Callisto II 7" Tube Type Dobsonian Telescope
Page Created: 2003 August 11
Page Last Updated: 2003 August 11
The original Callisto was a two arm truss style dobsonian
telescope built back in 1999. Some weaknesses presented themselves and I decided to
rebuild the scope over a year ago... and only now am getting started on the project.
The original was constructed out of 5/8" plywood. Overkill to say the least.
The new tube will be an 8.5" sonotube and 11mm (7/16") Russian birch plywood,
which is lighter and stronger than the standard 5/8" spruce plywood.
The primary mirror assembly could use a little
improvement but since the mirror was firmly attached already, I decided to only replace
the part of the mirror cell that attached to the telescope body itself.
Mirror Specs: 7" mirror = 17.8cm
focal length: 32.5" = 82.6 cm
focal ratio: 82.6/17.8=4.6
The first step was to cut a circle 8.5" in diameter out of the 11mm Russian Brich Plywood
(a 2'x4' sheet costs about $15 at Home Depot, Kingston). I drilled a small 1/8" hole in the centre.
marked the circle with a compass, and cut it out by hand with a jigsaw.
Using a protractor, I measured out 60 degree angles and from those points measured
out a 2.25" radius circle to cut out later. Cutting out sections of the cell
mount would allow airflow past the mirror and cut the weight of the whole assembly
down without affecting the structural strength.
Cutting out the semicircles with a jigsaw by hand, slowly and carefully.
Finished cell mount.. sand around the outside with #50 until it fits into the sonotube.
Sand the inside areas with a sanding sponge.
On the top right is the mirror and it's mount covered in a plastic bag to protect it.
On the left is the old cell mount made of 5/8" plywood and square in design
(to fit into the old square telescope box) and drilled with holes to improve airflow.
Looks pretty ugly, no? :)
On the Right is the new cell mount.
We have to match the locations of the old mirror mount 1/4" bolts, but luckily they
were all 2.25" out from the centre. I drilled one size larger than 1/4" (=16/64"), which
was 17/64" to allow for painting the piece later and still allow the bolts to move
Once painted it will be difficult to identify a particular arm of the piece, which
info will be needed if it or other parts ofthe scope are not symmetrical. So I use a
flat screwdriver to punch an indentation into each arm and on both sides,
then filled it in with pen ink.
Doing preliminary measurements on the sonotube to find out how deep the cell mount
must go, leaving room for the calibration wingnuts and a possible black cloth light
shield at the bottom and a dustcap of some time. The mount assembly looked like it needs
3/4" so let's give it a full inch. Add another inch for a possible black light cloth
and or 12" muffin fan. Two inches from the bottom is what it will be. A little later...
let's make that three inches... I can always cut some off if it is too long.
A photo showing the various distances and sizes of the three mounting points.
And a closer photo of the same.
Time to take the tube back outside and do another coat of flat black spray paint inside
the tube. Where latex gloves, stick your hand and can in as far as they reach,
do a section, rotate the tube, repeat. Pull your arm back a bit and repeat.
Start at the other end of the tube and repeat. Let dry.
I thought of using other material or sawdust and glue to create a better light absorbing
or non reflecting surface inside, but could not come up with one in time.
Did I mention that the scope needs to be operational in 10 days to go to Starfest 2003 in Mount Forest?
Spray paint the outside with white enamel to help reflect light, enamel to
help harden the outside. Several coats later and I think this may need to be painted
on with a brush.
Steps to go: Remount the spider, measure it, drill holes in tube to mount the primary mirror
cell assembly, drill holes the appropriate distance up for the spider and secondary mirror assembly,
drill holes for the 1.25" focuser with curved base.
Then build altitude bearings and a rocker box.
It should end up looking something like this:
, a telescope made by Dave Pianosi.