A 27 pound travel scope



 
 
 

Before
After

This telescope was designed around a 13.1" f/4.5 mirror and the desire to get it inside a suitcase that meets the requirements for carry on luggage of most airlines so that no part of the scope would have to be checked.  There is a lot of variation from one airline to another so I decided on 14" x 9" x 22".  My weight goal was to keep it under 40 pounds.  The weight, as pictured above right, ended up at 27 pounds.  The 22" diameter of the altitude trunions and the very low rocker box height necessitated a very low center of gravity which I could only achieve with the use of a 32 pound counter weight.  This is the configuration I use at home.  When traveling I use (actually I should say I will use, if I ever fly anywhere) spring virtual counter weights to balance the tube assembly on the rocker box and 2 six packs of soda to balance the rocker box on the ground board.  With the 32 pound counter weight the total weight comes to about 60 pounds.  With the springs and soda the weight comes to just under 40 pounds, but the scope can be transported without haveing to lug around 2 six packs of soda resulting in a 30 pound package.  The truss tubes will fit in a long garment bag which I believe can also be carried on to most airlines and hung on a rack between seating sections.  At any rate, I've lost the desire to segment them so they'll remain single piece for now.

Tools
 
Hacksaw, jigsaw, drill, dremmel tool, screw drivers, 
clamps, SAFETY GLASSES ... 

 
 

Materials

43 feet of 3/4" aluminum square tubing with a wall thickness of 1/16".  Cost:  $32.25 @ $0.75/foot from a local metal supplier.  Much thanks to the employee at the hardware store who advised me I could do a whole lot better than $2.25/foot!

12" x 24" x 1/16" aluminum sheet for secondary support.  Good thing I bought enough for 2 because I screwed up the first one.
                                                                                                                                                         Cost:  $7.00
2 feet of 1/8" x 4" aluminum sheet for corner braces.  Cost:  $8.20

24" square 1/8" aluminum sheet for altitude truniuns.  Cost:  $24.54

24" square 1/16" aluminum sheet for mirror cover.  Cost:  $13.50

I used 3/16" screws and nuts with nylon inserts throughout except for the altitude bearings and attachment of the counter weight.  I also used quite a few t-nuts that were attached by 56-20 screws.  Cost:  $26.83

20" x 10' aluminum flashing for secondary baffle.  Cost:  $9.49

Four 1.25" rollers (used on sliding doors) to support altitude truniuns.  Cost:  $7.98

Springs for virtual counter weight.  Cost:  $6.31

32 pound counter weight, 2" x 4" x 13.5".  Cost:  $51.14 (ouch)

Total cost of materials:  $187.24.  If I had investigated the possibility of spring counter weights first, I probably would have skipped the steel counter weight and saved myself $51.14.
 

The telescope
(a larger version of most images can be seen by right clicking
on the desired image and then left clicking on "view image".)
 
 
These two images show the scope pointed straight up from the front  and from the side.  I made the scope with 5 trusses and a very simple upper tube assembly to keep the weight down.  Since this photo was taken I have added a secondary baffle and finder scope.

 
The scope was not yet finished when these photos were taken but they are of better quality and help to show the configuration of the truss tubes.

 
This view shows the scope from the top.

 
 
These views show the secondary baffle and the finder.  The inside of the baffle has since been painted black.  I was amazed at the size of the baffle required by this scope.  It is about 36" in diameter.  The focusser baffle and secondary baffle calculations can be done at Mel Bartels site.  A secondary baffle is a must from a light polluted site and also helps more than one might think from a dark site.  But for casual observing from a relatively good site the secondary baffle is not necessary.

 
The upper tube assembly, including telrad, weighs 2.75 pounds.  I used an astrosystems focusser.  The quartz secondary has a minor axis of 2.1" and is .5" thick.  It was made by Bill Marriott and performs very well. 

 
 
Here's another view prior to painting and installation of the telrad.

 
The secondary holder is 1/8" aluminum bent to about 45 degrees.  The secondary is attached by a 3/4" blob of silicone that is 1/8" thick.  If I had it to do over again I would use a thinner blob.
The secondary support is 1/16" inch aluminum.

 
 
My original plans called for bracing of the altitude truniuns.  They are about 2" wide, 22" in diameter and are 1/8" thick.  Each truniun is attached by 2 screws and turned out to be very stable with no additional support.  The 32 pound counter weight can be seen attached to the back of the mirror box.

 
 
The primary is a 13.1" f/4.5 that is 1" thick and weighs about 10 pounds if I recall correctly.  I was fortunate to have John Hall refigure this mirror for me a few years ago.

 
 
The primary is attached to three 1/8" thick aluminum triangles that are attached to an inverted "T".  The upper leg of the T is fixed and is not adjustable.  Collimation is done by turning the screws that attach the horizontal component of the T to the mirror box.

 
 
Collimation of the primary is done by adjusting a screw on either side of the primary.  These screws can be turned while the laser spot is observed returning to the focusser tube.

 
 
Without the clamp the whole scope would tip over.  When horizontal, the center of gravity is well ahead of the forward bearings as well as the feet of the ground board.  For more on virtual counter weights read this from Tom Krajci.  I also recommend Toms website.

 
 
Two six packs of Mountain Dew weigh 10.5 pounds.  When located 13" behind the rocker box they perfectly counter balance the scope when it is horizontal.  When the scope is vertical the counter weight is not needed and the scope tips over backwards.  To solve this problem I will attach a simple linkage between the mirror box and the sliding tray to move the 6 packs closer to the scope when it is vertical.  Why go through all this trouble?  The 32 pound counter weight makes the scope to heavy to carry on to an airplane.  The springs, tray, and tray support/guide are all very small and light, and I can buy 2 six packs to act as counter weights just about anywhere I go.  As you can see in this photo the counter weights can also be used to quench your thirst after a long night of observing.  (The 32 pound counter weight  would normally not be in place when the springs are in use.  I just didn't want to remove it to take this photo.)
It has also occurred to me that if a curved counter weight support/guide were used and correctly linked to the mirror box, no springs would be needed.  I may investigate this further in the future if I get the time. 

 
 
A view of the non virtual counter weight.

 
 
The truss tubes attach by means of t-nuts that are located inside the square tubing of the mirror box.  The t-nuts are attached to the square tubing by means of very small flat head maching screws and nut. The heads are counter sunk into the tubing.

 
 
The camera was held up to the focusser for this photo.  The vanes of the secondary support are not as thin as most 3 or 4 vane supports, but are much thinner than the original equipment coulter support I had been using before I built this scope.  A little bit of the focusser baffle can be seen on the left.

 
 
This photo is a close up of the center spot on the primary with the laser collimator in place and turned on.  The scope had just been collimated while at an altitude of about 45 degrees.  When the scope was lowered to a horizontal position the laser spot moved down to the position you see here.  Similarly, when the scope was raised to a vertical position the laser spot moved up an equal amount.  I will eventually try and track down the source of this movement.  I think it is almost certainly in the trusses.  But it could also be in the secondary holder and/or support.  (The laser spot is not actually a spot.  I use a very cheap collimator that projects a short line instead of a spot.)

 
 
The cover for the primary has been cut out and is waiting to be bent.  The secondary support was so easy to bend I anticipated no problems with this cover.  I couldn't have been more wrong.  The cover is much wider than the secondary support and, though only 1/16" thick, was extremely hard to bend. 

 
 
 
The components of the mirror box being assembled.

 
The secondary support after being cut.

 
Bending the secondary support.  I had no problem with this but will bring all such jobs to a machine shop in the future if they deal with larger pieces of metal.

 
I haven't had the whole scope in the carry on case yet but I have had individual components in the case to see if they fit.  This is the ground board and rocker box.  The case measures 22" x 14" x 9".  I am confident the scope, except for the trusses, will fit.  One of these days I'll get around to segmenting the trusses so they'll fit in the case as well. 

 

This scope did not turn out quite as well as I would have liked but I'm generally happy with it, and it was a lot of fun to build.  The split ring equatorial weighed about 120 pounds resulting which resulted in the scope not being used on some nights.  I mainly use this scope for planetary observation.  The best seeing in central Missouri comes at about 3:30 in the morning and the thought of carrying 120 pounds out to the driveway often caused me to stay in bed on good nights.  Now when I start my Jupiter observations the first week in September I won't miss any nights due to a heavy telescope.  I also hope to complete a cylindrical bearing equatorial platform by then.  Much thanks to Jerry Kelly for suggesting I use aluminum instead of wood.  I have never worked with metal before, but found it to be not much harder than wood to work with.  The idea for the basic design came from several scopes built some time ago by Thane Bopp.  Thanks to Clive Milney and Bruce Sayre for making their expertise available to the ATM community.  The idea for the cell came from an unkown member of the S.T.A.R. astronomy club.

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