My amateur telescope
making page
The 24 inch
"Deep Space Scope" Design
Home | Astronomy |
DS-5
Overview
Requirements
Explanation
of
Deep Space Name
Why
make lightweight telescopes
Current
design
Newt
inputs
Newt
results
Design
methodology
Telescope
weights and height (theoretical)
Overview
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:
- Weight, within reason, is irrelevant. Since a large scope
will require a heavier mirror than I want to lift, I will use a wheel
barrow type arrangment. Thus, within reason, weight is
irrelevant. A lightweight ramp will be designed and created.
Weight should still be under 100 pounds.
- 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 24" f4.0.
- Height at the eyepiece. I want the eyepiece to be viewable
by a 6' 0" man standing on a folding step stool. Home Depot has a three
stair stool with a top step of 29 inches.
- 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.
Current design:
- Primary mirror: 24" f4.0, 2" thick, enhanced coatings
- Secondary mirror: 4.5", enhanced coatings.
- Secondary diameter percent of primary diameter: 19%
- Weight of heaviest piece (Mirror
box with mirror): unknown (irrelevant). Too heavy to lift.
- Secondary cage weight: 9.8"
- Truss length: 84"
- Height of eyepiece: 90"
- Finder: Unknown. Possibly a Telrad or a Rigel
Quick Finder
- Focuser: Feather touch 2"
- Packed size: tbd
Inputs to NEWT:
- Primary diameter: 24", 2" thick
- Biggest mirror that doesn't require a big ladder.
- Focal ratio: 4.0
- This is as fast as I think I can go. Coma will
be
an issue, so a paracor will need to be used. Any slower, and
my
eyepiece requirement is not net.
- Diagonal minor axis: 4.5"
- Tube inside diameter: 25"
- 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: 3.0"
- Irrelevant. Used by NEWT.
- Focuser height: 3"
- Thickness of the focuser from the base plate to the top
of the
tube.
- 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.
This says that the 75% ray will have a very, very small
amount of
the light blocked by the upper ring. I can live with this.
- 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: 15.7
- Obstruction of the primary by the secondary by area: 4%
- Obstruction of the primary by the secondary by diameter: 19%
- Theoretical resolution: 0.19 arc seconds.
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, with no voids.
- 1/2" composite weights about 14 lbs/4X8 sheet.
Thus, 0.44
lbs/sq foot is used for calculations. In this case, composite
means thin plywood skins over a paper core. Check out Tricel
Honeycomb,
marine version for details.
Design
methodology
- Select the mirror. I am going
to go with a 24"
f4.0. This is the largest mirror that I can make and only
need a step stool. It also
will fit in a car. So, DS-5
will be
a 24" f4.0 truss based dob telescope.
Count
|
Name
|
Dimensions
|
Material
|
Weight
|
Distance from
bottom of OTA
|
1
|
Primary Mirror
|
24" f4.0, 2.0"
thick
|
Pyrex
|
72 lbs
|
????
|
- Select the building materials. Initial
design will use
1/2" composite 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, working from the mirror DOWN:
THIS SECTION NEEDS TO BE CHANGED FROM DS-4. (To Be
Done. Assume 4 lbs.
- Mirror. 2.0" thick.
- Plop inputs: diameter: 609mm (24"), thickness: 50.8mm
(2"), focal
length: 2438 (f 4.0), secondary diameter: 114.3mm (4.5").
- Modified 6 point suspension. These will be 6
round disks
of 1/2"
plywood, attached to the mirror with double sided sticky
tape.
Each will be ALAN" in diameter, centered at a distance of ALAN" from
the
center. These disks will be ALAN" from each other, at 60
degree
angles from each other. Each plywood disk will be double
sided
sticky taped to the back of the mirror. 1/2" thick
- These 6 plywood disks will each have a small bolt through
them,
which will go through an arm created of 1/2" plywood. There
will
be three arms, with the bolts from the disks going into the
arms.
Each arm bolt will have a small rubber O ring separating the round disk
from the arms. Each arm will be ALAN " in length,
and ALAN"
wide
(ALAN - TEST FOR STRENGTH) 1/2" thick.
- Through the center of the plywood arms will be placed a
columnation bolt, with a spring around the bolt. The bolts will then go
through the base of the box, and nuts will be placed on the bottom of
the bolts. 1" thick.
Count
|
Name
|
Dimensions
|
Material
|
Weight
ply
|
Distance from
bottom of OTA
|
9
|
Mirror cell
contacts
|
5" diameter
|
1/2" ply
|
1.2 lbs
|
2.25
|
3
|
Mirror cell
triangles
|
?" X ?" |
1/2" ply
|
0.8 lbs
|
1.75
|
?
|
Misc hardware
|
|
Metal
|
1.0 lbs
|
1
|
|
Total weight
|
|
|
3.0 lbs
|
|
- Design the mirror box. The mirror box
will be like
DS-4, although may be a bit deeper. It will use 8
trusses. It will measure 26" X 26"
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 OTA
|
1
|
Mirror box bottom
|
26"x26"
|
1/2" ply |
7.0 lbs
|
0.25
|
| 1 |
Mirror box bottom (hollow) |
25"x25" |
1/2" ply |
3.5 lbs |
|
| 1 |
Morrir box stiffining splines |
24"x24" |
1/2" ply |
1 lbs? |
|
2
|
Mirror box long
side
|
26"x4"
|
1/2" ply |
2.2 lbs
|
2.25
|
2
|
Mirror box short
side
|
25"x4"
|
1/2" ply |
2.1 lbs
|
2.25
|
1
|
Mirror box top
|
26"x26"
|
1/2" ply |
1.9 lbs
|
4.5
|
4
|
Corner triangles
|
3"x3"
|
1/2" ply
|
0.2
|
|
|
Total weight
|
|
|
16.9 lbs
|
|
- Design the Bearings. The bla
Count
|
Name
|
Dimensions
|
Material
|
Weight
ply
|
Distance from
bottom of OTA
|
2x2
|
Bearings 1" thick
(here for
weight)
(Use DS-4 design)
|
36"x3"
|
1/2" ply |
4.5 lbs
|
6?
|
|
Total weight
|
|
|
11.5 lbs
|
|
- Design the secondary "cage".
The secondary cage will
consist of two plywood rings. They will have an inside
diameter
of 25", and an outside diameter of 28". At present, a 3 wire
spider will be used. . See DS-3 and DS-4
for details.
Count
|
Name
|
Dimensions
|
Material
|
Weight
ply
|
Dimensions from
bottom of OTA
|
| 1 |
Secondary mirror |
4.5" |
glass |
5 lbs |
|
| 1 |
Spider stuff |
?? |
aluminum/wood |
1.0 lbs |
|
2
|
Secondary rings
|
28" od, 25" id
ring
|
1/2" ply
|
2.0
|
|
1
|
Secondary focuser
board
|
5 1/2" X 5 1/2"
|
1/2" ply
|
0.3
|
|
1
|
Focuser
(Starlight
Featherweight)
|
|
|
1.3 lbs
|
|
1
|
Rigel Quickfinder
|
|
|
0.2 lbs
|
|
|
Total secondary
cage -
figured, bare
|
|
|
9.8 lbs
|
|
|
ParaCor |
|
|
1.0 lbs |
|
|
Heaviest eyepiece
that I own
|
|
|
1.0 lbs
|
|
|
Telview Barlow
|
|
|
1.0 lbs
|
|
|
Total secondary
cage -
figured,
with eyepiece
|
|
|
12.8 lbs
|
|
|
Total ring -
rough guess -
Includes 2Xds-3, 8mm+barlow, baffles theoretical, MAX
|
|
|
13.2
|
67
|
- Trusses. Trusses for this truss
telescope will be eight
aluminum
poles 1" in diameter. They will split in the middle, with a
wood dowl glued in the upper section. The lower wood dowl
will slip into the lower sections, and then be a friction (flex) fit.
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 84"
in length - minus the height of the secondary cage. ALAN - needs work..
Count
|
Name
|
Dimensions
|
Material
|
Weight
|
Dimensions from
bottom
of OTA
|
8
|
Truss poles
|
1" X 84" long
|
Aluminum
|
13 lbs
|
46
|
- Bearings. Bearings will use
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
ground
|
Moment arm
Weight * distance
|
Mirror
|
72
|
3.25
|
84.5
|
Mirror cell disks
|
1.2
|
2.25
|
1.8
|
Mirror cell arms
|
0.8
|
1.75
|
0.7
|
Mirror box
hardware
|
1
|
1
|
1
|
Mirror box bottom
|
4.5
|
0.25
|
0.73
|
Mirror box long
sides
|
2.2
|
2.25
|
3.38
|
Mirror box short
sides
|
2.1
|
2.25
|
3.15
|
Mirror box top
|
1.9
|
4.5
|
5.4
|
Bearings (guess,
used for
calculations)
|
10
|
10
|
31.5
|
Secondary ring,
focuser,
eyepieces, baffling, etc
|
13.2
|
89
|
435.5
|
Truss tubes
|
13
|
47
|
180
|
Total
|
121.9
|
|
819
|
Center of gravity = totalMomentArm/totalWeight, or2144/121.9 =
17.5". 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 13.5". This will leave the bearing attached to the side of
the
mirror box for ALAN", and have at least ALAN" of material resting against
the side of the mirror box for ALAN". ?? alan, check.
- 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..
Count
|
Name
|
Dimensions
|
Material
|
1
|
Base plate
|
Triangle, ALAN"
middle to tip
|
1/2" ply?
Aluminum?
|
1
|
Rocker base plate
|
ALAN
|
1/2" ply
|
2
|
Rocker base
stiffener
|
ALAN
|
1/2" ply?
composite?
|
2
|
Rocker side plate
|
ALAN ??
alan - check this
|
1/2" ply
|
6
|
Magic sliders -
above and
below base plate,,
Teflon, bolts
|
|
|
|
Total weight
|
|
|
Total
telescope (theoretical):
Dimensions
Height
of eyepiece at zenith
|
90",
7'8"
|
Size
of mirror box and base, no bearings
|
28x26x8"
|
Length
of trusses
|
84"
|
| Length of trusses - folded (approx) |
45" |
