Building 20 inch truss-tube Dobsonian telescope

The courage to undertake this project resulted from successful completion of 24 inch f4.5 Venor Telescope by fellow members of RASC - Kingston Centre. While many construction steps were carried alone in my basement, I have enjoyed a tremendous help of Doug Angle. His persistence and inventiveness cannot be overstated!

To construct my truss-tube telescope, I have generally followed instructions outlined in the book "The Dobsonian Telescope: A Practical Manual to Building Large Aperture Telescopes" by David Kriege and Richard Berry (Willmann-Bell, 1997). While great on woodworking and other construction techniques, it contains only short (though very useful) chapter on making large aperture optics. To tackle mirror making, I dipped extensively into vast knowledge compiled in "Amateur Telescope Making" edited by Albert G. Ingalls (Scientific American, editions: 1928, 1933 and Book Two - 1949). Making a flat diagonal mirror is described on the separate page.


Scope overview

  1. red-dot 1x finder

  2. homebuilt 6" f4 scope (primary and secondary mirrors coated with enhanced aluminum) with 25mm eyepiece (24x)

short distance - everything rolls together in one compact package (including step stool)

First light

After some testing and adjustments, the first official night of observing started on the evening of July 30, 2002. After setting up and cooling the scope, I have re-colimated it and then star-hopped to a newly discovered comet 2002 O4. It was noticable in 6 inch finder but 20 inch showed it a7 133x as a quite large oval with point nucleus (no tail, though). Below are two images taken about an hour later with a Cookbook CCD camera on Celestron 8 inch f6.3 SCT (both are 30 sec. unguided exposures). Motion of the comet is quite apparent - UT of midexposure and coordinates of the nucleus are listed. North is up. Only part of the frame is shown. The scale was reduced to 75% as well.

Some other objects observed that night included:

I have also located Pease 1 planetary in M15 - that was my second challenge for the first light observing! Blinking with nebular filter confirmed sighting of this extremely difficult object.

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Jan Wisniewski