20" Ursus  - Building the Telescope

[ Home ]

dobs_tot.jpg (389958 Byte)
Figures to the telescope:

Primary mirror (self made):
Material: TERMISIL (similar to PYREX)
Diameter: 20 (508mm)
Thickness at the edge: 1,5 (38mm)
Aperture (effective): 506mm
Focal length: 93,9 (2385mm= = f/4,7

Secondary mirror:
3,5 minor axis, 96% reflection (dielectric coating)

Total weight:
Telescope: 89 lb without eyepiece, without finder scope

4,2 (106mm) Newtonian, 17,2 focal length = f/4,1; Rigel Quickfinder at the secondary cage


The components:

Primary mirror cell: made of 25mm*25mm (1 * 1 ) square aluminium pipe, 2mm (0,02 ) thick. Lateraly struts made of 5mm*25mm aluminium. It's a classical 18- point flotation cell like described in David Kriege's book, but completely made of aluminium.  I ve sawed and drilled the struts and fixed in a jig for welding done by a small company. 


All sawing was made with a simple and cheap mechanical fret saw, using a thin saw blade this is possible  If you have enough endurance. In difference to Kriege I took a thin metal sheet as sling. Left and right beside the mirror 8mm+20mm (0,3 * 0,8 ) aluminum pieces were welded at the middle strut. Through 5mm (0,2 ) bore hole the ends of the metal sling will be put. The ends are soldered screw threat pieces.  To avoid mirror warping at the inner side of the sling adhesive tape is affixed.

The geometry of the cell was optimized with the program PLOPP  from David Lewis. I m using pieces of screw threats with 8mm (0,3 ) diameter as collimation bolts, for screwing I fixed stopcock-wheels at the ends, these are lightweight (aluminum), flat and cheap. On the other side they emphasize the individual telescope. The triangles are made of 3mm (0,12) aluminum.


Altitude Bearings: Similar to the ones of PHOENIX, my 14 Telescope. This time I tool a cheap router to achieve accurate radii. The diameter of the bearings is 700mm (27,6 ), 21mm (0,8 ) thick made of Baltic birch plywood. The slide is made of 1mm (0,04 ) aluminum. I could get this at a maximum length of 1 meter (40 ) only. After some doubts I decided to add a piece. Using a good adhesive and some clamps it came to a good end.


The rings of the secondary cage are also made with the router, using plywood. To connect the rings I tried to buy 10mm * 2mm  (0,25 * 0,08 ) square pipe, but unfortunately without success. Then I took 12mm * 1mm (0,5 * 0,04 )  pipe. The threat inlays I made myself form massive aluminium bar and fixed with pins.


The Spider is made of 0,8mm  (0,02 ) sheet metal, in the middle a 1,4 piece of a square-pipe tube with threated hole , 4,5mm(0,18 ) offset included. To fix the mounting bolts I took M5 (0,2 ) metal wall plugs and soldered it at the ends. It are lightweight, and have already a placket at the end to be fixed at the sheet metal.


The Secondary holder  is made of 3mm (0,12 ) thick aluminum and was bended by hand in a bench vice. The holder can be collimated with 3 pcs. M4 (0.16")  knurled nuts. It is connected by a 8mm (0,31 ) bolt to the spider and can be fixed with a butterfly nut.


How to connect the secondary cage with the truss poles: finally I took the following solution: I connected trusses by pairs at the upper side. With  M6 (0,24 ) screws with self-clamming bolts are these connected with aluminium brackets, 3mm (0,12 ) thick. I ve  sawn them with the mechanical fret saw from a bar. A  Key-Hole houses the quick release skewer bolt. It looks like it will work very well.


The mirror box  is made of 12mm (0,5 ) plywood. Placing triangular reinforcement braces into the corners increases stability. For transportation purposes I built in carrying handles, as seen at Stathis nice 24 Kyklopas telescope. As the mirror box weights only 53 lb, I think I will not use these handles, but for sure is for sure! 


I m taking again the fine combination Quickfinder at the secondary cage and finder Newton at the mirror box. While the finder allows to see more or less all Messier objects, you ll find almost every corner at the night sky. Connection  with  Keyhole principle.


I took a sheet of hard mansonite for the cover, 3mm (0,12 ) thick an varnished it against humidity. In the top I milled 2 8 holes that way, that these are outside the shadow of the secondary mirror and the spider. I m interesting about the images of a two-times unobstructed 8 view.


The rocker  is built as flat as possible. The groundboard is 24mm (1 ) thick, left and right 21mm (0,8 ), front and rear 12mm (0,5 ). To save weight and for nice looking purposes I cut out some pieces. I took glassboard for the azimuth bearings, fortunately there is a small specialised company in my village. It took me 10 Euro to get a cut piece in the needed dimension.


Baffles: At the secondary cage baffles from 5 mm (0.2") insulation foam material are fixed to the cage rings with Velcro fastener. To fix it  sewed I to the foam at the one side and glued it with  two components stuff at the rings.


To top of this page