Andromeda Galaxy

Given that it’s another ‘faintish fuzzy’ the C8 should win here, but, strangely, it doesn’t. True, the C8 delivers a slightly brighter image at 40x with the 50mm e.p., but for some reason it’s just a fuzz. By contrast, the APO delivers just a hint of the galactic structure. Perhaps it’s the wider field of view in the APO, perhaps the contrast effects of the central obstruction with the Celestron, I don’t know.

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 e.p. which works superbly with the TMB) and it’s one of the most beautiful sights available with the scope. Jewels in velvet etc – you’ve heard it all before. But, really, the pin-point stars, the perfect colours… The C8 looks dull by comparison and once again the lack of field-width lets it down.

Another easy win for the APO on star-fields.

Double Stars

I’m no expert here, but I’ll give it a shot. Polaris resolves easily in both scopes, no surprise there. The double-double is much harder. There’s a lot of moonlight on the night I try it and the image in the APO at 220x is just too dim. The C8 is much brighter and the stars just resolve into dumbells. Actually, the stellar images are much smaller in the APO, so maybe under better conditions it would give a better showing. Jury out on this one and sadly I’ve no more time to re-run the comparison.

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. 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 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 comparo’:

  1. The superb, uncompromising design of the TMB and near-perfect optical figure made up for the aperture difference and then some.
  2. 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 third the price of a TMB 100 and still a nice, very portable telescope. What this does suggest, though, is that 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.

So where does this leave me ?

I enjoyed doing this test, but it has left me puzzled and with a real dilemma. I bought the APO expecting ease-of-use and convenience. I did not expect it to replace the C8, my intention being to keep the larger instrument for nights when I had time to use it. On the basis of these tests, though, there seems little point in keeping my C8, other than for nostalgic reasons. In most ways the APO outperforms it and for deep sky I’d be better off with a big, cheap dob’.

Then again I had, one day, been hoping to replace the C8 with a top-quality larger Maksutov for planetary work, but I’m now seriously doubting if such a scope would ever perform under my skies. Instead, I find myself reading APO reviews again. I started off my astronomy ‘career’ as a ‘reflector man’ all those years ago, but the superb TMB has converted me.


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