To: "J&S" , ,
        , 
From: tkrajci{at}san.osd.mil
Reply-To: tkrajci{at}san.osd.mil


From:  "J&S" 

>> I'm trying
>> to do it with monocoque fork arms.  If you know of a lighter-weight
>> construction approach, please tell me.
>
>     Monocoque and semi-monocoque construction works very well for
airplanes
>which are strong and light but flexible. For a telescope designed to be
>light weight, with little or no cantilevered loads (alt-az variants) I
guess
>it would be suitable, but I would imagine that fiberglass-covered
stryofoam
>would be much stiffer and just as light.

The way I see it, monocoque construction is essentially a shell.  You get
more stiffness for a given weight when you have such a shell.  (Same with
thin walled tubing...it's stiffer than solid rod of the same weight-per-
linear foot.)

Your proposal of fiberglass shell over foam is substituting one material
for another...so it strikes me as essentially the same approach...but I
don't know the modulus of elasticity of fiberglass, so I can't say if it
will make for a stiffer structure compared to steel or aluminum.

>     I thought this scope was to be permanent and weight was not an issue?

Weight is *always* an issue.  If you make a scope with lots of weight the
fundamental vibration frequency gets lower.  That tends to make for slower
damping qualities after a gust of wind.  If the pro's can make enormous
truss telescope structures with fundamental vibration freq's around 5-10
Hz...why can't amateurs, on smaller structrues...do the same, or better?
Shells and trusses tend to have the most stiffness for the lowest
weight...or is there an approach that is fundamentally better than shell or
truss?

>But for photometry,  image
>rotation shouldn't be an issue anyway.

That's true for photoelectric photometry.  For CCD photometry I want no (or
a tolerably small amount of) image rotation in exposures of 120 secuds
duration.

>     I am still seeing a split ring or horseshoe in your future
>though.  :)

There are fabrication challenges I am wary of...making a 'round enough'
large ring...putting the thrust bearing at the bottom (south end of a
horseshoe) in an accurate enough placement that the runout is tolerable and
the mount doesn't have a 'wandering' axis at different orientations. And
there are probably other challenges I have not thought of yet.

Tom Krajci
Tashkent, Uzbekistan

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