Tech – The Telescope

The telescope and mount (as described in Tech – The Telescope Mount) are not new. The telescope was built around the turn of the millennium (1999-2000) and was since used in a small observatory north of Linz.

The telescope was mainly used to track and hunt minor planets. Some new minor planets were found with this telescope from.

The LAG (Linzer Astronomische Gemeinschaft) got hold of the telescope in early 2018. Short after obtaining the telescope, the work on the Kepler Remote Observatory began.

But before we come to the tech of the telescope let us talk a little about its history.


The observatory in Davidschlag already existed since 1978, was built by Erwin Obermair and Erich Meyer, and was equipped with a 30cm f/4.5 newtonian reflector till 1983. From 1983 on it was equipped with a 29cm Schmidt-Cassegrain telescope before the new computer controlled Cassegrain telescope was installed in 1999.

The observatory was listed in the IAU observatory catalogue with the number 540 Linz. Quite a few minor planets were discovered at this location (find the complete list here). The observatory also played a role in refining the impact trajectory of the comet Shoemaker-levy-9.

This is the first light image of the telescope (without the corrector installed) taken in Davidschlag on October 26, 1999 by Erwin Obermair and Erich Meyer. The image shows the Dumbbell Nebula, Messier 27. The image is a single shot 60sec exposure, captured with a SBIG ST-6 CCD.

Between 2005 and 2016 the telescope was not really used. In 2016, the LAG started a project to revitalize the observatory and the telescope itself. The project was called LAGST540 (LAG Starlight Team 540). Shortly after starting this project we got the opportunity to join the Star Park Hohe Dirn project and we decided to change the project goals: 2018 the KRO project was born.


First of all, to be on the same page, we refer with tube to the white part of the telescope. The blue part is the fork mount and was discussed here. The tube holds the main mirror, the corrector and the camera.

The telescope during a night session. You can see the white tube and the blue fork mount. (Photo by Erwin Obermair, Erich Meyer)

The mount and the tube were designed together by Rudolf Pressberger, so they fit together perfectly and therefore the telescope can’t be used with another mount and also the mount can’t be used with another telescope.

The tube was built from light weight steel panel. A clever internal design makes sure, that the tube is also very stiff. The spider ring (holding the Keller corrector and the camera) is also built from light weight steel panels and is held in place by hollow steel tubes following the principle of a Serrurier truss. Only this design in combination with the very stiff spider ring, prohibits the tilt of the optical axis during a position change of the tube.

On the side faced towards the mirror cell, a similar construction as the Serrurier truss facing the spider ring, was used. This made it possible to bring the mirror cell as close as possible to the load-bearing part of the tube and thus to the DEC axis.

Due to this yet stiff but light weight construction of the tube, it only weighs a little bit over 100kg – of course without the mirror cell (the round assembly at the bottom of the tube, see picture above) and mirror.

Mirror and mirror cell

We are using a mirror cell which supports the mirror in axial direction with a 9-point whiffletree design invented by an Irish optician called Thomas Grubb. The radial bearings for the mirror are three temperature compensated ball joints which support the mirror on six sliding surfaces on the outer rim of the mirror. Due to this construction, the mirror can be centered and fixed to the optical axis. The ball joints provide free axial movement.

The studs holding the ball joints are made of zinc to compensate the temperature expansion of the mirror cell. This special construction prohibits the deformation of the mirror but locks the mirror into position, so that it can’t slide out of place.

The telescope lacks of a secondary mirror, instead there is a corrector and the camera is directly mounted to the corrector. So the camera is in prime focus which (in combination with the corrector) results in the quite fast aperture ratio of f/3.3.

A prime focus telescope design. The observer/camera + corrector is at the focal point (shown as a red X). (Image on Wikipedia by Oleg Alexandrov, CC BY-SA 3.0, Link to the image)

Another advantage of the prime focus design is, that the mirror do not need a hole in the center to be able to output the light path at the back of the telescope.

The disadvantage of this design is, that the spider ring needs to be able to take a lot of a load (Camera + quite large corrector). Luckily the spider ring design by Pressberger is really stiff and can handle this load with ease. And also the rest of the telescope design has to be stiff to minimize tube bending.

Keller corrector

Since the telescope is a standard Cassegrain type, we need some sort of correction due to the fact that a Cassegrain telescope tends to introduce distortions to the image. This is due to the fact, that the mirror is spherical not parabolic. Another important point for a telescope used only as astrophotography tool: the image area should be flat.

In our case this is achieved with a corrector mounted in the spider ring. It was designed by Philipp Keller, a physicist specialized on optical components. The corrector eliminates the coma of the optical system and also flattens the image (this is very important for cameras with a big sensor area).

Our corrector also incorporates the focus. This is done via a slide in/out lens at the camera faced side of the corrector. This focus is motorized and will be described in a future post.


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