Ross Sackett's amateur telescope making
Models and
prototypes
I think of models and prototypes as three dimensional sketches.  They are an
important part of my design process.  The two maquettes shown here were
crucial in the development of the Moonsilver series.  I made the 8" tall model
above while trying to figure out the arrangement of Moonsilver I, my first
single pole telescope.  Until I made it I couldn't wrap my head around the
asymmetrical sculptural shape of the scope.  In frustration I started hot melt
gluing foamcore and aluminum rod in various arrangments, and after about
15 minutes this emerged.  From there I went back to drawing, and after
another hour I had the design.

The blue model to the left was important in building the 18" Moonsilver III.  I
had already worked out the general design on paper, and I even knew what
color I wanted to finish the telescope.  But I just couldn't convince myself that
the design would work for a telescope as large as 18".  If it was too large to
carry there was no point in building it.  This model took about half an hour to
make out of foamcore and wooden dowel, and built my confidence that the
design was really practical.  Also, it showed that the color would be
outlandish, but not nuts.
I was working on the truss design for Captain Nemo, and wanted to go with
six truss poles.  But I also wanted a hinged lid to protect the mirror.  The
difficulty is that in the conventional symmetrical geometry of a 6-pole truss
the poles prevent the lid from opening fully unless you make the truss much
wider that it otherwise needs to be.  In an online forum I mentioned this
problem, and ATM Dale Eason responded that in fact there was a way to do
it, but didn't elaborate.  I started fiddling with thin ply and bamboo skewers,
and built the conventional (left) and a modified truss (right) with more room
for a lid.  I later saw a photo of one of Dale's scopes, and it showed a truss
much like the one on the right, which is the design I ultimately used in Nemo.
Prototypes often show you what not to do.  This alt-azimuth head was meant
to be the original mounting for the Stellascope.  Testing showed it to be
much too flexible for even a lightweight telescope like that one.  In trying to
work out that bug I came up with the "hybrid dob-fork" design I ultimately
used in Stella and the Moonsilver scopes.  I later found out that British ATM
Francis Milsom had designed a similar hybrid '3 point" mount several years
earlier, an interesting example of convergent evolution in engineering.  
Curiously, he later independently built a single pole travelscope similar in
some ways to Moonsilver, but didn't employ his clever 3-point mounting.
I always prototype new mirror cells and cell parts.  Cells are a three
dimensional jigsaw puzzle, and you have to get both the shapes of the parts
and their interrelations right for the mirror to be supported well and be easy
to collimate.  I find my tendency in cells is to start out overly elaborate and
complicated, then to simplify, simplify, simplify.  The fewer the parts, and the
fewer the points of adjustment, the less that can go wrong and the better the
scope can maintain collimation.
Trusses are another can of worms to engineer, especially the clamps.  I think
they resist simplification more than any other telescope part except...
...secondary assemblies.  I am an aficionado of elegant secondary holders,
and I hate kludgy designs.  Unfortunately, it is very hard to get secondary
assemblies right, as these prototypes show.
But occasionally you stumble onto something that works well.  These are the
prototype experiments that led to the secondary holders I used in Moonsilver
III and IV and in Captain Nemo.  I like this design very much, and will use it in
other telescopes.
Here are some experiments with tube rings.  The one in the middle is a
single-piece stiffening endring for cardboard tubes, cut with multiple router
setups.  Clever and elegant, but I haven't used it in a telescope yet.  The ring
on the right is a prototype for a small truss UTA with a wire spider, also not
yet built.
I often prototype construction methods if I don't have experience with them.  
Here I have been fiddling with stiff-but-light sandwich construction with
plywood faces and lightweight core materials (paper honeycomb on the left,
balsa on the right).  Similar sandwich panels and rings were used in
Moonsilver IV and Nemo.
Here is a demonstration showing a manufacturing sequence for building a
ball bearing primary mirror edge support.  I even varnished the wooden base.
 Pointless, as it turns out, since I now use commercially made transfer balls
for this purpose, which work much better.  But it was fun to make.
My telescopes and ATM Projects