8" F/5.4 Travel Scope

A couple years ago I built a 6" F/6 travel scope.  It works well, provides good images and is compact when disassembled, but it took forever to assemble and this was tricky in the dark.  It also had a bungee-cord spring counterweight system that worked, but I wanted to avoid that if possible in my next travel creation.

My friend Bob recently completed an 8" F/6 mirror with my help, and in thanks he gave me his old 8" F/6 plate glass primary, which had been damaged on the optical surface near the center.  So, I decided to use it in my next travel scope.  I remove most of the surface damage by deepening the mirror to approximately F/5.5, which shortens the scope.  This was done with 220 grit, and then I fine ground it on my machine.  Figuring was simple, and done with the 75% diameter lap used to polish it on the machine.  After ~40 mirrors or so, making a nearly perfect 8" F/6 is simple and quick for me... but still fun!

With the mirror done, I made the cell first to fit it and then sent it off to get coated.  The rest of the scope took shape fairly quickly after I got the basic concept in my head - it would have two six-pole trusses that would collapse to go in my carry on bag or in my suitcase.  I first set out to build the mirror box and secondary cage, and my concept was that the wire spider would be removeable so that the mirror box could next inside the secondary cage, which is the reverse of what many do.  This saves space, because the minimum size of a secondary cage is larger than that of a mirror box to allow clearance for the light cone to fully illuminate the field.

Below is a photo of the secondary cage under construction.  The cage has 1/4"-thick sides, and a 1/2"-thick baffle set 5/8" down from the front of the cage.  The corners are mitred at 45º and glued, and the baffle is simply glued inside (after careful fitting).  The inside of the cage is only about 1/16" larger than the outside of the mirror box.  The mirror box, picture at right below, has 1/2" thick sides (all wood is Baltic Birch) with a 1/2"-thick top baffle that also serves to connect to the truss pole system.  (The thicker mirror box reinforces the thinner cage in transport.)  Some plywood triangles are installed in the top corners of the mirror box near the back.  T-nuts in those serve as attachment points for two collimation bolts.  The bottom of the mirror cell pivots on a rubber washer.  One screw holds the cell against the washer.  As shown below, the mirror box is upside down.

Cage in clamps Mirror box

The bundles of poles are just like that of my 10" F/8.8 (see my page here or see my article in the March 2006 issue of Sky & Telescope) except shorter, and I used 5/8"-diameter tubing instead of 1".  A friend made the "hinge blocks", which connect the ends of the poles, out of aluminum channel welded at the proper angle.  They work great and are stronger and less bulky than wood (thanks Andrew!).  The bundles were assembled with the scope held in a jig to ensure the structure would be straight.  To join the two bundles of poles, a central baffle board is installed.  Each hinge block on the bundles has a plastic "peg" installed in the bottom, which fits into corresponding holes on the mirror box, baffle board, and another piece that sits agains the secondary cage.  Here is a photo of the scope in the jig, with the last few poles being installed.  Note the extra 1/4"-thick baffle on the right side of the secondary cage - this provides a point where the trusses can attach, and it is removeable so that the cage and mirror box "nest" together.  The center baffle where the trusses join is simply a piece of 1/2"-thick plywood.

Scope in jig

You might wonder how much slop is in the two bundles of poles - the answer is "some".  While my 10" F/8.8 and F/5.5 have no slop, this scope does.  The key is that the whole structure is held in COMPRESSION, which makes slop permissible - the slop disappears when the structure is compressed.  This means the cage is simply pulled against the thinner baffle, which has small protruding blocks to locate the cage against it.  The tension is provided by a nylon strap, connected to the secondary cage, which is pulled tight around the bottom of the mirror box, holding the scope together with no latches, catches, knobs, etc.  I've never seen such a design, and it was inspired by trying to avoid installing latches at all truss attachment points and making slop completely irrelevent.

Here's a photo of the scope assembled, partially painted, showing the strap holding it together.  Also, there is a photo of the completed cage, with focuser (attached to a piece of wood which attaches to the cage with three thumbscrews) attached and wire spider installed.  The eye bolts slip into slots in the 3/4" aluminum angle that serve as spider attachment points.  The two grey "nuts" installed on the eye bolts at the bottom left and right allow the spider to be installed and tentioned with no tools, and also de-tentioned and removed easily.  It works.

Structure under tension Cage, focuser, spider

The bundles of poles were made slightly different lengths because I didn't know where the balance point would be.  Once I checked the balance point, I installed the trusses so that they would provide the most convenient point for attachment of the bearings, which attach to both the central baffle and the mirror box, thereby further strengthening the structure.  Thus, the two-truss structure has many benefits:  1) it provides triangles with wider legs for more rigidity, 2) it helps baffle the optics, 3) the baffle provides an additional attachment point for the bearings, and 4) the system shortens the truss tubes by nearly a factor of two.

Below is a photo of the side bearings attached to the mirror box and the center baffle, with the upper truss and cage removed.  Three thumbscrews attach each bearing.  On the top of the center baffle you can see the three holes that the pegs of the upper truss fit into.  The bearings are 1/2" plywood with 1/16" thick aluminum epoxied to the bearing surface.  The wooden mirror box cover is in place.  Also shown at right below is the stand that I built for my 6" travel scope - it is quite up to the task of supporting this scope, too, and I simply moved the Teflon pads out a bit to support the azimuth bearing better.  The legs unscrew for transport.

Bearings on Travel scope stand

Below is a photo of the simple rocker box.  One side is removeable to save some more space when the scope is broken down and packed up.  At right is a view of the painted scope looking down the tube.  The three secondary tilt thumbscrews are visible (no tools needed), as are the attachment points for the nylon strap that holds the scope together.  Yes, there is probably a better tensioner, but I used what I had on hand.  Focuser is a 1.25" reverse Crayford from JMI, chosen due to its light weight.

Rocker box View down the barrel

Here are photos of the items that go in my carry-on bag, at left, including the primary, focuser, spider, trusses, and foam light shield, and at right, the pieces that go in my suitcase.  The mirror box is stuffed full of clothes.  I also have eyepieces and a laser collimator to pack up.

Carry on parts Suitcase parts

Finally, here's a photo of the finished scope at its first destination, a beach in Negril, Jamaica.  Conditions weren't the best for observing, but it was still an excellent vacation without observing with the scope, though I did see Alpha Centauri for the first time.  Ya mon!

Scope on beach

I am reassured by the fact that I now have a travel scope that I enjoy using and which can be ready to go at a moment's notice.  Of course, the thought lingers in the back of my mind - how large can I take the design?  It is possible I could use the same sets of trusses on a larger scope of slightly shorter focal length, so a 10" or 12" is quite possible.  The same bearings might even be useable!

In conclusion, this design has really given me some ideas.  I have a thin 13.1" F/4.5 mirror that I plan to build a similar scope structure for, and hopefully I can improve on the tensioning method.  That scope will be light and I suspect I will use it a lot because of that!  Can't wait.

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