ObservatoryMain telescope

Main telescope


The main telescope of the Hans Haffner Observatory is the Astrograph CDK 20 from PlaneWave-Instruments with a primary mirror diameter of 50 centimeters. The telescope is carried by a GM 4000 HPS II mount of 10 microns and is complemented by the apochromatic lens telescope ED 120 from Sky-Watcher with an aperture of 12 centimeters.

In contrast to microscopy, magnification is not as important for the telescope. Instead, the light collecting surface of a telescope is usually of greater scientific importance because many astronomical objects appear extremely faint when observed from Earth. The larger the diameter of the mirror or lens, the brighter the astronomical object appears and the more details can be seen.

For example, the CDK 20 has a light collecting surface that is about ten thousand times larger than the human pupil.An astrograph is a telescope that is optimized for astrophotography. It produces the largest possible, flawless and flat image, so that the flat camera chip is illuminated as evenly and distortion-free as possible up to the edge.

© From ArsMechanik, Fabian R: https://de.wikipedia.org/w/index.php?curid=1834874
Ray path in a Cassegrain telescope. In the CDK, the correction lenses are located in the beam path just before the focal point.

The CDK 20 from PlaneWave Instruments is a Dall-Kirkham modified Cassegrain telescope with an additional corrector (CDK: corrected Dall-Kirkham).

A Cassegrain telescope is a reflecting telescope, i.e. a telescope in which the objective is a concave mirror. The incident light is reflected and bundled by this primary mirror (also called primary mirror) and directed to a second, smaller mirror (secondary or intercepting mirror). This mirror directs the bundled light through an opening in the primary mirror onto the camera chip or into the eye of the observer.

In the CDK, the primary mirror is elliptical and the secondary mirror is spherical. In addition, there are two correction lenses in the beam path in front of the focal point, which ensure good imaging quality outside the image axis. In this way, an image that is as error-free as possible is also achieved in the peripheral areas of the CCD chip.

The mount not only supports the telescope but also compensates for the rotation of the earth so that the telescope remains pointed at the same point in the sky for as long as possible. This compensation, called tracking, is achieved by very precise stepper motors and high-precision mechanics. Modern mounts also have a so-called Goto feature, i.e. they can drive to saved celestial objects automatically.
The GM 4000 HPS is a high-performance mount, which is characterized by high positioning speed and accuracy. At the tip, it positions the telescope at a speed of ten degrees per second and has a positioning accuracy of less than twenty seconds of arc. Absolute encoders in both axes align the actual positioning with the required one. From this, the control computer calculates the necessary corrections in the tracking system. As a result, this mount achieves a better tracking accuracy than one arc second in fifteen minutes.

The Apochromat, which is additionally supported by the mount, serves both as a guide tube and as a telescope that can image a much larger section of the sky than the reflecting telescope.

Installation of the new mount (GM 4000 HPS II)

Installation of the new mount (GM 4000 HPS II)

With simple glass lenses, the incident light rays are refracted to different degrees depending on their wavelength, which is reflected in color fringes and blurred imaging, the so-called chromatic aberration. With an apochromat, these color errors are corrected for three wavelengths by cleverly selecting different types of glass for the individual lenses, so that hardly any chromatic aberration occurs. In addition, the ED 120 with its 120 millimeter aperture has good light collection properties.

The CDK 20 was initially mounted on a GM 4000 QCI. Thanks to the generous support of Baader-Planetarium, this mount could be replaced by the GM 4000 HPS II in 2018. This now enables many objects to be approached and recorded quickly and precisely in one night. On a winter night, the "AGN Monitoring" project can now measure the brightness of up to twenty active galaxy nuclei.