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Zeiss Telementor

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Zeiss Telementor

By PJ Anway

Figure 1. (Presented with permission of Carl Zeiss)

The Telementor refractor is a telescope that many amateur observers have heard about, but relatively few have actually owned. It has a well-deserved reputation for being a quality refractor and that is not surprising, due to its pedigree - Carl Zeiss. Zeiss is known for producing quality instruments in almost every field involving lenses. I have owned five different Zeiss Telementors over the years and have not been disappointed by the performance of any one of them.

My reason for writing this article is because the Telementor is relatively rare in the US and this has resulted in a bit of misunderstanding of the different models and their features. For instance, recently, on one Internet auction site, a well-known and experienced telescope dealer had to pull his ad for the sale of a Zeiss Telementor on three different occasions. The reason? He kept misstating information about which model he was actually selling. After that, I thought an article might be of help. Let me start with a little history.

In a way, the Telementor got its start in 1949 in Germany. In that year, its predecessor, the AS 63/840 refractor, was first produced. The "AS" stands for ""Astro-Spezialobjektiv" or "Astro-Special" Objective. Though considered an achromat, the "special" part of the AS objective came from the Steinheil design lens with a KzFN2/BK7 lens configuration. The Zeiss AS lens is a "flint-leading" doublet using the KzFN2 ''short flint". By using this lens design, the chromatic aberration (color error) is helped somewhat. In his landmark article in Applied Optics (entitled "Planetary Telescopes"), James G. Baker termed the Zeiss AS lens a "semi-apochromat". I have owned one of these scopes and I felt it did show some improvement in color correction over the traditional achromat. The AS 63/840 was advertised as the "school and amateur scope" but this is not the telescope referred to when we speak of the Telementor. That scope would not be seen on the market until the 1970's.

The Carl Zeiss Jena refractor that came to known as the "Telementor" would make its appearance around May of 1972. It was produced with the idea of placing one in every secondary school in the Republic of Germany, thus the title - "School scope" or "Telementor". This version of the refractor would be a completely revised edition from the AS63. It would house a typical Fraunhofer design doublet objective - an achromat consisting of crown-flint lens elements BK7/F2. It would also be a much more compact cell design than that of the AS lens and Zeiss would be give it the designation "C63/840".

Obviously, objective would also have a diameter of 63mm and a focal length of 840mm (f/13.3). Even though this lens is inferior to the AS lens as far as color correction, maintaining the long focal ratio of f/13 would enable the Telementor to show only a small amount of chromatic aberration on bright objects. As with all Zeiss lenses the excellent figure and polish makes it a great performer. The lens I have in my Telementor gives sharp views with excellent contrast and allows for magnification exceeding 80x per inch.

Over the years the Telementor has been basically the same telescope built in 1972, with just a couple of changes to the body of the OTA:

  1. The Telementor-I came with a helical rear focuser. The Telementor-II and Telemator came with an internal focusing system (I'll discuss the focusers later).
  2. The Telementor-I and Telementor-II did not come with a finder scope. The Telemator was equipped with a 7.5 x 42 finder scope.
  3. The Telementor-I and Telementor-II came on the standard equatorial “T-mount”; the manual mount pictured above. The Telemator came on a motorized “TM-mount”.
  4. The exception to these rules came with the last installment of the Telementor in 1995, which marketed as the “C63/840” and came with a finder scope and gave the purchaser the choice of a “T” or “TM” mount. It is the scope pictured above.


Now for the features that all Zeiss Telementors and Telemators have:

  1. the C63/840 lens and cell
  2. the one-piece tube that extends a few inches beyond the lens cell forming a dew shield
  3. the "sighting device"
  4. the M44 rear threads
  5. the dovetail bar

Let's look at each of these on the Telementor-I that I currently own.

Figure 2. My Telementor-I on a Zeiss TM mount

Figure 3. (1) the C63/840 lens and cell

This is the heart of the Telementor, the C63/840 lens and cell. A cemented doublet achromat, it is nicely tucked into a compact cell. Though common to all Telementors, it is placed differently into the OTA's of the Telementor-I and Telemento-II/Telemator. In the Telementor-I, two small screws inserted into the OTA hold the cell in place. It can be removed for cleaning, but is somewhat tricky to reinstall. In the Telementor II the lens cell is screwed onto an inner tube (more on that later). The label is the famous Zeiss Jena logo – with the name “Carl Zeiss Jena” in an achromat doublet lens outline.

Figure 4. (2) an OTA that extends a few inches beyond the lens cell forming a dew shield

The OTA for the Telementor-I extends about 3-1/2" beyond the lens cell forming in effect a dew shield. This makes for a classic look that appeals to me, but without a sliding dew-shield, it does not allow any shortening of the tube for travel.

Figure 5. (3) the "sighting device"

All Telementor OTA's have two small "peep holes" on the top of the OTA. When aligned by sight, the tiny section of sky seen through these is where the OTA is pointing. These are surprisingly effective in accurately pointing the refractor, in my opinion as good as red dot finder. The one drawback to them is that under very dark skies, the front sight can be difficult to see.

Figure 6. (4) the M44 rear threads

Zeiss maintained a unique way of attaching accessories to their refractors. It is called the M44 thread. M44 comes from the fact that the diameter of the thread is 44mm.

All of the Telementors (as well as many AS and APQ scopes) use this thread to mount just about everything - including:

Figure 7. Zenith prism diagonals and eyepiece turrets

Figure 8. Barlows

Figure 9. Extensions in lengths of 20mm to 80mm

Figure 10. Eyepiece adapters

Zeiss made a .965" eyepiece adapter (right) for the Carl Zeiss Jena .965" ortho and mono eyepieces. Also, many after-market eyepiece adapters were later made for 1.25" and 2" eyepieces. All attached using M44 threads.

Figure 11.

Changing from one threaded accessory to another at times can be a pain, when it requires you to unscrew one accessory and screw in another. To alleviate this somewhat, Zeiss (and several after-market suppliers) came up with "quick-change" adapters. These make changing accessories much easier. It also doubles nicely as a camera angle adjuster.

Figure 12. Here is pic of the changer in position.

Figure 13. (5) dovetail bar

Most Zeiss refractors have a dovetail bar bolted to the bottom of the OTA. These vary in length depending on the scope, but are consistent in width and angle. The Telementor dovetail is 9” long by 1-3/8” wide. It also has pin at each end to prevent the scope from accidentally slipping from the mount clamp in the dark.

(An additional note: Astro-Physics makes a dovetail clamp (DOVE08) that fits the Zeiss dovetail perfectly. This makes it possible to use Zeiss refractors on other non-Zeiss mounts.)

That covers the similarities of the Zeiss Telementor design. Let’s next take a look at the main difference, the focusing mechanism.

Focusing mechanism

Figure 14.

The Telementor-I uses a helical focuser at the rear of the OTA. It is a little over 2-1/2” (65mm) in diameter and has a travel of 1-3/8” (35mm). I personally love this design. The one on my Telementor-I is very smooth and brings all my eyepieces to focus. Though I am sure there are eyepieces out there that the 35mm of back-focus might not accommodate.

Figure 15.

With the Telementor-II and Telemator, Zeiss removed the helical focuser. In fact they did away with a rear focuser altogether and replaced it with a very unique design.

The diagonal (and therefore eyepiece) is screwed to the rear of the OTA. It remains stationary, while the objective is moved back and forth to obtain focus. A second tube inside the OTA accomplishes this. The objective cell it attached (screwed) to the front of this internal tube and a knob on the side of the OTA moves the internal tube along slides located between it and the OTA. This focusing system works very well and is also smooth, allowing close to 100mm of travel.

Most used Telementors sold today are of this variety.

Below are pictures of the Telementor-II and Telemator for comparison. In it you see the focusing knob that moves the inner tube (and objective) on the side of each OTA.

Figure 16. (Presented with permission of Carl Zeiss)

The features of each refractor are noted in this ad from the early 1990s:

Figure 17. (Presented with permission of Seiler Instrument and Manufacturing Inc.)

Some additional notes about the mounts:

  • The T-mount is a manual equatorial mount, with slow-motion knobs. These can be turned to track the scope for about 30 minutes. After that, they have to be reset by loosing another locking knob.
  • The TM-mount: tracks automatically with a motor. However, the motor that drives the mount is the European 220v 50hz power standard and will not track correctly on the US 60hz power standard. You have to purchase a voltage converter to use it in the US.

The last note is about labeling. Zeiss did not label the Telementor OTA for years. Most have no labels, but it isn’t hard to recognize the unique look. However, I have seen a couple of labels from OTA produced in the 1990’s. The first is the label from a Telementor II I used to own.

Figure 18.

I contacted Dr. Wolfgang Wimmer from Zeiss about this label and he dated its use to 1990 and 1991. The only other label I have seen is the “Zeiss Germany” in the box pictured on the brochure pictures above.

The Telementor is a unique refractor that beckons back to days of old, when a “60-something”mm refractor was a common choice of many amateur astronomers. It is still a wonderful instrument that works very well for the moon, planets and double stars, my main targets. It has also achieved a following that is well deserved. In a tongue-in-cheek article by Jay Freeman entitled: “You know you are a planetary observer when...”, one of the lines states: “You know you are a planetary observer when, you actually own a Zeiss Telementor”……… enough said.

  • ngc2289, starmason, eyekazoo and 11 others like this


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