My amateur telescope making page

The Upsized "Deep Space Scope" Design

Overview

DS-3 has ended up being such a successful telescope that I have been thinking of creating 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-4 requirements are as follows:

• Light weight. Weight of the heaviest single part of this lightweight dob should be less than 50 pounds, with a desired weight of under 40 pounds.  The base, mirror box and secondary cage should weigh less than 60 pounds, with a desired weight of 50 pounds.  The scope will be stored in a garage, so it can split into as many parts as needed.
• Composite materials will be analyzed as a possible construction material.  Weight and cost will be analyzed.
  
• Small size when stowed. The telescope MUST fit in the trunk of a mid size hatchback.  My target car is a Toyota Prius, Honda Civic, or VW Golf.  I also want space left over for carrying 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 16" f4.5 or a 18" f4.5 mirror.
• The primary mirror should be f 4.5 or slower. This is a slight compromise, but should still allow the design of a great scope.  I am trying to keep the amount of coma to a minimum.
  
• Height at the eyepiece. I want the eyepiece to be viewable by a 5' 10" man standing on a one step or two step "step stool" that folds.  That means that the eyepiece should be no higher than 7' max, and preferably 6'.
  
• Setup time should be under 5 minutes, and must be under 10 minutes.  Setup of this truss dob should require no tools and include a minimum amount of setup or tear down.  
• This telescope will be able to carry a half height or full height shroud.  It will also be usable without a shroud for observing during windy nights.
• Easy to build.  If possible, no laminating, metal machining,  or  home made composites will be used.  Easy to build.
  
• As a secondary design goal, this scope should be cool looking!

• Mainly, it is because I am a Star Trek Deep Space fan.  The name Deep Space 9 has been taken, so I chose Deep Space 3 (DS-3) - and now Deep Space 4 (DS-4).
• NASA already used up the names Deep Space One and Deep Space Two.  I would be honored to have NASA share the name Deep Space 3 or Deep Space 4 with me!
  
• DS-3 was my third telescope.  My first was a Meade 10" dob with terrible optics, and my second was an 8" Orion dob with great optics.  DS-4 will be my forth telescope.
• DS-3 and DS-4 are primarily designed to be "Deep Space" telescopes, as opposed to planetary scopes.  They both have big, fairly fast mirrors.
• Doesn't the secondary cage look like the docking ring of Deep Space 9?
  

• Weight.  It appeals to me to have my telescopes easy to lift, easy to use, and easy to setup and tear down.
• 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 ramps.  By design.
  

• Primary mirror: 16" f4.5, 1 5/8" thick (Already acquired)
  
• Secondary mirror: 3.1"
• Weight: Total weight, including trusses: 60.4 lbs
  
• Weight of heaviest piece (Mirror box with mirror):  35.2 lbs
  
• Weight of primary mirror and mirror cell:  28.2 lbs
  
• Secondary cage weight: 4 lbs? (Needs to be refined)
  
• Truss weight: 7 lbs
  
• Base weight: 10.1 lbs
  
• Height of eyepiece: 71", or 5' 11"
  
• Finder: Rigel Quick Finder
• Focuser: Current plans are for a feather touch two speed.
  
• Packed size: tbd
• Size of this ultra light telescope if compressed for airline travel by removing the bearings (approximate, without poles): alan

• Primary diameter: 16"
• Biggest mirror that doesn't require a big ladder.
  
• Focal ratio: 4.5
• Much faster than this and coma becomes an issue.
  
• Diagonal minor axis: 3.1"
• Tube inside diameter: 18"
• As narrow as possible, while not causing venyetting.
  
• Tube thickness 0.5"
• Thickness of the wood plate that holds the focuser.
  
• 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: 2.5"
• 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.
  

• Diagonal too small to admit 100% ray?  NO
• Vignetting of 75% ray at front of aperture?  None
• Vignetting of focuser at 100% ray?  None
• 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: 14.8
• Obstruction of the primary by the secondary by area: 4%
• Obstruction of the primary by the secondary by diameter: 19%
• Theoretical resolution: 0.29 arc seconds.

• 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

1. Select the mirror.  I am going to go with an 16" f4.5.  I believe that a 16" to 18" is about the largest mirror that can be made into a car portable telescope, not need a ladder, and have an f ratio of 4.5 or slower.  I have picked up a used mirror that was made be Meade.  I had the opportunity to have the local ATM club test this mirror, and they gave it a very large thumbs up.  So, DS-4 will be a 16" f4.5 truss based dob telescope.
   

1. Select the building materials. Initial design will use 1/2" apple ply.  This wood was chosen due to weight and ease of use.  If weights are not acceptable, composite materials will be used.  For details on composite construction of telescopes, see my Astronomy Links page, Albert's Telescopes.
2. Design the mirror cell. The mirror cell will be a modified version of DS-3.  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:
   
1. Mirror.  1.625" thick.
2. Plop inputs: diameter: 406mm, thickness: 41.3, focal length: 1830, secondary diameter: 78.74.
   
3. 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 4" in diameter, centered at a distance of 4.66" from the center.  These disks will be 4 2/3" 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
   
4. 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 4 2/3 + 2" in length, and 2" wide (ALAN - TEST FOR STRENGTH) 1/2" thick.
   
5. 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	Weight composite	Distance from bottom of OTA
6	Mirror cell contacts	4" diameter	1/2" ply	0.8 lbs	0.2	2.25
3	Mirror cell arms	2" X 6 2/3"	1/2" ply	0.4 lbs	----	1.75
?	Misc hardware		Metal	1.0 lbs	----	1
	Total weight			2.2 lbs	1.6 lbs





3. Design the mirror box. The mirror box will be like DS-3, although may be a bit deeper.  It will also use 8 trusses.  It will measure 18" X 18" 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.)
   

Count	Name	Dimensions	Material	Weight ply	Weight composite	Distance from bottom of OTA
1	Mirror box bottom	18"x18"	1/2" ply	2.9 lbs	----	0.25
2	Mirror box long side	18"x4"	1/2" ply	1.5 lbs	----	2.25
2	Mirror box short side	17"x4"	1/2" ply	1.4 lbs	----	2.25
1	Mirror box top	18"x18"	1/2" ply	1.0 lbs	----	4.5
4	Corner triangles	3"x3"	1/2" ply	0.2
2x2	Bearings 1" thick (here for weight)  (Use DS-3 design)	36"x3"	1/2" ply	4.5 lbs	----	6?
	Total weight			11.5 lbs	9.4 lbs



4. Design the secondary "cage".  The secondary cage will consist of a single plywood ring.  It will have an inside diameter of 18", and an outside diameter of 21".  At present, a 3 wire spider will be used.  See DS-3 for details.



Count	Name	Dimensions	Material	Weight ply	Weight composite	Dimensions from bottom of OTA
1	Secondary ring	21" od, 18" id ring	1/2" ply	1.0	0.3 lbs
1	Secondary focuser board	5 1/2" X 5 1/2"	1/2" ply	0.3	0.1
1	Focuser (Starlight Featherweight)			1.3 lbs	----
1	Rigel Quickfinder			0.2 lbs
1	Shroud			1.0 lb?	----
	Total secondary cage - figured, bare			3.6 lbs	2.7 lbs
	Heaviest eyepiece that I own			1.0 lbs	-----
	Telview Barlow			1.0 lbs	-----
	Total secondary cage - figured, with eyepiece			5.6 lbs	4.7 lbs
	Total ring - rough guess - Includes 2Xds-3, 8mm+barlow, baffles theoretical, MAX			6.5	----	67



5. Trusses. Trusses for this truss telescope will be eight 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 5 above. Truss lengths should be APPROXIMATELY 62" in length.



Count	Name	Dimensions	Material	Weight	Dimensions from bottom of OTA
8	Truss poles	3/4" X 62" long	Aluminum	7	36



6. Bearings.  Bearings will use the same design as DS-3. 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 for plywood only	Distance from ground	Moment arm Weight * distance
Mirror	26	3.25	84.5
Mirror cell disks	0.8	2.25	1.8
Mirror cell arms	0.4	1.75	0.7
Mirror box hardware	1	1	1
Mirror box bottom	2.9	0.25	0.73
Mirror box long sides	1.5	2.25	3.38
Mirror box short sides	1.4	2.25	3.15
Mirror box top	1.2	4.5	5.4
Bearings (guess, used for calculations)	4.5	7	31.5
Secondary ring, focuser, eyepieces, baffling, etc	6.5	67	435.5
Truss tubes	7	36	180
Total	53.2		819


Center of gravity = totalMomentArm/totalWeight, or 819/53.2 = 15.4".  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 2.5", and will have a radius of 13".  This will leave the bearing attached to the side of the mirror box for 16", and have at least 1" of material resting against the side of the mirror box for 12". ?? alan, check.


7. Rocker box and ground board.  DS-4 will use the same rocker box design as was used by DS-3.  Additional stiffening will be added to the rocker box. along the back edge.  The side plates will have weight reducing holes in them.
   



Count	Name	Dimensions	Material	Weight ply	Weight composite
1	Base plate	Triangle,  10" middle to tip	1/2" ply? Aluminum?	1.4 lbs	----
1	Rocker base plate	18" X 20"	1/2" ply	3.8 lbs	1.3
2	Rocker base stiffener	6" X 19"	1/2" ply? composite?	2.4 lbs	0.9
2	Rocker side plate	6" X 18" ?? alan - check this	1/2" ply	2.3 lbs	----
6	Magic sliders - above and below base plate,, Teflon, bolts			0.2 lbs	----
	Total weight			10.1 lbs	6.2 lbs

Height of eyepiece at zenith	71", 5'11"
Size of mirror box and base, no bearings	20x18x8"
Size and weight of mirror box, for airplane travel	18x18x5", 39.7 lbs
Length of trusses	62"