I
initially planned to build the tertiary mirror/focuser from scratch, but had on
hand two old 90 degree prism diagonals from Orion and small helical focusers a
friend had machined from aluminum. The
prism diagonals would have worked “out of the box” to provide the needed 90
degree redirection of the light path and a means to focus by sliding the
eyepiece barrels up and down as needed.
I installed the 1.18 diagonal mirrors in place of the prisms, and
replaced the eyepiece holders with the helical focusers. Each diagonal is mounted via a 1.25 inch hole
in a small rectangle of our ugly plywood which is attached to the upper cage
wall with small bolts and t- nuts.
drilling
1¼” hole matching secondary
center ugly plywood squares installed with bolts
t-nuts/sheet
metal screw mounted
assembly mirror replaces
diagonal diagonal w/helical focuser
The
wood block is held in position by (a) the small bolts and t-nuts (b) a small
sheet metal screw at center bottom and (c) the diagonal tube which extends into
the 1 ¼” hole in the upper cage. The
block can be adjusted to center the diagonal on the secondary horizontally by
tightening/loosening the two bolts and vertical centering is accomplished by
moving the secondary vanes up/down as needed.
Tertiary/focuser assembly is held in the 1 ¼” hole in the plywood square
with a ¼” nylon bolt which is threaded directly into the plywood. Threads are cut in the plywood by running a
steel ¼” bolt into an undersized 7/32” hole.
Prism
Diagonals from Orion cost $29.00 and surplus diagonals can be found for about
$15.00. The helical focusers can be
purchased from http://www.lumicon.com/
for $70.00 each and the bases can be cut off and the barrels hand fitted ,
replacing diagonal eyepiece holders – a bit of work, but the result is worth
the effort. The diagonals and focusers
add another $170.00 to our total cost.
Paul
Rini http://www.ganymedeoptics.com
sent me a pair of 30 mm eyepieces fabricated from surplus optics – nice flat
field, sharp images, good eye relief, but no field stop – edges of view are
“fuzzy” At the end of the day they are
quite nice and cost $72.00 for the pair, with shipping. I also have a pair of 16 mm French WWII tank
erfles which provide 65 degree apparent field and very good images at field
center – these are very easy to look through and were purchased from Apogee,
Inc. http://www.apogeeinc.com Both sets of these 1.25” eyepieces work well
with shorty 2x barlows, My old pair of 21 mm Televue plossls provide sharper
images, but are a bit demanding to keep “centered.” It is best to try a variety of eyepieces as
the results can be surprising. Note that
most 7x50 binocs have kelner eyepieces – which also work very well in
binoscopes.