68cm (27") multi purpose telescope

in dobsonian configuration.

Johan Van Beselaere, Antoon Maesele, Johan Coussens, ACG

For several years now, the Astronomical Contact Group (ACG) at Ieper (Belgium) is working on a 27 inch mirror. Now the mirror is in a final stage, polished and in latest stage of figuring.

To finalise this job, there were already made several test setups in the optical lab to determine the final shape of the completely self-grinded mirror. However, the best test is still the image a mirror produces under the real sky. To accomplish this we needed a star-test-setup. Such a star-test-setup could be made in several ways: the only restriction was that it must be stable enough to get all the optics perfectly collimated.

Several options were considered, from a easy to build temporary setup to polaris (fixed approach) to a fully computerized dobsonian mount. We had already some experience in the group: two members already build several telescopes at home, the third designed a complete german equatorial mount for ACG wich is approaching its final stage of construction. The final descision was to make a dobsonian type of mount that was temporarely placed on an existing computer controlled alt-az platform. Knowing that this platform was to small for this measure of mirror, we had the advantage of starting the star-test much faster and we had the advantage of the computer controlled function for slewing, tracking and guiding an object.

The basic component: obvious the mirror, grinded bij Johan VB and collegues of ACG

First, we started cutting wood to make the side bearings of the mirror box

Then the bearings must be cut perfectly circular on the milling machine

The four wooden bearing sides were glued together to give a construction that was way more ridgid. between the wooden plates we mounted a metal framework to hold the mirrorcell

To hold the secundaire mirror and focusser, we needed a stable top ring, Antoon made a beautiful piece of work.

We could not resist, after having the basic components, to put the parts temporarely together.

Also the alt-az platform of Johan C, made for a 40 cm scope was transferred to accommodate the larger scope

The side support for the mirror were made of 2 mm diameter hardened steel.

The mirror is supported on 18 points flotation cell.

The whole construction gives us a possibility of testing the 68 cm mirror on a computer controlled alt-azimuth mount.

Finaly, the whole construction seems to be portable, however some construction parts must be redesigned to make transport more compact.




some links may be also interesting:

computer control for telescope designed by:Mel Bartels.

Astronomical Contact Group



last update: August 29, 2010