Sunday, April 21, 2013

Big Bertha Diet

The semester almost being over, it is time to implement the rebuilding of Big Bertha using a half-Suerrier truss, using Moonlite Optical's truss components.  While doing all the measurements yet again, I noticed some more opportunities to remove weight.

Right now, there is a 72" long C-channel that mounts the mirror assembly and the eyepiece/diagonal mirror assembly to the dovetail plate that goes into the telescope mount.  The plan was to cut this down to 24" long because the half-Suerrier truss does all the holding of optical components in place; the C-channel will only connect the mirror assembly to the mount, which saves 4.8 pounds.

It turns out, however, that this C-channel has 2" "legs" and is 4" wide.  The legs are so that the tube is supported by the legs, and screws in the middle of the C-channel to the tube keep it from moving.  But really, there is only a need for 1/2" or less legs.  Trimming this down will reduce stiffness a bit, but stiffness isn't really an issue here -- and it will knock off another pound of weight.  I don't know if the bandsaw will do this well or not -- but if worst comes to worst, I will put it in the vertical mill on its side, and use a fine end mill to slice the legs down to size, then turn the C-channel on its back, and use an end mill to trim everything to a consistent length.

Then I noticed that the dovetail plate (which looks like this, although longer)
that connects the C-channel to the mount saddle is quite long -- about 21" long, far longer than needed to properly support the telescope on the mount.  What if I found a shorter dovetail plate?  If I found one that was 12" long, it would save 2.5 pounds.  What I dispensed with the dovetail plate completely, and cut the dovetails into the bottom of the C-channel?  (I have a dovetail cutter, although I have yet to use it.)  That's six pounds saved.

I am thinking that I can knock this down about eleven pounds -- down to perhaps 48 pounds total or less for a 17.5" reflector.  That is light enough that the CI-700 mount should be able to handle it -- and the half-Suerrier truss will be stiffer than what I have now.  It also lets me remove about 11 pounds of counterweights as well.

UPDATE: My mistake: I have a slot cutter, but not a dovetail cutter.  This might be the thing to buy next.

UPDATE 2: What I have now is a 19" long dovetail plate: the DP19, which is 3/4" thick.  It turns out that the existing C-channel that I have is only .16" thick in the base, and I would need to cut a dovetail that is .25" deep.  Yes, that is not going to work.  I may use the Losmandy DUP dovetail plate instead. It is only 1/2" thick, weighs only 2.4 lbs. (partly because it has holes everywhere -- rather Swiss cheese like), and is 14" long.  This is still a savings of 2.6 lbs. from the current dovetail plate.  It looks like I am going to save at lest 7.4 lbs. from a shorter, thinner dovetail, shorter and lower C-channel.  I may also save fractions of a pound from being able to use shorter bolts to connect the lower cage to the C-channel, and dispensing with the connections of the upper cage to the C-channel.  To my surprise, the Moonlite Optical truss connectors and tubes are roughly a wash with the existing square tubes and guy wire scheme.


  1. But will you ever be able to fold it up and carry it in a rucksack?


  2. I may be able to fold it up and carry it in the Corvette!

  3. You sound like the racing engineers looking at the chassis drilling lightening holes. . .
    Do post pictures when this is finished (or done enough to use).



  5. So to solve your rolling on a rough surface problem try taller wheels. Wider will help but taller will work the best.