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Home / TMB152 and GR-2XXL Alt-Az mount
by John Ford 09/27/03 | Email Author

Having been duly impressed with the invention of the Swiss Army Knife, I have always tried to manage all my equipment to eliminate redundancy and maximize flexibility.

Accordingly, I currently enjoy both wide-field low-power viewing, higher-power viewing of solar system objects (without being a planetary nut, just yet), and some "see-if-I-can-find-it" star-hopping for deepsky objects. The latter is especially enjoyable when I can find stuff quicker (albeit not as brightly) than the folks with the bigger dobs or GOTO's, and invite them over to see what they are looking for. Hey, it's the spice of life!

My second criteria is that I really don't enjoy spending one hour loading the van for a night's observing. Heavy and complex equipment is not good, particularly when carrying it to a remote site. I need to make no more the three trips to the van; scopes, mount, and eyepieces/tripod.
Finally, I can't tolerate a shaky mount.

So my "ultimate" system has come to consist of the following:

a) TMB152
b) TMB105
c) Tele-Optik GR-2DXXL alt-az mount (dual arm) and Casady 23lb counterweight.
d) Losmandy G-11 tripod with "refractor extension" on the mini-pier.
e) TeleVue eyepieces (Naglers/Radians)

For the "grab-and-go" or traveling sessions;

a) TMB105
b) Tele-Optik GR-2 Deluxe alt-az mount (single arm)
c) APM wood tripod
d) TeleVue Eyepieces

I'll review the TMB152 as well as the mount system I've assembled, so that you can appreciate the simplicity and functionality of this setup.

Here we go…

Review of TMB 152mm Apochromatic Refractor

The TMB152 was my second TMB purchase in 2001 and intended to supplement my TMB105 and cover the range of observing activities that I really enjoy. Proverbially, I have sold all other toys to buy these optical jewels and have no regrets. I consider my equipment as being part of a "system" which allows me to do the types of observing that I enjoy most.

Getting back to the TMB152, Tom Back has again shipped an outstanding instrument. This particular specimen is my second personal close encounter with a 6-inch refractor, the last one being an f/12 hand-made doublet. Otherwise, I have meandered around numerous other 6-inch refractors over time and without getting into comparisons and shootouts, I can again say that I've acquired one of the best currently-available 6-inch refractors. If you really want to know why I think that way, just read on.

The Box, the Scope, the First Impression

The box arrived the day after Tom shipped it, since we only live 3 hours drive from each other. Tom packed the scope in a sealed plastic bag, sitting in a bath of peanuts that did not allow any movement of the scope. The scope was also held captive by some styrofoam blocks on both box ends. FEDEX treated the box well, and my wife had to ask the delivery person to take it the extra mile down to the basement, since it was quite heavy.

Opening the box, the first thing that hit me was the size of the scope. I had debated about acquiring the 7-inch, but at this moment, I realized that the 6-inch teetered on the brink between "portable" and "transportable". The TMB152 is a large instrument and is quite hefty. The specs say it tips the scales at about 45 pounds, which is not so bad, however, it is somewhat awkward to handle. I took off my wrist watch before wrapping my hand under it to prevent scratching the tube's beautiful paint finish. Believe me, you need to wrap yourself around this scope to feel you have a good grip.

Feeling very uneasy about carrying this scope, I fabricated a carry handle out of half-inch welded square steel tubing that fastens to the tops of the tube rings and turns the awkward tube assembly into a very easily carried object. This handle also helps when lifting the scope onto its mount.

Finally, the scope was on the bench. What a beautiful instrument. The white paint is the same as the TMB105, but the focuser flange and dewcap flange are a shiny metalic silver that looks better, in my opinion, than the greenish teal color on the TMB105. (Of course, this is like commenting on which of your kids is cutest!) The dewshield is huge, having an outside diameter of almost 9 inches, and the lens cell is quite impressive. Everything is clean, sharp, and in good condition.

Starting at the focuser end.

The focuser is the usual signature TMB focuser. Machined aluminum, completely rotating, precise, smooth, and without any slop or play. (Actually, I discovered some slop in the focuser when using heavier eyepieces, but this situation is easy to remedy by adjusting the focuser. A complete description of this easy procedure can be found at my website, at www.angelfire.com/mi3/astropattern/) The APM/TMB focuser is a massive affair that features six user-adjustable tensioning pads that guide and support the focuser drawtube during its travel. These tensioning pads are adjustable by tightening or loosening the small metric Allen screws that are positioned at 120-degree intervals around the outer focuser body.

Without taking the focuser apart, only the three outer adjusting screws are accessible, whereas the three inner ones are factory-preset. Adjusting these screws will give you a focuser tension range that feels either very silky or very solid. I also found that loosening or tightening these screws does not have any apparent effect on the centering of the drawtube; only on the friction drag. Therefore, there is apparently no danger of losing collimation by radial displacement of the drawtube during adjustment.

As with all true quality equipment, the focuser drive shaft mechanism is also cleverly mounted within an eccentric housing that allows minute adjustments to the rack and pinion backlash, to a level of precision well beyond the detection limits of your fingertips. On my specimen, I found that the focuser backlash was factory pre-set and did not need to be adjusted and likely never will.

As with the other TMB's, the focuser drawtube consists of a screw-in extension to supplement the optical path length of the "active" drawtube portion. This focuser extension is a larger diameter than those for the TMB105, so if you have any extensions for the TMB105, they won't fit the TMB152 focuser. On the other hand, the TMB152's focuser has more than enough backfocus necessary to offset the absence of the mirror diagonal, in the event you wish to use a CCD camera. On the other hand, removal of the extension should allow use of bino-viewers without a problem.

The focuser is very smooth to the feel, and the torque required to adjust focus is firm and steady. It is simply a pleasure to feel, particularly with the large rubber-ringed knobs.

The finderscope bracket is the same as the TMB105 except that the standoff is longer. Very hefty and overbuilt…just the way I like it. Two self-centering locking knobs hold the finder bracket in place and allow the finderscope to return to exact alignment every time it is removed and replaced. The finder bracket is also a convenient slewing handle for the scope.

The focuser drag/lock adjustment knobs and focuser rotation friction/lock screw are upsized for this scope and are in the same convenient position. I did notice that when you loosen the friction knob, it wobbles noticeably, denoting a somewhat coarse threads. On an instrument of this quality, I would have expected tighter, finer or superfine threads that keep the screw centered until the very bitter end. This doesn't detract from the overall function of the focuser, but it is an item for Tom's and Markus' notebook.

The tube itself is very sturdy, and the finish is excellent. The mating and fit of the parts is outstanding. There are no detectable mechanical or cosmetic flaws of any kind in the tube assembly.

Although the triplet lens is the same design as the other scopes, the machined temperature-compensated lens cell is different in design from the TMB105. The inscriptions are different, and the overall aspect is more like an observatory-class cell than a commercial cell such as was the case with the TMB105 cell with its rubber knurled contours.

Inspection with a flashlight revealed no coating flaws, sleeks or dirty spots. The coatings appeared to be more uniform thickness than on my TMB105, although both scopes are better than some other premium scopes. The edges of the lens elements are clear and the inside barrel of the lens cell is not blackened and carries some pencil inscriptions, which gives it that "made-one-at-a-time" look. The overall aspect is, as I said, very "professional observatory" style. Built for serious work.

The inside of the tube assembly is, of course, heavily baffled. Five baffles are supported by the usual TMB standoffs. Looking down the tube, everything is black except for the faces of the focuser flanges and extensions.

This issue has not been corrected since I reviewed the TMB105 earlier this year. If you look up the tube from the focuser, it is obvious that the ray-traced baffle system is properly designed since you can see the edge of the objective lens no matter where you place your eye in the focuser opening.


This is less than optimal for visual use, but it was meant to accommodate larger format cameras. This scope, as all TMB's are design to deliver absolutely every possible usage capability except larger aperture.

Finally, the tube rings are the same TMB rings as on other scopes, cast out of some sort of corrosion-resistant aluminum alloy. They are felt-lined and perform very well. Some people have commented on the roughness or lack of a polished/anodized finish, but I have experienced no such impressions. If I walked into a professional observatory, I would expect to see this style of functional grace instead of what I consider to be the "toy and jewel" aspect of the more commercial offerings.

The rings mate very easily to the Losmandy DAP6 plate, which in turn slides onto any mount fitted with a Losmandy G-11 saddle plate. As described later, I fitted the G-11 saddle plate to my APM/TeleOptik GR2XXL alt-ax mount for use with the TMB152.

So much for the equipment, now let's look at the performance.

First light and subsequent star testing.

As a previous reviewer once said, it is difficult to write a lengthy report about excellent optics. The optics of the TMB152 are simply better than other units I have seen. The criteria for perfect optics includes not only the figure and optical design, but also the coatings, collimation, performance, startest, and workmanship. I have become a fan of TMB products, but after seeing more of them and having been involved in several comparison tests, my elation have been tempered by a more critical standard than I have used to evaluation optics in the past. In site of that, I still can't find anything wrong with these scopes that doesn't justify their cost and claims of performance. So, if I sound unreasonably positive about the scope, it's not for lack of trying to find faults.

Its important to remember that the TMB152 is a slower scope than the TMB105, so the expectations of the startest can not be the same for both scopes. The faster triplet scopes, no matter how perfect the design, will allow the deep violet to focus longer than other wavelengths, which accounts for the "hazy" aspect of the outside focus diffraction pattern, which differs from Suiter's "perfect" startest.

As well, the effects of spherochromatism are worse as the scope gets faster, so that outside focus, you get a series of overlapping diffraction patterns from the different colors, namely the deep blue to the near ultraviolet, which also softens the image.

Nonetheless, the person evaluating the scope needs to concentrate on the relative brightness of the intra- and extra-focal diffraction rings and their relative sizes even though the effects described abov e as well as the effects of upper atmospheric seeing will make the outside focus diffraction rings seem fainter or softer. Often, this is mistaken for spherical aberrations.

If I may get on the soapbox for a moment, allow me talk about the unfortunate misperceptions of optical test data, since I have answered numerous questions from prospective TMB owners this year.

I told you my TMB105 had essentially perfect optics with a very high Strehl ratio. I also told you that Strehl ratio and actual scope performance don't always match up. I wasn't joking, and this TMB152 is a good example. During my first observing sessions, it jumped out at me immediately that the star test on this scope was even better than on my TMB105, notwithstanding that the Strehl on my TMB105 is pretty much as high as it can possibly be.

Tom Back is using an interferometer measuring protocol that is, in my opinion, extremely unforgiving. For some reason, this has led to the popular misconception that a Strehl 97% TMB is somehow better than a Strehl 95% TMB scope.

My point is that you aren't going to increase your chances of getting a "better" TMB by making sure your numbers are equal to or better than someone else's. To illustrate my point, I will tell you that my p.t.v. (on both scopes) is "worse" than others I have seen, although my rms value is virtually the same, and my Strehl is only slightly higher. Knowing what I know today, I pay much more attention to the concept that the three numbers are reasonable and not suspiciously high or low relative to each other. Then I spend time at the eyepiece.

If you were able to look at all the TMB's ever sold by Tom, you would see that the p.t.v., rms, and Strehl numbers all vary within a narrow band, but they aren't necessarily grouped in tidy mathematically-tight "threesomes". Tom initially uses the numbers to make sure the optic rises to his own standard before taking it from the manufacturer. Unfortunately, some folks have begun "Strehl peddling" based on Tom's test results, which is why he doesn't provide printed reports anymore.

Most people don't realize that these measurements tell you something about the quality of the optic, but they aren't absolute, and are subject to test instrument errors, human errors and processing choices during the test. In the end, you need to use the startest to try and tell the difference, if you can. On the other hand, Tom will always feed you a scope that will be right up there when you figure out how.

Enough soapbox. Back to the scope performance.

I expected that the TMB152 would probably differentiate itself by virtue of increased aperture over my TMB105. Of course, the TMB105 is a much faster (f/6.2) scope than the TMB152 at f/7.9, so some improvement on the startest could come from that fact alone.

Whatever the actual Strehl on this scope, it outperforms my TMB105 on the startest. The first indication was that the intra and extra focal diffraction patterns were virtually identical. Quite frankly, I'd never seen that degree of symmetry. The outside focus pattern on other superb scopes I'd seen was always slightly fainter, although the rings were identical and the relative amplitude of the rings was the same. I had attributed this phenomenon to upper atmospheric seeing, and some spherochromatic effects.

On this scope, I can hardly tell the intra from the extra focal ring pattern in spite of having tested the scope in a variety of seeing conditions. It really is like the textbook patterns. I'm impressed. It raises my standards for "perfect optics" by another notch.

As is the case for all other TMBs, AP's, and occasional Tak specimens, there was no perceptible color either in focus or out of focus. The Moon's limb immediately falls off to a deep black. (Except, of course, if you are using a Radian eyepiece where you see the eyepiece's significant lateral color. Refer to my TMB105 review for more explanation of this).

At this point, you would expect me to launch into a detailed account of how I diagnosed and assessed pinched optics, miscollimation, astigmatism, zonal flaws, roughness, or other optical defects. I can't, simply because I couldn't find any such defects on this scope. I have spent numerous hours looking at Vega to try and measure the spherical aberration. It is beyond my ability to detect differences in brightness of the inner and outer rings. I can normally detect about 1/8th wave spherical and make guesses beyond that.

The five baffles are very effective, and flare from a bright object just outside the field of view can be reduced to almost nothing simply by changing eyepieces, which is consistent with the conclusion I came to with the TMB105…this scope puts the burden of optical performance on the eyepiece.

In terms of notable sights, I enjoy spending countless hours searching the sky for faint open clusters and globulars, simply by opening the atlas to any page and star-hopping across the page to see how many I can detect. I guess I'm a tourist.

The beauty of this instrument lies in the tack-sharpness of the optics and the baffling that provides high resolution, high contrast star images against a black background. In most cases, the scope tends to "disappear" and I personally get a sense of three-dimensional depth to the images of certain open clusters. This isn't an effect unique to this instrument, it is simply better in this one.

Recently, I was in the presence of two NexStar GPS-11's and to my surprise, it was difficult to decide which scope was providing the best view of the Trifid Nebula and the Ring Nebula. The TMB152 provided a better contrast against the SCT's traditional "grey" background. The sharpness of the field stars enhanced the setting of the Nebulae to the point where spectators preferred the refractor over the SCT.

I am very pleased with the planetary views, while not claiming to be a planetary observer. Provided conditions allow, there is no image breakdown at 400x, with the new Nagler 3-6mm Zoom. (I haven't pushed it higher than 400x yet.) Even Jupiter's traditionally soft features looked crisp and contrasty. One of the most satisfying moments was to have the Encke Minima appear as clearly as Cassini's Division.

What I liked….

As usual, I don't do comparisons, but my wanderings have put my eye in the eyepieces of the competitor scopes and the TMB152's comparative performance meets or exceeds anything else out there, both past and present. Period.

The overall fit and finish of the scope is very good, not to mention beefy, professional, and impressive. I also like the fact that everything on the scope is user-adjustable, such as the glass elements in the lens cell, the lens cell in the tube, and the focuser. I think that as these scopes survive into their vintage years, their retained value will be enhanced by the fact that they can be disassembled and restored without the need for special tooling or proprietary techniques.

Depending on the size you wish to buy, delivery varies from several weeks (some may sometimes be in stock at the time you call) to several months (a year or so for the bigger ones), but in fact the scopes are generally available within a time frame that lies within my attention span, which is a valuable feature, in my opinion. I like equipment that was built to be used, with that frugal "NASA-whatever-it-takes" look to it. I like being proud of my equipment and I like when spectators don't need to look at the nameplate to understand that this scope is a breed apart.

What I'd improve…

Between my TMB105 and my TMB152, the only feature that I would rate quite poorly (I would even go as far as to say "disappointing") was the objective lens cover. Both scopes are supplied with a steel press-fit cover that fits over the lens cell. However, the problem is that the fit is very loose and requires the owner to be extremely careful when removing or replacing it (because there is no way to grip it) such that the cover does not inadvertently tip backwards and strike the lens glass. Furthermore, the tiny friction pads on the inner rim of these covers quickly crush down to the point that the lens cover will fall off the cell under its own weight, which has happened to me on the TMB152. To fix this, I replaced the small friction pads with leftover adhesive friction pads from an Orion solar filter package. On the TMB105, the solution was an after-market cover that fits over the dewshield but that had to be purchased from TMB as an extra. The final solution for all TMB's might be a "deeper dish" cover with foam rubber friction pads and a finger handle for positive control.

I think that lays the TMB152 pretty open, and gives you about as close a tour of the product without actually sending it to you.

Sub-report - The GR-2XXL Alt-Az mount and the alternate tripod.

This mount, as described by Dave N. is extremely solid, smooth and worthy of being spoken of in the same breath as the AP900 or AP1200, etc. Just like it's little brother, the GR-2Deluxe, the "XXL" is very simple and straightforward to use. It has impressed me by being the embodiment of both precision and light weight/compactness. For what it does, it is less than half the weight of the Losmandy G-11 equatorial head, or the AP900 head. It is essentially a two-dimensional, round object that doesn't have any angular protrusions. Also, it pleasant to lift and handle comfortably due to its "arms". Each of its arms can easily be drilled and tapped to accept virtually any mounting system on the market today. I chose the Losmandy, simply because all my scopes and friends' scopes are G-11 mounted. However, APM sells a dovetail saddle plate that is Losmandy compatible. Since I already had spare Losmandy saddle plates, a drill, and a set of taps, it just made sense to use this stuff instead of ordering more.

The XXL mount has two friction drag adjustments that are easy to operate. These drag mechanisms consist of plastic inserts that have been pushed into the threads ahead of the adjustment knobs. When the adjustment knob is tightened, the plastic insert applies pressure on the shaft. That's it. Quite simple, and it can't be broken, no matter how tightly you turn it.

The specs say that the design of the XXL is completely different from the GR-2D, suggesting that there are bearings of some kind in this design. Judging by the smoothness of the action, I was almost convinced of this, yet when large unbalanced loads were applied (scope with no counterweight), the friction increased quite significantly, while still remaining very smooth and useable. So, I did the only logical thing…I took it apart. As you can see from the picture, there are no bearings in this design. In fact, it is exactly the same design as the GR-2D, only bigger.

Having disassembled both versions of this mount, I was able to measure the fit of the shafts and bores. The GR-2D components have about 3.5 thousandths of an inch looseness on both the altitude and azimuth axes, whereas the XXL displays only about 1.5 thousandths of an inch on both axes. My GR-2D has some slop that I can easily control by keeping a little tension on the friction knobs. My XXL is slop-free regardless of the knob tension. Clearly, if there is any slop or wobble on the axes of the mount, it is because the tolerances on the shafts and bores have not been held to standard.

In summary, the concept is sound and simple. On the issue of an advertised difference in design between the two versions, I'll let you be the judge.

The issue that needs to be addressed more in depth by APM is the total lack of instructions for the unit. True, it is a simple unit, but this mount does not place the weight of the scope directly over the tripod. Due to this fact, it is possible (especially with the APM wood tripod) that the whole system will tip over when loaded in certain ways.

It is therefore important to install enough counterweights on the counterweight shaft BEFORE installing the scope. The good thing about this system is that there is quite a bit of room to maneuver with balance, since the center of gravity only needs to be somewhere within the "tripod footprint" in order to avoid being tipped over. Caution: I define the "tripod footprint" as the area under the tripod that would be defined by drawing a straight line from one tripod foot to the next! The footprint is NOT the "circle" defined by the tripod feet. After the scope is secured on the mount, the counterweights can be shifted around for best overall balance.

Best overall balance is another issue that could be addressed by instructions. The XXL mount comes with a 20-inch counterweight extension shaft that accepts the standard Losmandy or Casady counterweights. To fully balance the TMB152, I needed to put 20 pounds out of the very end of the counterweight shaft. To my dismay, the whole system would swing back and forth, making it impossible to focus at high power. After putting a lot of thought into the issue, I decided there were two components to this problem…

1) position of the counterweight and,
2) rigidity of the tripod.

Up until now, I had been using the APM wood tripod under APM's advice that the wood tripod was strong enough to hold everything up to the 8-inch TMB refractor. What wasn't expressed with that advice, was that even if the tripod could physically support the weight of these large scopes, it had to be fully collapsed to provide any degree of observing stability. My idea of observing comfort doesn't include laying on my stomach (or on my knees) to look into the eyepiece, which is what happens when looking at the zenith with the TMB152 on a fully-collapsed APM wood tripod.

With the APM wood tripod extended to about 55 inches to put the scope at a comfortable height, the tripod was allowing the whole system to twist very slightly, which affected the stability dramatically with the 20-pound weight at the end of the shaft. Damping times were over 15 seconds. This was not at all acceptable.

What was needed was a solid tripod that had little or no twisting motion.

The only other options I could see that would have the level of portability that I needed would be the AstroPhysics portable pier, or the Losmandy G-11 tripod with the 12-inch "refractor extension". The AP pier is more costly, and from my experience, works best with heavier loads that increase the stability of the "seating" on the ground. While the TMB152 is heavy, the scope and DXXL mount still isn't heavier than 75 pounds, which is like having only a counterweighted G-11 or AP900 head on the pier without any scope.

The Losmandy G-11 tripod is very stable with a wider variety of loads, and costs less. It also allows the use of a removable 12-inch extension to raise the scope height. Finally, it is compatible with my venerable GM-8 head. So that was my choice.

The tripod arrived and the only problem to solve was to figure out how to adapt the XXL to the Losmandy pier. The solution was relatively simple and elegant.

I obtained a base ring plate for the Losmandy equatorial heads, and well as the capture plate. (If you remove the big hex cap screw from the bottom of your Losmandy mount, everything that falls off is what you need to get from Losmandy.)

The only thing I did then was to machine the Losmandy base ring plate bore from 2 inches to 2-3/8 inches to accept the XXL footprint. I made a spacer ring out of hard plywood to allow the capture plate to tension properly and that was all.

As a fortunate coincidence, the outside diameter of the XXL is virtually identical to the diameter of the Losmandy pier, so everything looks like it was made to fit together.

The performance of this tripod with either both scopes or only with one is very acceptable, with damping times of less than 2 seconds at 400x. The tripod is easy to setup, and only about five minutes is required from the time you park your car to the time you take your first look in the eyepiece. It was exactly what I wanted.

With both scopes on the mount or only with one, the system is smooth and positive, with virtually no backlash. Once you push the scope to an object, it stays there…just like a good dobsonian. The mount is very well matched to the high power capability of fine apochromats, enabling one to admire fine details without having to wait for the image to settle down from focusing or panning vibration.

Balancing the scope with a counterweight is also worth mention, since I have found it best to have some unbalance in the system. This seems to minimize the moment arm of the counterweight and consequently reduces the vibration level quite significantly. When using the TMB152 by itself on the XXL, I usually run the 23-pound counterweight only half way out on the shaft. If I use only the TMB105, I use no counterweight at all (also true for the GR-2D mount)

Cost-wise, the mount asks you to choose between this and a G-11. The tripod and pier extension is $500, and the GR2-XXL with dual arms is about $1000. The Losmandy parts and the machining required to complete the adaptation is about $140. The Losmandy saddle plates(or the APM equivalent) will need to be purchased regardless of the tripod you use.

However, I must say that the APM wood tripod and GR-2 Deluxe mount is the very best mount combination I have seen or owned for scopes up to 25 pounds or so. By comparison, the TeleVue Gibraltar is acceptable for scopes up to the weight of the TV-101 and such, but clearly inadequate for anything like the TMB105.

After eight months of intensive use, the APM tripod, the GR-2D and the TMB105 is my all-time favorite grab-and-go setup. Without a need for counterweights, this system is rock-solid, smooth, handy, and top quality.

But alas, I found that the APM wood tripod, when extended for comfortable use with the TMB152, to be very shaky at even moderate magnifications, which is not very good for a planetary refractor. I decided that I was not going to actually sit on the ground to observe the gas giants as they culminated!

Conclusions

My preference was for the lighter weight, simpler setup, and elegance over the rather heavy complement of equatorial head, wires, batteries, in addition to the tripod. The issue of dew heaters is also simplified, since the mass of glass of the refractors and the deep dew shield effectively prevents dew formation for entire sessions. If needed, dew heaters for the finders and eyepieces can be used in very heavy dew conditions, but I usually get away with covering the eyepiece with its cap between viewing periods.

So there you have it. I think there is little that I haven't revealed about this equipment and how I have chosen to use it.

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