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TMB Optical 175 f/8 Apochromatic Refractor


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Introduction:

I have had a lifetime love affair with refractors, ever since I first had an opportunity to view Saturn through the 20" Alvan Clark at Denver University's Chamberlin Observatory while I was still in High School. Since then, I have been an amateur astronomer for most of my life, even though not always active as such. After owning many laughable improvisations, I acquired my first "real" telescope in 1982, a Celestron Schmidt-Newtonian 5". With it, I was able to find and view some of the brighter Messier objects and become frustrated with the design. More recently I have done most of my observing with a pair of tripod mounted Schneider, Kreuznach 18x80 binoculars that were restored and modified for astronomical use.

Two years ago, after having waited unsuccessfully on the Astro-Physics list for several years for an opportunity to purchase an EDFS 130 Starfire, I fulfilled my lifetime dream and moved into the world of APO refractors by purchasing a Takahashi TOA 130 visual model mounted on an EM-200 equatorial mount. With that telescope, I logged over 70 Messier objects, and enjoyed the December 2003 appearance of Saturn. However, it didn't take me long to want more aperture than 5". After doing my Internet research, the TMB Optical APO refractors appeared to be the only answer that would result in a larger refractor in my lifetime, and certainly the only one available in the larger aperture range. I ordered my TMB 175 f/8 in March of 2004 and received the scope in May, a wait of only 7 weeks from order placement! Below is my evaluation and field report on the performance of the telescope.

Disclaimer:

I am not employed by TMB Optical and have received no compensation for my preparation of this report. I do not have any financial or investment interest in TMB Optical or APM Telescopes, Germany. The evaluation below is entirely the work of the author, and any errors his alone.

Description and First Impressions:

The TMB 175 is an apochromatic refracting telescope that utilizes an air spaced triplet objective lens designed by Thomas Back. The lens and cell are manufactured to TMB Optical standards by LZOS in the Ukraine, which are then installed by TMB Optical in CNC machined tubes manufactured by APM Telescopes (Saarbruecken, Germany). The new "lightweight" model described in this report utilizes a 3" "Feathertouch" focuser.

The telescope as received weighed in at 42 pounds for the bare optical tube assembly. The f-ratio was as advertised by Thomas Back : f/8, with a focal length of 1400 millimeters. The Strehl of this lens was reported as 0.985; about as good as you can get in a lens of this aperture. The telescope came with a quick-detachable finder scope bracket (Japanese manufacture) and a 100-millimeter extension (manufactured by Intes in Russia). A set of mounting rings was included, and were of APM Telescopes manufacture.

The OTA is painted a beautiful creamy white, with a contrasting black trim ring on the sliding dew shield. The "Feathertouch" focuser, bearing the TMB logo, is deep black anodized, and is fully rotatable, with two stage focusing. The APM manufactured mounting rings are also black powder painted. Overall length is just under 60 ", with the dew shield retracted. All in all, it presents a very striking appearance.

The objective lens is the heart of the telescope, and the surfaces are deeply antireflection coated. I could not see any reflected image of my finger held only 2" from the lens surface, even under strong illumination. A glance down the tube through the lens is like looking down a dark well. All of the internal surfaces of the well-baffled tube are blackened to enhance contrast by eliminating any stray light and internal reflections. The dew shield is lined with a black-flocked materiel, and is very adequate for reducing dewing on the lens.

I chose a Takahashi NJP-160 Temma 2 "go-to" mount based on my satisfactory experience with the EM-200 mount. The weight of the telescope, base plate, mounting rings, diagonal, and Panoptic 35 eyepiece was just a hair over 50 pounds; well within the weight limit of 65 pounds stated by Takahashi. This (as an aside) seems to be an almost ridiculously conservative figure for the mount.

Performance:

I really enjoy observing deep sky objects ; Globular Clusters and Planetary Nebulae are my favorites. Conventional wisdom asks "Why are you using an APO for DSOs?" In the part of the world where I observe, the combination of wind and dust, in addition to the atmospherics, demand a closed tube design. The shorter focal length of an APO also generally gives a nicer and wider field view than an SCT or a Mak-Cas. Besides, I happen to like refractors!

Since I received my TMB 175 in late May, and finally got the mount, mounting plate, and all the other paraphernalia together by the end of the month, I was able quickly able to resume where I left off on my Messier project.

First light:

I was faced with a dual problem in my efforts to get a first light: an unfamiliar new mount in addition to a new telescope. My approach was KISS (keep it simple, stupid!). I decided that a few double stars would get me off on the right foot, so my first target was Mizar in Ursa Major. I installed my Televue 2" dielectric coated diagonal and a Panoptic 35 ep , my Takahashi 7X50 finder scope and waited for darkness to take hold. After polar aligning the mount, I also aligned the finder on Polaris. It took me a while to get the controls of the mount figured out, but shortly I was viewing Mizar. Well, yes , Mizar was in the FOV , but I could not achieve focus. I needed the 100 mm extension and then focusing was a snap. It then became very obvious that the TMB was waaay ahead of my Tak TOA 130 in the area of ergonomics ; having the finder bracket on the rotating focuser allowed me to have the finder scope in an absolutely perfect position at all times. The dual range focusing is very, very nice! Score a big one for the TMB right off the bat. Was Mizar split? Hell yes! The stellar images were needle sharp at 40X.

Albireo was my next target, and the TMB yielded my best-ever view of my favorite double star. The colors were a beautiful contrast (as usual), but the absolute perfection of pinpoint stellar images with a DARK sky was surreal! Epsilon Lyrae was well positioned and became the next object on an increasing difficulty scale. Even tho' my skies were not ideally stable, the Double-Double resolved well at 117X using my Nagler 12 Type 4 ep. I needed perfect seeing (Wyoming standards) to see the split in my TOA 130. Since I was already in the neighborhood, I switched over to Vega, and tried (in vain) to see any evidence of a halo or false color. Once again, the contrast was "knock your sox off". At that point, the evening twilight was past, and time for some DSO fun. My first DSO was M57, the Ring nebula. I achieved an excellent view at 140X with a SMC Pentax 10 mm XW. I could actually see the "Ring" more as a bass-clef ÔC', than as a smoke ring or donut. Quite a bit of textural detail was visible. On to M13 , the great Hercules globular cluster! My TOA 130 was able to resolve quite a number of tiny stars in this object, but the TMB really was starting to show it's potential here. The Nagler 17 Type 4 was the best overall ep for resolution and brightness on this globular cluster. The cluster was extensively resolved across the face, and many peripheral stars seemed to resemble a swarm of glowing gnats around a brighter core even with direct vision. I finally tried a bright galaxy, M104. At my latitude the Sombrero galaxy never gets very high above the horizon. Even so, the dust lane was faintly visible.

Throughout all of these initial observations, it was apparent that the collimation was right on the mark, and all of the stellar images snapped into pinpoint focus simultaneously. A slight de-focusing showed all of the images as round across the entire field of view; not a trace of astigmatism was present. I have not "star tested" the telescope a la Suiter, since I do not believe myself and my old eyes capable of discerning a 1/8th wave objective from a 1/6th wave or such. I leave that to the self styled "experts".

Subsequent observations:

Over the next several months, I used the telescope every clear night. I continued my pursuit of the Messier objects and globular clusters. M4 in Scorpius was extensively resolved; the "bar" seen in small telescopes resolved into a chance alignment of 8 to 12 stars (seeing dependant) at 117x. M22 was well resolved into stars even at only 40X. M17: the Horseshoe, or Omega nebula, resembled an inverted numeral "2". All of the Sagittarius Messier globular clusters were easily in magnitude reach of the TMB 175, although some of them could not be resolved. I completed the entire Messier list and also landed quite a few Caldwell objects by early Autumn, 2004.

Update:

As of January 2005, I have observed the companion to Rigel, split many difficult double stars, and observed several other Caldwell objects satisfactorily (NGC663,theSaturn Nebula, and the "Blinking" Planetary). The performance on many of the brighter galaxies, M81/M82, M51, and the M31/M32/M110 group was excellent. I haven't had the appropriate combination of clear, dry, and moonless conditions to make an adequate statement about either M33 or M74. The view of the Orion Nebula, M42/M43 was absolutely breathtaking! I was finally able to see Theta-1-Orionis "F" in the Trapezium after several years of frustration with other telescopes. Also, I was able to resolve NGC2158, the open cluster visible in the same field as M35. Misty appearing NGC7789 in Cassiopeia dissolves into a brilliant dust of diamonds and sapphires against a fantastic star field at 82x in a Nagler 17 T4. Wow!

After a slight "planet drought" at the end of 2004, I finally was able to attempt Saturn with the TMB 175. Unfortunately for me, this is the worst seeing time of the year (December). In moments of good seeing (comparatively) I have easily observed the Cassini division, 5 moons, and a orange-brown darkening at the polar region and several tan cloud bands. The contrast was excellent and the position of the rings in-front of and behind the planet were obvious and sharp. I am not an experienced planetary observer, so I will stop here.

Cold weather performance is also proving to be excellent, and there is no indication of any problems caused by pinched optics at temperatures down to 5 degrees F. I think the scope is able to go lower than that , but I can't!

Summary and Conclusions:

The TMB 175 is an expensive instrument, but I consider that fact secondary to the performance obtained. Image sharpness, contrast, and resolution of detail were found to be absolutely first class. The light weight and retracting dew shield allow occasional field use; the telescope seems to be reasonably portable, and one person can manage setting up and taking down. A comparison with the Takahashi TOA 130 indicated that the optical quality of the TMB was equivalent or superior, and the ergonomics were far better. The dew shield seemed to work very well in preventing both dew and frost from forming on the objective. The only weakness of the TMB is really a nit-pick: the black paint used by APM on the dew shield and tube rings seemed to be a bit brittle and chipped off easily. Any bumping of the white tube in the dark while installing the scope in the rings resulted in black marks on the tube.

What I liked about this telescope:

1. Superb uncompromising optical quality.

2. Excellent mechanical components.

3. Wonderful ergonomics.

4. Performance!!!

What I disliked about this telescope:

1. Having to stop using it and put it away!

2. Black paint on rings seems a bit soft and chips easily. (Not really the telescope!)

Would I recommend this telescope to a prospective buyer? Yes. But at the time of this writing, APM has announced that the TMB 175 f/8 has been discontinued and is to be replaced by a new TMB 180 f/7 lens and tube system; only a few remaining TMB 175 f/8 scopes remain available.

I conclude by saying that the TMB 175 f/8 APO is in a class by itself. To the best of my knowledge, there are no other equivalent aperture apochromatic refracting telescopes in serial production anywhere else. There are a few older A-P 180 telescopes sometimes available on the used market, but these instruments command astronomical prices (pun intended).




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