This mount was inspired by a photograph in the December 1987 issue of "Sky and Telescope". When I saw the picture on the bottom of page 594, I was quickly taken with the idea and I knew I would one day build a scope like it. The Brown's built a mount that always places the eyepiece at one height. So, when our cottage Observatory became in need of a telescope, I set out to build a mount based on this design. A test scope using the tube from an 11cm f8 (a Tasco on parabolic steroids) was used and when this passed with flying colours, plans were drawn up for a 20cm f6. The mirrors are works of art by Barry Arnold of Edmonton RASC Centre.
Here are some pictures of the project during construction:
The first task was to cut and fit each part. The scope was fully assembled and prepared for construction. Then the scope was taken apart. Reassembly involved the use of a fast setting West System's epoxy. This meant that the project was broken down into assembly groups. Since the working time for the epoxy was around 10 minutes, each step had to be carefully planned so as to maximize the use of the epoxy.
The scope was assembled on a back plane made from 19mm plywood. Then the baffles were attached to plane to form the frame for the door-skins. Finally the interior was spray painted flat black and the door skins were epoxied into place. Here is the entire unit assembled on the backplane with half of the door skins epoxied in place. The small circular hole on the top is where the focuser will be mounted. The opening along the top of the scope will just be screwed in place and an access port to install and maintain the mirror is at the far left of the picture. The overexposed part of the photo washes out details of the counterbalance arm.
Sticking up to the right of the picture is the altitude counterbalance arm. It balances the weight of the tube and mirror using three gel cells and some lead shot. The gel cells provide power for the dew heaters and for the Rigel Quickfinder that is used as the primary pointing device, also dew protected. By putting the power on the scope, there are not any external cables to wind up or trip over. Here we see the scope from above in its stored position. You can see that it fits perfectly, an f6.1 or a 21cm scope would not fit into the confines of the room!
This next image shows detail of the altitude shaft during construction and the one below that shows it in the bearing in place on the pier. All surfaces are epoxied together and then pinned with both 10cm screws and a piece of 13mm dowel epoxied in place. All this was done to make sure that there was not any slop in the bearing under load:
Since the scope is completely balanced, this bearing clamp actually does very little work, it is just there to keep the scope where it is pointed. The plumbing pipe extends out beyond the counter balance arm so that lead shot can be put in the tube to balance the weight of the scope on the azimuth table. Finally, an image showing the ocular part of the scope. A 50cm helical super low profile focuser puts the eyepiece as close to the small secondary as is possible.
For more information send mail to Mark Kaye at: email@example.com
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