TMB 105/650 review
By John Ford (jsf@kc.rr.com)
A reason to buy…
The expectation of workmanship, fit, finish, and optical quality for cutting-edge apochromats is rising very rapidly. The history and reasons for the rise in popularity of these fine instruments will not be re-hashed here, but suffice it to say that there is no question that they have carved a niche market for themselves with today's amateur astronomers.
The TMB 105/650 is not the first (nor the last!) such refractor that I have owned. My love affair with fine refractors began more than 30 years ago when I was being mentored by a gentleman who created his own observatory-class 3 and 6-inch f/15 achromats, one of which I used rather extensively for planetary observation.
Admittedly, I am a deep sky nut, logically attracted to large reflectors. However, I have had the opportunity to sample the world of refractors through shared observing sessions with owners of Televues, APs, and Taks. All of these are fine instruments, and I even owned a TV-101, valued in my eyes for its ideal combination of flat wide field, color correction, and availability.
Being the typical suburban type, burdened with the obligation to trek into parts unknown in search of dark skies, I have come to value portability over aperture. Consequently, my demands for optical quality have increased. The TV-101 was an optically acceptable design, but portability was surprisingly awkward for my purposes, due to the size of the carry case. Nonetheless, the side-by-side performance of the TV-101 with 6-11 inch reflectors certainly gave me a lot to think about when it came time to load the van. With the refractor, I could still see the faint fuzzies, maybe not so brightly, but the tradeoff of convenience and simplicity pushed me into leaving the 8-inch at home on more than one occasion!
The TMB web site certainly made a solid case for uncompromising optics packaged in an incredibly short optical tube and compact carry case. The sparse reviews given by Todd Gross and others at the time were encouraging, so I took the plunge and sent Thomas a check.
First impressions
The aluminum carry case is a very sturdy unit that weighs very little when empty, so the weight you carry is mostly the weight of the scope. The carry case itself is barely larger than my aluminum eyepiece case, being only two inches longer and two inches deeper, while being the same standing height. (If James Bond needed a 4-inch APO, this is probably what "Q" would supply him with.)
The carry case has pre-carved slots for the OTA, the 100mm extension tube, the finder bracket, and my AP Maxbright diagonal. With some additional hand carving, the Lumicon 50mm finderscope, and a Rigel Quickfinder were also made to fit in the case. Even though TMB supplies a Sky Instruments finderscope, I decided to pass on it in favor of a Lumicon 50mm right-angle finder, equipped with an Antares mirror diagonal and a 25mm eyepiece. In my opinion, this finder is more compact and more convenient to use than the straight-through versions, with the added flexibility of being able to use any eyepiece. For star hopping, seeing the same image in both finder and scope is essential to me.
The scope sits in the carry case with dew shield retracted and focuser extension tube removed. The tube rings are stored on the tube next to the focuser, allowing just enough room for the retracted dew shield to fit into the case. Lifting the tube out of the case is a two-fingered operation, with one lifting the dew shield, and the other lifting the 4-inch focuser. The dust cover for the objective lens resides on the lens cell at all times regardless of the dew shield position.
Laying the OTA on the table allows you to loosen the tube rings and lift the OTA out, leaving the tube rings ready to be installed on the mounting plate.
Interestingly, with the tube rings removed, the dew shield can be retracted even further to reveal the entire lens cell. When this is done, the three sets of push-pull collimating screws are visible as well as the entire outside of the lens cell, which sports a Zeiss-like knurled rubber grip zone. The front of the lens cell is embossed with the lens' pedigree and serial number…mine is "No. 011".
The lens triplet itself is, as expected, flawless and pristine. The coatings appear to be quite good, judging by the fact that the reflected color across the face of the lens is uniform and the glass surface itself tends to disappear at certain viewing angles. Always a good sign.
First Light
The first impression I had was the surprising realization that my eyepieces weren't good enough.
The remarkable thing about this triplet, is that its optical quality can readily highlight the corresponding textbook imperfections in various eyepieces. By switching eyepieces, I could observe the characteristic lateral color of the Radians, and the edge distortions of the Nagler Type 4 and Type 5s.
I chuckled that in most commercial scopes I have used, these premium eyepieces tend to "take charge" of the particular optical system and draw maximum performance from the scope. In the case of the TMB, I would conclude from my findings that the triplet is setting the near-perfect optical standard, and the eyepiece tries its best to follow suit.
Star-test
As far as the star-test is concerned, I found that the scope requires time to equilibrate temperatures and settle down. I found no evidence of tube currents, as expected, but stars obviously don't focus down to a clean point until some cooling has taken place. Testing conditions varied from a low of about 10 degrees F, to about 30 degrees F, and in general, 30-45 minutes after being outside, the images were up to expected "planetary/double star" quality, although very acceptable deep sky views were available at any time during the cool down period. At no time during the cool down, did the optics appear pinched or display an irregular figure.
I am very discriminating on collimation (SCT's do that to you), and there was no reason to be upset here. The helical-tooth rack-and-pinion focuser was smooth enough to allow me to progressively "blossom" each Fresnel diffraction ring and collapse it while watching the transition zones appear and disappear. Consequently, it was easy to see the center spot exactly centered within the first diffraction ring. No question, this scope was delivered with exact collimation.
The relative energy distribution in each of the Fresnel rings is textbook, and is a mirror-image from inside to outside focus, although the outside focus ring pattern was fainter. There was no out-of-focus color on Sirius either.
I have to admit that the textbook first ring next to the Airy disk was beyond my ability to see easily. At best, it was very faint, which suggests (favorably!) that most of the energy is found in the Airy disk. Mathematically, the first ring has to be there, I just had a hard time seeing it.
The scope displays a good snap test. Under stable skies, there is no detectable ambiguity of focus. The field curvature was evident from the snap test. With the Nagler Type 5/31mm eyepiece, the snap test showed the stars in the outer 40% of the field of view to focus in progressively different positions. With the Radians' narrower field of view, however, everything was always very sharp to the edge.
One of my favorite tests for optical sharpness is to challenge myself to detect a difference in focus between the limb of the Moon and the close surface. I was impressed that there was indeed a detectable difference in focus between the two points. Contrast and absence of color gave it the "hover-over-the-surface-of the Moon" impression.
So, after the preliminary evaluation, I was satisfied that the scope displayed very good optics.
My more detailed optical assessment
The optical quality certification is carried out on the Twyman-Green Interferometer (D-304), which provides an optical figure map (3-D surface deviation plot) of the optic, and subsequently calculates the optical figure accuracy expressed in RMS.
From what I understand from TMB's test protocol, and my own understanding of the Twyman-Green test, Tom and Markus have picked a set of testing parameters that are quite airtight and exhaustive. Optical testing staff people have the option of choosing from several interferometric test protocols, and it is possible to choose one that will best "ignore" the flaws of a particular optic.
There is a debate over the number of test points and fringe lines needed to accurately measure an optic, but in my opinion, (you are free to choose your own side of the debate) TMB has chosen an unforgiving tests, given the large number of data selections in the test protocol. In my eyes, this puts a lot of credibility in the numbers shown on the test report, and given that I bought the scope sight-unseen, this was the factor that tipped the scales for me.
Of course, in a scope that comes with the expectation that there will be no detectable Seidel, or monochromatic aberrations (except field curvature), the final test of "human" appreciation is that of secondary color, or the absence thereof.
In all the stars I looked at and used as star test targets with the TMB, I was not able to find any on-axis secondary spectrum, either at focus or out of focus.
Looking at the Moon provided an opportunity to evaluate the entire field image plane. As I stated before, the Radians' lateral color generated plenty of detectable lateral color in the edges of the field of view, so I needed a better test.
In order to convince myself that the triplet was indeed free of color, particularly in the field edges, I devised a flat piece of metal with a 1.25 inch hole in it so that I could slide the eyepiece back and forth across the image plane in the 2-inch diagonal opening, while remaining at focus. My goal was to see if the lateral color followed the eyepiece. If it stayed fixed, that would indicate there was secondary color in the triplet. Test results…the yellow fringe on the moon clearly moved with the Radians, indicating that the lateral color was from the eyepiece, and there was no secondary spectrum from the triplet.
I then decided to test the degree of coma correction. Using the GM-8 drive corrector, I moved a defocused diffraction-ring around the field of view perimeter edge at 217x and the diffraction rings stayed round and concentric. Impressive. Obviously, coma correction is excellent, as advertised.
As for zonal aberrations, I could not detect any differential amplitude between any two diffraction rings. I was also able to use my "sliding plate" experiment to examine the entire focal plane while looking at a defocused star (Sirius). The diffraction rings remained steady, even, and symmetrical. If there is a zonal deformation, I can't find it.
Even with very unstable seeing on one of the testing nights, I was able to split Rigel-B away from Rigel-A (Strehl at work here!). On subsequent nights, Rigel-B was always an obvious target, as were the E and F stars of the Trapezium. Jupiter and Saturn were very contrasty and crisp, without any halos, to the point where the added contrast provided impressive 3-D hues on the limbs of these planets.
The sky next to the planets is deep black, indicating that the baffling of the TMB 105 is quite adequate. Internal inspection reveals 3 deep knife-edge baffles.
The baffles are obviously designed to accommodate the un-vignetted use of larger format photographic equipment, but still provided state-of-the-art suppression of internal reflections for the narrower field of views. It was easy to see faint stars next to the limb of the Moon, without any overglow from the moon.
On deep sky targets, the TMB showed its merits by providing quite satisfactory views of the Crab Nebula and the planetary NGC 2022 in Orion. Actually, I was side-by-side with an Orion XT-10 dob and an 8-inch ATM dob. The Crab was not as bright in the TMB, but every bit as easy to spot as it was in the dobs! Once again, the legendary contrast of the refractor gives bigger reflectors a run for their money.
The most impressive sight on one of the nights, was the Double Cluster with the Nagler Type 4/22mm. I had never seen the progressively fainter stars in so much pinpoint detail, gradually disappearing into the black background! A breathtaking sight that captured my attention for an extended period.
Mechanical Assessment
The weight of the OTA has been the subject of much debate. According to my scales, the OTA weighs about 15 lbs, and the finder bracket, finderscope, and focuser extension adds another 4 lbs. The whole carry case, loaded and ready to travel shows about 22 lbs on my scales.
The compact size makes you ignore the weight, and everyone has been amazed at the scope's ability to disappear into such a small case.
The scope's weight comes into play when it is mounted. The short tube length doesn't cause much of a torque problem, however, the tube weight certainly will cause flimsy mounts to swing, particularly when you are focusing at high altitudes with a locked down focuser to prevent the draw tube from pulling out under gravity. The TMB features focusing knobs that are oversized to help alleviate the "hand torque" required, but your mount will need to hold things steady when focusing.
The scope looks like it was built around the focuser, which itself tends to draw all the attention. The fabrication of the focuser is very good, and the machined finish was left unpolished, giving it an industrial look. There is a friction adjustment knob for both the rotatable body and the focuser draw tube. The draw tube itself can be fine-adjusted for fit within the focuser body by three axial metric set screws that change the pressure of what I assumed to be the centering/drag pads on the inside bore of the focuser body.
I must say that being able to easily rotate the focuser/finder was quite convenient. It made observing a very comfortable experience, regardless of the orientation of the GM-8 equatorial mount. Also, for those who prefer to star-hop from a horizon reference, the rotating focuser will be a fantastic asset. By the way, balancing the scope in DEC was easily achieved, even with the Nagler Type 5/31mm.
The focuser draw tube is a two-part component. For visual use with a diagonal, you must screw in a 100mm extension tube section that brings the diagonal and eyepiece to the proper focus plane. If you choose to use a bino-viewer, or you are using larger format photo equipment, the 100mm extension is not installed. If you are going to use a CCD camera, such as the SBIG line, you will need to buy an extra extension tube to replace the optical path length of the diagonal. The 2-inch adapter screws into the draw tube, with or without the 100mm extension. TMB also supplies a 1.25-inch adapter, from either AP or Baader. The total focuser travel is about 70mm from the stop. My Radians come to focus at about 30mm from the stop. Of course, you can change the focus distance in increments of 100mm with the extensions, or ask Thomas to supply you with other extender lengths.
The last step in preparing the scope is to install the cast aluminum tube rings (I like these rings! Very understated, yet sturdy.) onto your choice of mounting plate. The open tube length on the TMB 105 is only about 7 inches, so the rings should be spaced apart by no more than that distance. One caution to observe here is to make sure the mounting plate does not extend past the front of the rotating focuser body, or the focusing knobs will collide with the mounting plate.
Sitting on my GM-8 mount, the TMB 105 looks like a piece of industrial artwork, as if someone tried to collect into one product all the finest features of modern machinery: fit, smoothness, paint finish, user-friendliness, and adaptability. Furthermore, there are no shortcuts or money-saving tricks used in the design. This scope is "brute-force engineered" to the max on everything. Period.
Comments from the peanut gallery
The scope isn't perfect…none are.
First, the objective lens cover is difficult to remove or replace when the scope is mounted with its dew shield extended. The photo and CCD people will need to do this a lot! I used a set of stick-on rubber knobs to provide a grip. As a standard feature, the dust cap should have a handle or knob designed into it for this purpose.
The gears on the R& focuser should be honed smooth to avoid the mild "buzzing" feel and sound when racking in and out, particularly with heavy eyepieces. This absolutely doesn't affect the operation of the scope, but would add to the refinement of the scope's "industrial artwork" appeal.
Next, the internals have been carefully flocked and baffled for maximum contrast, however, there are a few small surfaces that were missed, namely the machined faces of the focuser draw tube and of the focusing extension and 2-inch adapter. Again, easy home fix is possible with some flat black paint.
Last but not least, the coatings are certainly effective and the surfaces of each triplet component are pristine. From most angles, the glass tends to disappear, which is what you should expect from superior coatings. However, true to form, the Russians are still not "artists" in terms of making their coatings as esthetically pleasing as they are functionally excellent. There are no outright sleeks that I could find, but there are visible differences in the depth of the coatings across the surface that will draw comments from the "armchair astronomers". If TMB ever taps into the Zeiss coating process, the coatings may look better, but I doubt if optical performance will improve, so I wouldn't pay extra for it.
Conclusions
I wrote this review to express my "consumer" impressions and opinions. Clearly, the TMB triplets show state-of-the-art optical design and display a level of workmanship that challenges or exceeds the competition. Even though I used the TMB alongside several other refractors and reflectors, I will leave the task of exhaustive direct comparison to others. I saw pleasing optical performance in the TMB that I have not seen in the competitor products, which led me to believe that I had made the right choice.
All things considered, I would say that the AP scopes have met their match, and I would bet that half of the upcoming direct comparison reviews will favor the TMB. The Taks (102's, etc) have been easily surpassed in color correction (in all fairness, the FSQ-106 is an astrograph, not a fast APO), and the TV-101 continues to be surpassed on all counts by the TMB. Having said that, I believe that the owners of TeleVue 101's will find the view a little different. Once you are accustomed to the edge-to-edge flat field of the 101's flat-field Petzval design, the curved field of a fast triplet refractor may take some getting used to. However, the TMB's gain in contrast, sharpness, and planetary performance is quite noticeable to me, as compared to the TV-101. It all boils down to your preferences and priorities. Personally, I traded the flat field of the TV for the portability and optical rigor of the TMB.
Furthermore, I find the TMB optical test certificate to be a marketing masterpiece. Whether today's high-end consumer buys cars or telescopes, they are more than willing (and eager) to learn about the criteria that sets the threshold for excellence. For sure, those astronomers who decide that a high-end APO is the logical way to feed their hobby, may also be the ones who are curious enough to understand more about their equipment. I would venture to say that optical test certificates will become increasingly commonplace as more and more competitors enter into this high-end market. Of course, I think it is pointless to compare a Strehl of 0,95 to a Strehl of 0,99 and potential customers should realize from readings and experience that anything above 0,90 is the benchmark for "perfect" optics. I believe it is possible for experienced observers to tell the difference between a 0,85 and a 0,95 but not between the 0,95 and 0,99.
We will probably always be comparing the premium scopes to one another in the field, on the basis of optical performance, but comparing them purely on the basis of test data may fall short of the truth. It is possible, and actually quite common, for a 0,95 Strehl scope to present a marginally-better visual image than one with a higher number, but this is hair-splitting for sure. At this point, we begin to deal with the observer's eye quality as much as the scope's optics.
As for the physical scope, I really think the TMB is the hands-down, all-time winner in the "Built Forever" category. It is clearly built to be used, designed to be transported, and much thought went into its multi-use capacities. Providing a large aperture is the ONLY thing this scope can't do. Every other conceivable task or duty can be successfully undertaken by the TMB.
For many reasons, I believe that we are probably in a sort of "golden age" of APOs. In spite of their cost, I marvel at the workmanship and quality supplied to us at such reasonable price (it could be much more!).
As a closing comment, I would say that Thomas Back gives his products the finishing touch, by fussing over your individual scope as if it were his. Product support doesn't get any better. Furthermore, I learn something new every time I speak to him.
After all is said and done, I don't worry about coatings or Strehls, or RMS, or Fresnel rings, or Airy disks. I just enjoy it.
John Ford |
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