My amateur telescope making page

The 12 inch travel (Trill) Design

Overview

This is a design for a 12" travel telescope.  

DS Trill requirements are as follows:

• Must fit in a wheeled suitcase. 
• Must be the largest mirror possible.
• The telescope and suitcase must weigh under 50 pounds for airplane travel.  Must be able to reduce max weight to 44 pounds for train travel..
• Telescope and suitcase should be rugged.
• No electronics.
• Setup time is less important than other scopes, but should be under 30 minutes.
• Setup of this truss dob should require no tools, or have minimal tools included in the suitcase.
• Easy to build.

• Primary mirror: 12" f5.0, 1.5" thick, enhanced coatings
• Secondary mirror: 2.1", enhanced coatings.
• Secondary diameter percent of primary diameter: 18%
• Weight of heaviest piece (Mirror box with mirror):  unknown.
• Secondary cage weight: unknown
  
• Truss length: unknown
  
• Height of eyepiece: unknown
  
• Finder: Rigel Quick Finder
• Focuser: HC-1 helical Crawford 1.25"
• Packed size: 15" x 15" x 8".  22.5" trusses fit in the base of the suitcase.

• Primary mirror diameter: 12", 1.5" thick
• Biggest mirror that I can fit in luggage.
• Focal ratio: 5.0 
• Diagonal minor axis: 2.1"  
• Tube inside diameter: 14?"
• Tube thickness 0.5"
• This is for half inch plywood. 
• 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 for a HC-1.
• Spare focuser in travel: 0.25"
• I think that this is irrelevant.
  
• Focuser inside diameter: 1.25"
• Designed for a 1.25" 24mm Tele Vue Panoptic eyepiece, with additional 2X and 3X Tele Vue barlows.  Minimizes weight and size in my carry on luggage.
  

• Diagonal too small to admit 100% ray?  No.
• Vignetting of 75% ray at front of aperture? No..
• 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.  1.25" is needed for weight requirements.)
  
• Theoretical limiting magnitude: 14.2
• Obstruction of the primary by the secondary by area: 3%
• Obstruction of the primary by the secondary by diameter: 18%
• Theoretical resolution: 0.38 arc seconds.

• I am convinced that a 4 point suspension for this small mirror will be sufficient. I did not try Plop.  Further, no astigmatism seen at the eyepiece with star tests.

• 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

1. Select the mirror.  12" f5.0.  This is the largest mirror that works with the smallest secondary cage that fits in a suitcase.
   

2. 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. 


3. Design the mirror box and cell. The mirror box will measure 13.5" X 13.5" X 3"  The bottom of the box is also the mirror cell, and will have the mirror glued to this bottom piece in 4 places with silicone glue.  This box bottom/mirror cell is not glued or attached to the rest of the box, but is supported with 4 large bolts running up each corner of the box from the bottom.  In transport, these bolts hold the box bottom snugly against the box sides.  In use, the bolts are backed out about 4 to 8 turns (1/4"), the mirror and cell drop away from the box, and these bolts are used to collimate the telescope.  Gravity is used to keep the mirror cell in position on the bolts.  Stiffening triangles are placed in all of the box corners, and these triangles are used to hold the bolts mentioned above.  On top, the mirror box will attach to 4 trusses, i.e., 8 tubes.  Bearings are attached to the sides of the box. Laminate will be used for the mirror cover, to be held in place by velcro.

Note - Trick - During the calculation of the center of gravity of the OTA, and thus the diameter of the bearings, I subtracted one inch from the Z of the OTA, and thus one inch from the bearing radius.  This is the equivalent of needing to add 3 to 4 pounds to the mirror box.  This was done in order to make the bearings smaller so they fit in the suitcase.  Either ankle weights or socks filled with onsite gravel will be tied to the front trusses.

4. Design the Bearings - rough, for CG calculations. The bearings will consist of 1/2" plywood. The bearings will taper towards the top. They will also be designed to only allow the scope to go from 10 to 80 degrees from the horizon. (Note post build - the finished scope does to to the zenith, but only down to 10 degrees above the horizon.)
   

4. Design the secondary "cage".  The secondary cage will consist of two plywood rings.  They will have an inside diameter of 13.0", and an outside diameter of 15.0".  Use a commercial Astrosystems spider and secondary holder.



Count	Name	Dimensions	Material	Weight ply	Dimensions from bottom of OTA
1	Secondary mirror	2.1"	glass	2.0 lbs
1	Spider stuff	??	metal	1.0 lbs
2	Secondary rings	15" od, 13" 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 (KineOptics HC-1)			1.3 lbs
1	Rigel Quick Finder			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



5. 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



6. 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".


7. 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
	Total weight