Side view of the TMB Optical 175mm f/8 apochromatic refractor showing the 3.5" Feather Touch Focuser, the CNC mounting rings, and dew cap.

Overview

On May 2004 I took delivery of a TMB Optical 175mm (7") f/8 refractor. This is the first telescope I have purchased from TMB Optical. To provide some background information on these telescopes Thomas Back designs the Air Spaced SD Triplet lenses, which are then manufactured, multicoated and tested using an interferometer by LZOS, a Zeiss subcontractor, in Russia. The lenses are and shipped to APM Markus Ludes in Germany. Markus tests the lenses, and has the lens cells, optical tube assembly (OTA), CNC focuser's, and CNC mounting rings manufactured. Customers may pruchase telescopes from Markus Ludes or Thomas Back.

Note there are two versions of the 175mm f/8 available, a light weight (LW) versions that weighs in at 42 lbs, and a heavier CNC version that weighs around 5 lbs more. In general there are LW versions available for most of the other refractor telescopes in the TMB line. If the customer orders a CNC version of the OTA Markus Ludes ships the CNC focuser to Thomas Back. If the customer orders a LW version Thomas Back purchases the Feather Touch Focuser from Starlight Instruments.

Once Thomas Back receives all of the telescope components he assembles the telescope, and performs final optical and mechanical testing and quality assurance. This includes testing the telescope in the shop using an autocollimator and star testing under the night sky, which must pass his high requirements before he ships it to the customer.

Telescope:

The 175mm I received is one of the newer LW versions. It came with a 3.5" Feather Touch Focuser, CNC mounting rings, 8x50mm finder bracket, Intes focuser extension and 2"-1.25" eyepiece adapter, sliding dew cap with dust cap, and an inner dust cap. An owner's manual is provided as well.

The telescope arrived well packed in the shipping box with plenty of Styrofoam peanuts to protect the telescope during shipment and wrapped in a plastic bag. As I took the OTA out of the box I was impressed by the fine quality of the workmanship. Everything on it has a nice solid feel to it. The mounting rings come pre-drilled as well for mounting cameras, guidescopes, or other accessories.

The length of the telescope with the dew cap retracted is 48" long. With the dew cap fully extended the telescope is 58" long. In the above photograph the dew cap was almost fully extended and for a comparison the home made wooden box the telescope is shown sitting on is 27" long. The outside diameter of the OTA is 8.5", while the outside diameter of the dew cap is 10".

I noted when looking inside the OTA that there are five baffles spaced by three aluminum bars. Another baffle is located near the front of the dew cap. In addition the inside of the OTA and the dew cap is lined with black flocking or velveteen paper which helps to reduce light scattered and increase contrast.

I used flocking paper in a telescope I made many years ago for the same reason after reading an article by Richard Berry in the Winter 1983 issue of Deep Sky Magazine. The article was entitled "Of Stellafane, Optics, and the Art of Deep-Sky Observing", and in it he recommended ways to reduce light scatter and increase contrast a telescope so they would perform better when observing deep-sky objects. This included smooth, high quality optics, the telescope having efficient baffles, a roughened tube interior (in this case flocking paper), and an extra long dew cap that extends beyond the objective. In the TMB 175mm the dew cap extends 11" beyond the objective, which in addition to helps reduce the likelihood of dew forming on the objective. The smoothness of the optical surface is important because any surface roughness or micro ripple left over from when the objective is being figured, which by its nature leaves "texture" on the glass, results in more scattered light and reduced contrast.

After observing with the telescope I noted that it seemed to have very low light scatter and high image contrast, more so than I have noted in other telescopes. I surmised that the baffles, flocking paper, and longer dew cap contributed to this, but wondered if there might be other factors as well. So I corresponded with Thomas Back to see what other factors may play a role. He indicated that the figuring process used for the lenses were very smooth, which helped to reduce scattered light, and increase contrast. Also, the baffle assembly was designed so that any stray light that entered the OTA does not reach the focal plane and reduce contrast. The combination of reduced scattered light and increased contrast puts more energy into the airy disk and produces a brighter image. This is important not just for deep-sky observing but when observing the Moon and planets as well, as it will make it easier to see fine detail.

The 3.5" Feather Touch Focuser, with the 8x50mm finder bracket mounted on top.

As noted my 175mm LW came with the 3.5" Feather Touch Focuser. While I considered this to be more useful for those who do imaging in practice I found it very useful for visual observing as well. It has very smooth focusing, and the fine focus adjustment made it very easy to focus the telescope at high power, even when wearing gloves. Also the entire focuser assembly can be easily rotated while observing. I found this particularly useful when observing the Jupiter and the Moon, as I find it easier to concentrate on the fine detail when their orientation is correct, such as Jupiter's belts and zones being parallel to the line of sight. Also as the focuser is rotated the finder scope is rotated also, keeping it at a convenient angle. In addition the focuser has a gradated scale on the focuser barrel which makes it easy to note where my eyepieces reach focus and will be useful for future observing sessions. The Feather Touch Focuser doesn't use a set screw to hold a star diagonal or binocular viewer but rather a compression ring that is tightened by hand which works much better than a set screw.

I spent a couple hours testing the 175mm the first night, and six hours the second night. This gave me the opportunity to try it out on a variety of objects, including the Moon, Jupiter, stars, and deep-sky objects, using both a homemade Dobsonian-style mount and a German equatorial mount. Since then I have observed with the telescope on other nights and have added links below to these observations.

The 175mm f/8 lens with the dew cap retracted.

Optical Performance

Moon

One of the first things I noted when observing the Moon was how well controlled the light scattering was in the telescope. For example the sky next to the lunar limb was jet black. In addition the high contrast and sharpness of the lens made it easy to see subtle lunar detail such as the ray systems extending from craters including Copernicus across Mare Imbrium, as well as the wrinkle ridges and different shades of the lava flows across the various Mares at 60x. Isolated peaks in the northern portion of Mare Imbrium such as Montes Recti, Montes Teneriffe, and Mons Pico appeared to be 3-D. The eroded wall of Clavius was prominent also. I noted too that the telescope was able to hold detail in lunar features far away from the terminator and under high sun angle at magnifications of 80x and 100x. For example I could see small craters in Palus Somni (which is next to Mare Crisium near the eastern limb of the Moon) when the Moon was just a few days from being full and the terminator was near the western limb. By the way for those observers who don't have a copy of Rukl's lunar atlas I found that a lunar map produced by Rukl and sold by S&T was very handy in identifying lunar features at the eyepiece.

Jupiter

The seeing was only fair when I observed Jupiter and it was only around 35 degrees in elevation but I noted that it all four Jovian satellites were defined as disks and had noticeable differences in color even when using magnifications of 60x. For example Ganymede appeared to be the largest and brightest, while Callisto the dimmest. Also Jupiter was near eastern quadrature so its 99% phase was noticeable, as was limb darkening on the side of the globe. Higher magnifications helped to bring out subtle detail in its belts and zones, including festoons along the North Equatorial Belt south (NEBs), as well as ovals within the NEB.

Deep Sky Objects, Double Stars, and Bright Stars

To round out my testing I observed a variety of deep-sky objects including M4, M8, M11, M13, M17, M27, M31 and M32, and M57, as well as Alberio and Altair. While the transparency was very good during these tests, the limiting magnitude was only around 4.5 due to interference from bright moonlight as the Moon a few days from full phase. For M8, M17, and M27 I used an OIII filter to help bring out fainter detail that was being washed out by the bright moonlight.

M4 - The linear shape to the central portion of this globular cluster was well resolved even at 45x, as were the stars around the outer edge.

M8 - The shape of the Lagoon Nebula was well resolved at 54x as were brighter areas within the nebula and stars embedded in it.

M11 - This open cluster, also known as the The Wild Duck Cluster (although it reminds me of the shape of the Music Television or MTV symbol so I always think of it as the MTV Cluster), was well resolved at 45x, and the color of bright nearby yellow star was very pronounced.

M13 - The stars across the core and near the edge of this globular cluster were well resolved at 88x.

M17 - The shape of the Swan or Omega Nebula was very pronounced as the nebula appeared bisected by dark rifts with lighter areas. Also several stars appeared embedded within the nebulosity at 70x and 88x.

M27 - The Dumbbell Nebula showed many variations in brightness and tone at 100x, as well as a couple of stars embedded within it. The football shape extension to the nebula was visible as well.

M31 and M32 - Even though the Andromeda Galaxy and M32 were only 25 degrees in elevation when I observed it at 25x they both looked nice and showed some detail.

M57 - At 34x the Ring Nebula was resolved including the lighter inner portion of this planetary nebula. At 100x the nebula appeared elongated at both ends and bright.

Alberio - At 34x the colors of this double star were very pronounced and the stars well resolved.

Altair - Light scatter was very well controlled around Altair even at magnifications of 200x.

An observing report of deep-sky objects and the Moon using the TMB 175mm was added in August 2004, and an observing report of deep-sky objects, Venus and Saturn using the TMB 175mm was added in October 2004. Also a sketch of Comet Machholz, as well as an observing observing report of the comet using the TMB 175mm was added on January 3rd, 2005. An observing report of deep-sky objects, Moon, Saturn, and Jupiter was added on April 4th, 2005, as were sketches of Mars on September 25th, 2005 and October 22nd, 2005 showing a dust storm. A sketch of Comet Pojmanski was added on March 5th, 2006.

Summary

Overall I have been very impressed by the performance of this telescope. Mechanical features such as the Feather Touch Focuser makes it very convenient to observe with and aids in obtaining sharp focus at high power. Also the very fine quality of the lens, coupled with very low light scatter, high image contrast and brightness, has shown me detail on the Moon I have not seen before in over 30 years of observing, as well as the promise of fine detail on the planets and in deep-sky objects under more favorable viewing conditions. All of this leads to the telescope delivering more performance than I expected for its 7" aperture. Highly recommended.

Eric Jamison has been interested in astronomy since the 1960's, when, as a child, he witnessed two total lunar eclipses. You can find a lot of good information on his website, where he provides equipment reviews and observational reports . Eric has also written about the history of astronomy in America, and he devotes space in his site to define the various types of optical systems for people new to the hobby. It's a place worthy of saving in your favorites.

Eric Jamison's Astronomy and Photography Home Page

Back