My amateur telescope making page
The "Deep Space Scope" Design
DS-3 | Mirror cell construction | Secondary cage construction | Mirror box construction | Plop Design | NEWT design | Finish the mirror box | Truss construction | Bearing construction and mounting | Rocker box construction | Details, current issues and future enhancements | Baffling and Contrast |
In the last few years, I have decided that I needed a
specialized telescope, and that it was time to build it. The following
links deal with designing, building and using a homebuilt 12" ultra
DS-3 (Deep Space 3). Although this was my first amateur telescope
making adventure, it ended up working out quite well. Finally, I
want to give credit to my mentor and fellow ATM partner, Jim
Lawrence. Without Jim's help, as well as the advice of other
members of the local Albuquerque astronomy club (TAAS), this ultra
light telescope would
have been attempted. Links to Jim's telescope that served as a
for DS-3, along with his ongoing work on binocular telescopes can be
found in the Astronomy Links section of this web site. Be sure to
check out his 300mm Telescope
(my prototype) and his 300mm Binocular (my dream scope, if it would fit
into my car). Jim's site is also located here.
Design requirements of DS-3 were as follows:
- Light weight. Weight of the
heaviest single part of this lightweight dob should be less than 30
pounds, and the total
be less than 50 pounds. This is necessary since I live in a second
apartment, and must carry the telescope upstairs each time I use it.
- Small size. The telescope MUST
fit in the trunk of my car, which is a Honda Del Sol. I also want space
left over for carrying my eyepiece box and
an observing chair. Why not buy a bigger vehicle? See
my web page on Peak Oil.
- Large diameter mirror. Aperture
rules for deep space! I want at least a 10" (10 inch) primary mirror,
and preferably a
- The primary mirror should be about f 5.0. From
my experience, faster telescopes have mirror costs go up
disproportionately fast for a given
quality, and slower telescopes end up being too tall.
- Height at the eyepiece. I wanted the
height of the eyepiece to be less than 60" when looking straight up, so
that anyone could use
the scope without needing a ladder or step.
- Setup time should be under 5 minutes. Setup of this truss
dob should require
no tools and include a minimum amount of setup or tear down.
- As a secondary design goal, this scope should be cool looking!
Note - all pictures on this web
site are clickable
bring up full size pictures.
The finished product looks like this:
Two pictures of the completed truss scope, without baffling. DS-3
ready to play.
This lightweight dobsonian telescope torn down, and in the trunk of a
Honda Del Sol.
Final design specs (as of Dec 2004) are:
- Primary mirror: 12" f5.0, 1 1/2" thick
- Secondary mirror: 2.05"
- Weight: Total weight, including
trusses: 39 pounds
- Weight of heaviest piece (Mirror
box with mirror): 25 pounds
- Weight of primary mirror and mirror cell: 14 pounds
- Secondary cage weight: 2 pounds
- Truss weight: 4 pounds
- Base weight: 8 pounds
- Height of eyepiece: 58.5"
- Finder: Rigel Quick Finder
- Focuser: KineOptics Helical Crayford
- Packed size: width 17.5", length 21",
- Size of this ultra light telescope if compressed for airline
by removing the bearings (approximate, without poles): width: 15.5",
length 17.5", height 9"
under the stars - This web page describes how each section of
telescope performed, along with my analysis of the whole.
Generally, results have been spectacular.
This homebuilt telescope was designed and built in the following
order - the primary mirror
selection, primary box and mirror cell, secondary ring and secondary
cell, trusses, bearings, and last rocker box and ground board.
- Select the mirror. I knew from working with a friend that
a 12" lightweight telescope design would just fit my specifications.
a 12" f5.0 primary mirror was selected. This mirror size maximizes the
amount of light
received, while still meeting my size and weight requirements. This
mirror is from Gary Hand at Hands On Optics, and is of very good
quality - as per numerous "old salts" in the local club.
- Select the building materials. See below.
- Design the telescope using NEWT.
I used NEWT(actually, it was NEWTWIN) to check clearances and
secondary mirror size. See
NEWT Design below for details. See Sources and Supplies below to
- Design the mirror cell using
PLOP. OK, I didn't originally design the mirror cell using
plop. I used Jim Lawrence's experience and a good dose of common
sense. As per plop, I have about a 1/21 wave mirror cell.
Not terribly bad, but not good. See Plop Design below
- Design the mirror box and cell. The primary mirror cell
will consist of three pads of plywood, attached to the mirror with
double sided sticky tape. The mirror box will consist of plywood as
follows: a single bottom layer with air holes, single layer sides and a
double layer top plate (to add strength for the truss attachments).
Mirror box is 15 1/2" X 15 1/2" X 4". Vertically through the center of
the mirror box, this allows 1/2" for the bottom plate, 1/2" for the
mirror cell plates, 1" for the springs between the mirror cell and the
bottom plate, and 1 1/2" for the mirror. Total to the top of the mirror
is 3 1/2". This gives 1/2" of air between the top of the mirror
and the top of the mirror box. Horizontally, this gives 12" for the
mirror, 1/2"*2 for airspace around the primary, and 3/4"*2 of wood to
the edges. We need this much space for attachment points for the
- Design the secondary "cage". The secondary cage will
consist of a single plywood ring, with "stuff" attached to it. Using
Newt, an inside diameter of 13 1/2 inches was chosen for the upper
ring. This allows "stuff" to intrude into the ring by 1/2", and still
have 1/4" on each side of clear air. However, if the "stuff" is small
this gives a full 3/4" of clear air, which reduces vinyetting to a
minimum. "Stuff" is currently three brass threaded rods that attach the
secondary ring to the spider, and the focuser. The focuser only
intrudes into the secondary rings space when racked all the way in. We
use a wire spider, which is made up of guitar string wire. The
secondary cell will end up
being a custom, lightweight setup. Note on the secondary cage. Any
and all weight here will cause us trouble when trying to make small
bearings and a small rocker box. So, we will try to keep the secondary
"cage" weight VERY light. This also means that it is a VERY bad idea to
add lots of gismos to the secondary cage.
- Trusses. Trusses for this truss telescope will be aluminum
poles 3/4" in diameter,
set into holes drilled into the mirror box. They are then flexed
into place. The top end of these trusses will have a hole drilled in
them, which will be placed over the exterior portion of the brass
threaded rods from section 4 above. Truss lengths should be about 60"
(focal length) + 1/2" (1" into the mirror box, mirror is 1/2" into the
mirror box) + 3" (distance from center of the focuser to middle of the
upper ring) - 8" (distance from the middle of the secondary to the
middle of the focuser) + 1/2" (distance from hole at top of truss to
top of truss) = 56". Mine ended up being about 54 1/2" long.
- Bearings. With the telescope assembled, and with a guess
at the weight impact of the bearings, the center of gravity (CG) is
found. This will be the center of the bearings. The bearings can be any
size and work, as long as the center of the bearings is also the
CG. However, the size of the bearings changes the friction of the
altitude motion of the scope and size of the rocker box, so getting the
correct size is important. Basically, bigger bearings seem to have a
better "feel" in altitude motion. Also, for every inch increase in
diameter of the bearings, we decrease an inch in the height of the
rocker box sides and vice versa. I decided to attach the bearings
directly to the sides of the mirror box, thus getting rid of any riser
or additional structure to hold the bearings. This also makes the
rocker box sides very small. You probably also want the bearings to not
touch the ground when you set the scope on the ground without the
rocker box. As a guess, this makes the outside bearing surface be about
11.5" in radius. For strength, we will make these bearings 1" wide and
- Rocker box and ground board. The rocker box bottom is made
out of 1/2" plywood, backed up with additional plates of 1/2"
plywood. The ground board is also made from 1/2" plywood.
This ultra light dob was built using the following building
- Everything is made out of plywood. 1/2" very good quality plywood
was chosen. I am not sure if it is apple or birch ply, since a friend
picked it up at the lumberyard. It really doesn't matter - what you
want is good quality 1/2" ply without gaps or voids. A 4'X4' (4 foot by
4 foot) sheet should be plenty.
- 3/4" Aluminum was chosen as the materials for the truss poles.
- CA was used to glue everything together. CA comes from the
- DUCK brand double sided sticky tape was used to hold the primary
and secondary to their "cells". DUCK brand was chosen because it
is reported to be stronger, and I was able to find the working strength
specs on the net. When dividing the weight of the primary mirror
by the surface area of the primary cell, I am well within safe working
limits with DUCK brand double sided tape.
- All circular cuts were done with a router with a circle cutter
attachment. This special tool is found at a special woodworking tool
store. Lowe's, Home Depot and Sears did not have this tool.
However, the prototype for this scope had all circles cut with a jig
saw and cleaned up with a sander.
- Tools that were pretty much required: Table saw, drill press,
drill and drill bits, screw drivers, tape measure, key hole saw OR jig
saw, sand paper,
- Tools that were nice to have: Belt Sander (Used for about
everything), and circle cutting router.
Details of the steps to design and build this scope are in the