APM TMB 175 Review

When Patrick Moore started his astronomy career at a prestigious private observatory in the late 1930s, the main instrument was a six inch refractor. As late as the Sixties and Seventies, even a four inch refractor, from the likes of Unitron , was a large instrument few could afford.  The reason? Fraunhofer achromatic refractors over about 3 inches need focal lengths of F15 and above in order to control chromatic aberration, so even modest refractors were long and heavy and needed big mounts.

Of course, the “APO revolution” changed all that by allowing much shorter focal lengths and so much smaller and more easily mounted refractors. However, even today, refractors larger than six inches are rare because big APO lenses require expensive glass and are hard to fabricate due to the exacting tolerances required.  According to APO-guru Roland Christen, the high cost and variable quality of large ED glass blanks from the big manufacturers mean that only optical fabricators able to make their own glass, or those able to work with expensive crystalline fluorite, can make big APOs at consumer prices  today (various specialist optical contractors could make them at NASA prices). The main fabricator making APOs from their own glass is the Russian company LZOS.

The acronym 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; it still does. As a sideline, LZOS makes refractor lenses from the ground up, including melting the glass and machining the cell (it used to make the lenses for Zeiss telescopes).

Design and build

Lens

Over the years, LZOS has made lenses for a few different companies which integrate them into OTAs, but the most prominent has for some years been APM telescopes in Germany, with the lens specification by the (sadly now deceased) American designer Tom Back (TMB).  APM are able to order and integrate LZOS lenses up to about 21 inches, but the only large (over 6”) APOs made in anything like production quantities are the 7” models, the most common of these being this one - the TMB 175.

TMB 175 Lens. Note the baffled dewshield.

The TMB/LZOS lens is a 7 inch F8 triplet (1400mm F.L.) which uses LZOS’s own OK4 high-fluoride glass as the centre element. OK4 is a high-fluoride glass (it’s not fluorite!) very similar to FPL-53. As I explained, getting hold of quality glass in large blanks is a big problem for APO makers in general, but not for LZOS who make their own.

Quality is pretty much a given with LZOS lenses. Bench tests suggest they come at or close to the top of the league in controlling all aberrations. They are one of the few companies that provide a Zygo interferometer report. Mine comes in at a PV of slightly better than 1/6th wave with a Strehl of 0.972 – impressive for a big lens. Independent tests I have seen tend to yield results similar to, or often slightly better than, the test report.

The lens is mounted in a very sophisticated cell and is beautifully coated. Today the lens from APM alone would cost about 11,000 Euros and it certainly looks more like a research-quality item than your average consumer telescope lens.

In terms of weight, that lens cell dominates the TMB175, making the balance point awkwardly near the front - something I only eventually ‘solved’ by cobbling together a 10 Kg counterweight for the focuser end.

Tube

The first impression you get of the TMB 175 is the enormous aluminium case (“alucoffer” in German and it is indeed coffin sized!). In fact the OTA is much shorter than the case, looking quite compact, but this impression is deceptive. APM describe the 175 as their largest portable model and they have achieved this with some clever engineering.  The long dewshield slides right back over the tube and an extension is required to get any eyepiece to focus, so in operation the telescope is typically much longer than when packed away. Weight for the bare OTA is around 20Kg.

The TMB 175 at its minimum length, with dew-shield fully retracted and extension-tube fitted down-stream of the diagonal.

So what about fit and finish? I once owned an early TMB 100/8 which had a superb lens, but was compromised in terms of fit and finish, with various unresolved problems that made it more like a home build than a production OTA.

The new OTAs are CNC-made in Germany and I am happy to report that the finish is superb – much, much better than my earlier TMB. Although the lens, tube and focuser are all made by different companies, integration is flawless. The paint is deep and perfect and has not been marred by years of sliding the dewshield. All the fasteners are stainless, so that nothing has tarnished after years in my observatory. My only criticism is the soft paint on the rings, but it’s a very minor gripe.

The dew-shield is superbly designed, works smoothly and locks solidly in place. It is equipped with a front baffle that helps with stray light but also with dew – I have never seen dew on the lens which is thus completely free from dewspots, even after years of use.

The tube is very carefully baffled internally. A set of knife-edge baffles have been inserted on framing rods to keep them absolutely true and the space in between appears to have been flocked with a specialist coating, not just painted. Again, this is professional-quality attention to finish.

Focuser

The focuser is a massive 3.5 inch stainless-steel rack and pinion model from Starlight Instruments with built-in fine-focus and draw-tube scale. It has an enormous travel – 4.5 inches. This is quite simply the best focuser I have ever used. It is completely solid and smooth, is effectively free of image shift (it only shows the merest trace when changing focus direction at over 400x) and never racks out on its own, even under heavy loads. It has no locking mechanism because it doesn’t need one. It is just as smooth and perfect even when fully racked out carrying a big eyepiece, or a camera on extension tubes. I sold my electric focuser because I just never needed it – the big FT is that good.

The focuser attaches to the tube via a rotating mechanism which works via a capstan wheel. This is another marvellous feature: ease off the tension on the capstan wheel and the whole focuser rotates, but extremely smoothly and with minimal image shift. Again, weight is no issue when rotating. This is a great feature for changing camera-angle when imaging, but just as useful when getting the perfect eyepiece position for comfort.

Mounting

The big TMB may look quite compact in its case, but the first shock comes when you try to lift it out. Not only is it heavy (about 20kg), but all the weight is at the front in that lens cell, despite the massive focuser. Only the bravest would want to lift this telescope into its cradles on their own (especially when dew-covered, in the dark, with numb fingers): it may just be portable in theory, but it really needs permanent installation. Incidentally, if you do want to take it star parties, you’ll need a large estate car (or a hearse:  that alucoffer is big!).

So you need a big mount for the TMB 175. No, an EQ6 Pro will not do the job. The previous owner had used a Losmandy G11, but by the state of that mount, I’m guessing it had struggled. An AP900 would take it; my AP1200 is perfect and has the spare capacity for lots of accessories as well.

The CNC rings have the standard Astro-Physics hole pattern, which is ideal for me. I have found that fixing the rings to an AP plate via centring screws is the best way to achieve accurate alignment and orthogonality for GOTO pointing accuracy; it also gives better damping than using a dovetail. On the photo above, you will see that I use a massive AP Losmandy D plate inverted on top of the rings. This makes the assembly more orthogonal and reduces flexing whilst slewing, but also makes a rail for mounting cameras and smaller scopes.

An FS-60 mounted via a Losmandy D clamp atop the rings – ready for imaging.

In Use

Cool-down

If kept permanently outside in an observatory, cool-down is not an issue. On nights where the temperature drops rapidly, it can take up to an hour for tube currents to completely subside, but it is usually usable immediately.

Star Test

Once the lens has reached thermal equilibrium, the star test mirrors the test report – near perfect, with snappy diffraction rings that look identical either side of focus. The star test shows no evidence of chromatic aberration, either in or out of focus.

In use the TMB 175 is close to my definition of the Perfect Telescope. The big TMB really is just like a premium 4” APO, only bigger! Its strength is in the ability to everything well.

The Moon

In good seeing the Moon shows incredible detail in stark shades of black and grey: a mass of fine rilles and craterlets. Ethos – 13mm, 10mm, 8mm and 6mm - all work particularly well. The first time I saw the whole Moon with the Ethos 13mm, just before first quarter, I nearly shouted aloud. Even my wife and daughter were genuinely impressed and my wife lingered at the eyepiece for ten minutes or more and had to be persuaded away from it in the end, an unheard of situation. Here was that “Lunar Module Porthole” view I had always been hoping for with absolutely staggering, crisp detail from limb to limb.

Venus

The workmanship of the LZOS craftsmen really shows when you turn to planets. Venus at its brightest is a difficult subject and otherwise decent telescopes can show a mess of chromatic aberration and flare, but with the TMB 175 you get a perfect dazzling white crescent with no false colour and no flare or ghosting. A hint of cloud patterns are visible as well.

Mars

Mars is another difficult subject which many telescopes show as just a fuzzy orange ball, but with average seeing and a 4mm eyepiece giving 350x, the TMB can reveal surprising Martian detail: The polar caps with surrounding dark areas, Syrtis Major, Hellas covered in bright cloud, more bright clouds on the limb. In exceptional seeing, a 3mm eyepiece giving 467x gives increased image scale, but with no loss of contrast or sharpness.

On one magical, misty January night I went out expecting not much, but was met with stars that were absolutely steady. The seeing was, for perhaps half an hour, almost perfectly still before fog rolled in off the Bay and spoiled it all. At 467x, Mars was full of detail and the TMB175 felt like it would take even more magnification. Solis Lacus stood out like a big eye, with fingers of albedo detail reaching up from the south pole to the east and Mare Acidalium in the north split up into fine shadings that were impossible to sketch. In good seeing, exploring the Red Planet becomes a real and exciting prospect with the TMB 175.

Jupiter

Jupiter’s low contrast make it another challenging object, but the big TMB can show a wealth of detail on nights of good seeing (but quite a bit on poorer nights too, unlike a big reflector in my experience). A mass of festoons, white spots in the bands and hoods, fine partial belts and dark storms are all visible on a good night and shadow transits are beautifully defined. The GRS displays the mass of vortices that follow it.

The Galilean moons show up as hard planetary discs at high magnification, each different in size and colour. I have spent many happy hours at 400x plus trying to spot surface detail on Ganymede; think I might have … just once.

Saturn

Again, Saturn shows a beautiful prospect in the big TMB. The rings almost seem to stand out in 3D, the Cassini division is big and dark, the ring shadow precisely defined. On a good night, you can see subtle shading and bands in the polar hood.

Deep Sky

You might think of this as a purely planetary scope, but that isn’t so. The focal length of 1400mm is still short enough to manage wide views of star-fields and large nebulae. Put in a wide-field 2 inch eyepiece and the views of DSOs are simply stunning. The Orion nebula area shows a mass of detail and a hint of emission colours, reds and blues, with embedded stars like diamonds-on-velvet, the way it is in smaller APOs and never is in reflectors. Orion’s sword through the TMB 175 with a Pentax XW 40 is one of the most beautiful views I’ve ever had through any telescope, period. The whole of the Pleiades fits in the field: dazzling blue-white stars embedded in nebulosity. Even fainter nebulae, which are just a fuzz in smaller APOs, show shape and detail with the TMB’s bigger aperture and high contrast: The Dumbell, The Crab, Bodes Nebula, Andromeda all show detail and structure not apparent in smaller scopes. At 230x the Eskimo Nebula appears as a strange snowball in a field of pin-point stars.

Field curvature and off-axis coma are both superbly corrected in the TMB without a flattener and that really adds to the quality of star fields and extended DSOs, with Ethos giving you that incredible space-walk view that lesser scopes never quite seem to deliver.

Imaging

I am not an expert or committed imager, but I enjoy taking DLSR snaps of the Moon and DSOs. The big TMB is ideal for this – its absolute focus snap and that superbly smooth and stable focuser make critical focusing child’s play using a DSLR with live view.

There follow a couple of un-processed, single-frame prime-focus images of the Moon and M42, to give you an idea of what’s possible. I’ve also included a cropped image from the Moon photo to show the embedded detail in the same image. Both were taken with a basic Nikon 5100 DSLR.

 

Prime focus (F8) image of the Moon, taken with TMB 175 and Nikon 5100 DSLR, un-processed.

Gassendi and surroundings cropped from the same prime-focus image to show the detail embedded within the original.

Single, slightly cropped, but otherwise un-processed frame of M42 taken with TMB 175 and Nikon D5100 – 105s at ISO 800.

The following (short, to minimise bloat and tracking errors) image of Bode’s Nebula shows little or no off-axis aberrations in the corners of my 5100’s 23.6mm x 15.6mm sensor.

Short, un-processed image of Bode’s with Nikon 5100 shows clean star images right to the edge of the 23.6mm sensor.

Summary

Overall the big TMB is a total pleasure to use. You just forget about the optics and mechanics, because everything works just the way it should. Instead you just enjoy the view, which is always superb. I generally don’t use my other scopes much now, because this one is always my preference. Given the choice between a 12” Dall-Kirkham, a 16” Dob’ or the TMB 175, I found that I would always choose the TMB in practice. Having owned the TMB for several years, I have found nothing to rival it (let alone replace it) in my affections; my experience of it just keeps improving as I explore its capabilities.

So for me it doesn’t get any better than this and apart from cost, the only real criticism is size: a seven inch refractor will always be a big, heavy scope and this one is no exception. APM have adopted some clever design touches to make the TMB175 portable, but once it’s up and running it really needs a big mount and permanent installation in a big dome (I use it in an eight foot POD, which is all I have space for, but I wouldn’t recommend it).

The cost of making refractors is supposed to increase with the cube of the aperture. In the case of the TMB 175 it’s more like the fourth power: at around double the cost of the six inch version (the seven inch lens is exactly twice the price of the six inch lens), this is an expensive telescope by any standards.

In comparison with Newtonians, SCTs, even smaller APOs, you could scarcely accuse it of being good value.

And yet... this is a telescope which is almost uniquely versatile. With a wide enough field to encompass almost any DSO or cluster, its perfect correction for chromatic aberration and naturally flat field make it a superb imaging tool, even without a reducer. Yet you could enjoy this scope for the simple pleasure of sweeping star-fields, just like a small APO but with much more reach. Then again, the large aperture, high optical quality and complete freedom from false colour deliver planetary contrast that few telescopes can match.  Lunar and planetary resolution is at a level where the limit is the seeing most of the time and yet the unobstructed aperture and perfect optics make the best use of mediocre seeing. Many telescopes will do one thing well; very few will do them all.

So the TMB 175 is the kind of user-friendly, do-anything all-rounder that many people must hope for (but don’t really get, in my experience) when they buy an 8 inch SCT. What a shame, then, that the hard realities of optical fabrication costs make big APOs like this so rare and unattainably expensive for most (I was able to buy my TMB 175 used – I could not have afforded a new one).

After four years of ownership, the TMB 175 comes with my highest recommendation, despite its cost.