DS-3 and DS-4 have ended up being such a successful telescope
design that I have been
thinking of creating even a bigger ultralight telescope. This
web page
currently holds some of my design requirements, brainstorming ideas and
engineering solutions. I am putting it on the net in order to
share my ideas, as well as get some feedback. My e-mail
address
can be found in Astronomy, which can be found through the link above.
DS-5
requirements are as follows:
The heaviest single part of the telescope must weigh under
50 pounds, and be easily moved.
The scope will be
stored in a garage, so it
can split into as many parts as needed.
Small size when stowed is imperitive. The telescope must
fit in the trunk of a mid size hatchback. My target car is a
Subaru Impresa, Toyota Prius, Honda Fit, or VW Golf. I also
want space
left over for carrying a step stool, my eyepiece box and
an observing chair.
I want the largest diameter mirror possible while
fulfilling the
other requirements. This is believed to be a 20" f4.0.
Height at the eyepiece. I want the eyepiece to be viewable
by a 6' 0" man standing on a folding two step stool.
Setup time should be under 15 minutes. Setup of
this truss dob should require
no tools and include a minimum amount of setup or tear
down.
Easy to build.
As a secondary design goal, this scope should be cool
looking!
Why do I keep insisting
that these telescopes fit into a car?
Peak Oil. See my Peak
Oil Links
page for a description of why gasoline prices are so high, and why they
will continue to get higher.
Cost. Cars are cheaper than trucks and mileage is
better. Just because I can afford a gas guzzler doesn't mean
that I should buy a gas guzzler.
Stuff. If the telescope fits in a car, it forces
designs that are simpler. No big ladder and no big
ramps.
By design.
Secondary diameter percent of primary diameter: 20%
Weight of heaviest piece (Mirror
box with mirror): unknown.
Secondary cage weight: unknown
Truss length: unknown
Height of eyepiece: unknown
Finder: Unknown. Possibly a Telrad or a Rigel
Quick Finder
Focuser: Feather touch 2"
Packed size: tbd
Inputs to NEWT:
Primary diameter: 20", 1.625" thick
Biggest mirror that doesn't require a big ladder and that
I can lift.
Focal ratio: 4.0
This is as fast as I think I can go. Coma will
be
an issue, so a paracor may need to be used. Any slower, and
my
eyepiece requirement is not net.
Diagonal minor axis: 4.0"
Tube inside diameter: 22"
Tube thickness 0.5"
This is for half inch plywood. We may use a
plate of aluminum. Basically, as
thin as possible.
Focuser to front of tube: 3.0"
Irrelevant. Used by NEWT.
Mirror face to back of tube: 3.5"
Irrelevant. Used by NEWT.
Focuser height: 2"
Thickness of the focuser from the base plate to the top
of the
tube. This is from a feathertouch focuser.
Spare focuser in travel: 0.25"
I think that this is irrelevant.
Focuser inside diameter: 2.0"
Designed for a 2" eyepiece. Allows use of
better
eyepieces with a wider field of view. Requires better
baffling. Heavier.
Results from NEWT:
Diagonal too small to admit 100% ray? No.
Vignetting of 75% ray at front of aperture? Yes. (But not
by much).
Vignetting of focuser at 100% ray? No.
Vignetting of focuser at 75% ray? Yes.
(I believe
that it says that I
need a bigger focuser. 2" is probably big enough.)
Theoretical limiting magnitude: 15.3
Obstruction of the primary by the secondary by area: 3%
Obstruction of the primary by the secondary by diameter: 18%
Theoretical resolution: 0.23 arc seconds.
Inputs to PLOP:
Primary diameter diameter: 508 mm
Primary mirror thickness: 41.3 mm
Primary mirror focal length: 2032 mm
Secondary mirror diameter: 88.9 mm
Number of point suspension: 18
Results from PLOP:
Support ring diameters:
Inside ring 39.78%
Outside ring: 80.09%
Six triangles of 1/2" plywood
Triangle
dimensions: Point one: 0.0", 0.0", Point two: 4.2886", 0.0", Point
three
2.3", 3.62" , CG 2.196", 1.206", Radius of CG 6.452" from center of
mirror at 0, 60, 120, 180, 240, and 300 degrees. Triangles
oversized by 1/2".
Three bars of 1/2" plywood
Bar dimensions: Point one: 0.0", 0.0", Point two: 6.452",
0.0". CG 3.226", 0.0", Radius of CG 5.587" from center of
mirror at 0, 120, and 240 degrees. Bars
oversized by 1/2".
See details below.
These dimensions are a good starting point.
General building notes:
1/2" plywood weights about 47 lbs/4X8 apple ply
sheet.
Thus, 1.5
lb/sq foot is used for calculations. Apple ply basically
means
thin layered plywood, with no voids.
1/4" plywood weights about 0.75 lb/sq foot.
Design
methodology
Select the mirror. I am going
to go with a 20"
f4.0. This is the largest mirror that I can use, be able to
lift and only
need a step stool. It also
will fit in a car. So, DS-5
will be
a 20" f4.0 truss based dob telescope.
Count
Name
Dimensions
Width/thickness ratio
Material
Weight
Distance from
bottom of OTA
1
Primary Mirror
20" f4.0, 1.5"
thick
13.3
Pyrex
36 lbs (reported by
a mirror maker)
3
Select the building materials. Initial
design will use
1/2" and 1/4" apple ply. This wood was chosen due to weight
and
ease
of
use.
Design
the mirror cell. The mirror cell
will be a modified
version of DS-4. Be sure to read all of my detail on
running Graphical
Plop here.
The mirror cell is
currently
planned to be the following
Mirror. 1.5" thick.
Plop inputs: See above.
Modified 18 point suspension.
The 18 point
suspension will be 18
round masonite disks
3" in diameter, with a center hole the size of the bolt head.
There will have another piece of masonite (1") glued on the
back, with
a hole the diameter of the bolt shaft. There will be a metal
washer below both masonite disks..
There
will be 6 1/2" plywood triangles. See attached diagram for
sizes.
Note that the actual triangles will be oversized by 3/4" on
all
three sides. Pt1:0.0,0.0 Pt2: 5.88, 0.0: Pt3: 2.93,
5.09.
Masonite disks will be attached using a small nut and bolt.
There
will be 3 bars, one each passing between two triangles.
Ends
of the bar are attached to the plywood trianges with a small nut and
bolt. Middle of the bars have a threaded shaft extending from
them. They may also have a 2" 1/2"thick disk glued to them, with a 1"
hole drilled 1/2" deep. This is for 3 springs (we are hiding
the
springs in these holes); Center hole to outter holes are
3.25".
Count
Name
Dimensions
Material
Weight
ply
Distance from
bottom of OTA
18
Mirror cell
contacts
3" diameter
1/8" masonite
0.5 lbs
6
Mirror cell
triangles
1/2" ply
1.0 lbs
3
Mirror cell bars
2.5"X8.5"
1/2" ply
0.7 lbs
?
Misc hardware
Metal
1.0 lbs
Total weight
3.2 lbs
1.5
Design the mirror box. The mirror
box will use 8
trusses. It will measure 22" X 22"
X 5" I want it to not have any open holes, not including the
mirror box bottom. (The hole in the mirror box top is covered
with a piece of Masonite.) Also, a second mirror bottom will
be glued to the bottom, which will have numerous lightening holes
drilled in it. This stiffening plate will be inside the box.
Count
Name
Dimensions
Material
Weight
ply
Distance from
bottom of
and notes
Notes
1
Mirror box bottom
22"x22"
1/2" ply
5.1 lbs
Composite with 1/4" stiffener listed below.
Should this also be composite?
1
Mirror box bottom stiffening
21"X21"
1/4" ply
1.2 lbs
Composite with 1/2" stiffener. This
stiffener has 50% of it's material removed.
2
Mirror box long
side
22"x4"
1/2" ply
1.8 lbs
Composite of two pieces of 1/4" ply. Inside
piece 3/4 removed. Front and back piece.
2
Mirror box short
side
21"x4"
1/2" ply
1.8 lbs
Solid. Side pieces
1
Mirror box top
22"x22"
1/2" ply
1.8 lbs
Composite of two 1/4" ply. Inside piece 3/4
removed.
4
Corner triangles
3"x3"
1/2" ply
0.4 lbs
Total weight
12.1 lbs
2.5
Total weight of the mirror box
Pyrex has a density of about 0.081 lbs/cubic inch. Mass of a
disk
1 5/8" thick is 41.35 lbs. But, we remove about 5 pounds of
glass. Thus, about 36 pounds.
Name
Weight
Mirror
36 lbs
Mirror cell
3.2 lbs
Mirror box
12.1 lbs
Total weight of
heaviest telescope part
49.8 lbs
Design the Bearings - rough, for CG calculations.
The bearings will consist of a 1/2" arc on the outside, with a 1/4" arc
on the inside.The inside arch will be 1/2" larger than the outside arc.
The bearings will taper towards the top.
Count
Name
Dimensions
Material
Weight
ply
Distance from
bottom of OTA
2
Bearings
composite 3/4" thick
(here for
weight)
(Use DS-4 design)
36"x3"
1/2" ply
3.5 lbs
Total weight
3.5 lbs
8
Design the secondary "cage".
The secondary cage will
consist of two plywood rings. They will have an inside
diameter
of 22", and an outside diameter of 25". At present, a 4 wire
spider will be used. See DS-3 and DS-4
for details. Secondary will be an astrosystems design
(purchased or made)
Count
Name
Dimensions
Material
Weight
ply
Dimensions from
bottom of OTA
1
Secondary mirror
3.5"
glass
2.0 lbs
1
Spider stuff
??
aluminum/wood
1.0 lbs
2
Secondary rings
25" od, 22" id
ring
1/2" ply
2.4 lbs
1
Secondary focuser
board
6" X 6"
1/2" ply
0.4 lbs
Additional structure - 3
6" X 2"
1/2" ply
0.4 lbs
1
Focuser
(Starlight
Featherweight)
1.3 lbs
1
Telrad
0.7 lbs
Total secondary
cage -
figured, bare
8.2 lbs
ParaCor
1.0 lbs
Heaviest eyepiece
that I own
1.0 lbs
Baffle, large
1.0 lbs
Normal secondary cage weight with eyepiece
11.2 lbs
66+4+3=73
Trusses. Trusses for this truss
telescope will be eight
aluminum
poles 1.5" in diameter. They will be 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 5 above. Truss lengths should be
APPROXIMATELY 80(focal length)-10(mirror radius)-1(height above
mirror)-3(focal length into focuser)=66"
in length.
Count
Name
Dimensions
Material
Weight
Dimensions from
bottom
of OTA
8
Truss poles
1.5" X 66" long
Aluminum
11 lbs
37
Bearings. Bearings will use
close to the same design as
DS-4. First off, we have to calculate the center of gravity of
the system.
The formula for center
of gravity is
as
follows:
totalWeight * centerOfGravity = weightPartA * distancePartA +
weightPartB * distancePartB + ... + weightPartZ * distancePartZ
where distancePart* represents the distance in inches from the bottom
of the optical tube assembly.
Item
Weight
lbs
Distance from
bottom of OTA
Moment arm
Weight * distance
Mirror
36.0
3
108
Mirror
cell
3.2
1.5
4.8
Mirror box
10.6
1.5
15.9
Bearings (guess,
used for
calculations)
3.5
18
63
Secondary ring,
focuser,
eyepieces, baffling, etc
10
73
730
Truss tubes
11
37
407
Total
74.3
1329
Center of gravity = totalMomentArm/totalWeight, or 1329/74.3 =
18". This will be the distance from the bottom of the
mirror
box
to the center of gravity, and the center of the bearings.
These bearings will ride up on the box about 4", and will have a
radius
of 16".
Rocker box and ground board.
DS-5 will use the same
rocker box design as was used by DS-4. Additional stiffening
will
be added to the rocker box along the back edge. Bottom of OTA
will be 2.6"+1"+pad=4" above rocker base plate.
Count
Name
Dimensions
Material
Weight
1
Base plate
26" circular
1/2" ply
1
Rocker base plate
23.5"X23.5"
1/2" ply
2
Rocker base
stiffener
23.5"X
1/2" ply?
composite?
2
Rocker side plate
23.4"X6"
1/2" ply
6
Magic sliders -
above and
below base plate,,
Teflon, bolts