Both
tubes are now ready for fittings. Let’s
proceed with the secondary holder or
“spider.” John Dobson’s “classic”
shingle and dowel spider has several things going for it – inexpensive and
(oddly enough) very stout – no flexion, no vibration. It begs to be adjusted once and then anchored
in place – and that is what we’ll do when the time comes. I fudged by using 1/8” masonite instead of
cedar shingles.
Dowel,
masonite, pattern used to mark
dowel for slots @ 120 deg then cut dowel end at 45 deg
cut
1/8”x1.5” masonite strips sand strip
ends to fit slots in dowel then
glue and then center dowel
cut
vanes to length and sand for snug fit
The
spider vanes can be rotated and moved up/down to align the secondary mirror
with the tertiary and with the secondary, then the vanes can be glued to the
tube wall. Take a look at http://www.cdcc.sc.usp.br/cda/telescopios/tie-jpl-nasa/page12.html
for further details on the spider layout.
The dowel cost $2.45 and I had some scrap 1/8” masonite. A regular masonite clip board available at Office
Max for $2.50 is a great source for the vane material.