Tech – The Telescope Mount

Each telescope needs a mount. In our case, this is a self-built equatorial fork mount. It is called “Österreich Neu” or ÖPFM, and was developed by Rudolf Pressberger.  He also developed the telescope housing which is an integral part of the complete mount. This makes the mount very stable and effective. The downside is, you can’t use other telescopes with it.

Only a few of these mounts are produced till this date, and the LAG owns two of them: the public telescope in Linz located at the Johannes Kepler Observatory and now the KRO telescope.

pressberger
The KRO telescope on its old location (Observatory Davidschlag) shortly after mobilization in 1999. The Photo shows (from left to right) one of the owners, Erich Meyer, the developer of the mount/telescope, Rudolf Pressberger and the second owner Erwin Obermaier.
lndm_stw_600px
The telescope (50cm f/10 Cassegrain telescope) at the Johannes Kepler Observatory in Linz during a public viewing. (Photo by Herbert Raab).

What is now so special regarding this mount? Basically it was designed to be built by one self – surly with some skill in welding and general metal works – and for precision.

Pressberger developed two versions, one more conventional one, with worm gear drive and rolling contact bearing and one refined version with friction wheel drive and special Teflon ball joint bearings. The second version is the one we use for the KRO and the Johannes Kepler Observatory.

The basic advantages of the version 2 mount are:

  • The special ball joint bearings are superior in terms of friction compared to really high quality pendulum rolling contact bearings. A nice side effect is, that you can easily make them by yourself.
  • There is no conventional RA axis, with its usual big and expansive rolling contact bearings, attached to the fork. The big friction wheel itself will become a part of the bearing. Therefore, you do not have the usual eight-part cantilever strain of the RA axis.
  • The weight distribution of the mount and tube is optimized for low vibration.
  • Friction wheels are generally not so easy to handle, but in combination with the special bearings mentioned above, they can be used successfully.
  • The necessary contact pressure of the drive shaft on the friction wheel is applied in RA of the weight of the fork and the tube. Thus, the remaining cantilever load of the hour axis is used meaningfully.

 

coll_teleskop_neu_bau800
The 50cm telescope of the Johannes Kepler Observatory during construction (around 2000). It is similar in construction to the KRO telescope. Only the fork width and mirror cell are a little bit bigger in the KRO since its main mirror is also bigger.

The main disadvantage of the mount are its, “virtual” axis. Due to the special construction of the bearings with ball joints, you do not have to use right angled axes (in relation to the fork and tube), but instead you can use angled (thus “virtual”) axes. Unfortunately this makes an update we have planned for the KRO more complex and more expansive: adding absolute encoders.

We can’t use encoders you simply connect to the rotating axes, thus we do not have such. We have to use quite big ring encoders mounted directly on the RA friction wheel and the tube (DEC). We will discuss the usage of encoders on the future in a separate post.

So one can ask now, why the heck to use such unusual axes? For the DEC axis it is mainly to provide an easy adjusting mechanism to adjust the orthogonality error.

And for RA axis (which is also hold by a ball joint bearing) it was mainly done to overcome the big and expansive rolling contact bearings. Due to the special bearing construction, the friction wheel itself becomes part of the bearing, and minimizes, for fork mounts typical cantilever load, to nearly zero.

Another advantage of the ball joints (for RA and DEC axes) developed by Pressberger, are the materials they are built from. The balls are hard chrome plated steel balls, sitting in a Teflon bearing cup. This type of bearing looks like an artificial hip joint. The pair of materials (hard chrome plated steel balls, Teflon) make up a really tight fit, since the Teflon, at high bearing pressure, bends around the ball (due to the flexibility of Teflon) and reduces the friction to a minimum. The ball more or less, swims on the Teflon. And there is another advantage: the initial breakaway torque, which is quite a problem for friction wheel drives (slip), will be reduced to zero.

The pole block shows, that the mount is clearly designed for stationary usage. Not only due to the weight of the whole thing, but also due to the static angle of the pole block. The angle of the pole block should be 90° – geographic latitude and is welded in this way. But there is the possibility to adjust the mount with four push/pull screws in the pole block. So the mount/telescope can be oriented exactly to north and geographic latitude.

Current Problem with the mount:

During inspection and testing at Davidschlag, we noticed that the friction wheels are corroded on their bearing surface. The surface there is quite the same as the ball joints: hard chromed plated.

Removing the chrome from the steel wheel is quite difficult (turning it down) and will result in a smaller friction wheel which will not fit anymore.

So we decided to rebuild the two friction wheels (RA and DEC axes) completely and scrap the old ones. The positive side effect: we can also turn the required encoder mountings directly on the new friction wheels.

But all in all, the mount and the telescope are in quite a good shape.

After finalizing the last tests in Davidschlag, we will dismantle the whole thing. We will report in a future post about this major project milestone.

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