magnilux - Astronomy Technology Today

A 3D CAD PRIMER
Image 1
focal-length value will rise. Click on that
and change the value from 120 inches to
60 inches and, when you hit the return
key, the mirror geometry will still have the
parabolic profile, but of a much faster f/5
mirror.
And, if you have assigned a density
value of Pyrex to that mirror (0.0805 per
cubic inch), you will see the weight go
down as a consequence of the deeper
sagitta. And, if the mirror just happens to
be nested in a telescope assembly, you will
see the center of gravity (CG) shift forward
to a new position – all automatically
and without having to use a calculator!
Builders of Dobsonian telescopes, please
note; never again need you guess where
your bearings should be or have to add or
remove lead ballast from the mirror box –
not if you designed it in a 3D solid modeler
and built it as you designed it! The
CG will be where the software said it
would be.
If your dream scope is going to reside
in a fork mount, again, you must know
where it will balance. If the tube is 30
inches long and the CG is about 10 inches
forward of the primary mirror, will the
back end of the tube clear the base of the
fork when it’s swung No How much
longer must the tines of the fork be in
order to swing the OTA without interference
Or, how much ballast must be
added to move the tube to clear and
Image 2
where
The scrap bins of the world are filled
with the guesstimates that failed to work.
Material can be expensive. Your time is
too. Invest time in designing it properly
before you step into the shop and all these
concerns cease to be. The consequences,
for even the most complex structures, are
faster construction, fewer makeovers, and
more certainty with no surprises.
Faster or Slower
In the beginning, using 3D CAD
software will not necessarily get your
scope built faster. It will likely be a better
design with fewer surprises, but the time
invested while seated at the computer has
to be considered as part of the dues
that must be paid. However, it is fun and,
after so many years of designing optical
systems, I have accumulated a library
of virtual parts that makes the design
process both faster and easier. In addition
there are user groups who share part
geometry with others. That speeds
Image 3
things up too.
Let’s say I need a model of a secondary
mirror with a minor axis (MA) of
three inches and one-half inch thick, but I
don’t have one. I simply copy an existing
model of a two-inch mirror to a new file,
open that one, click on the thickness and
click on the MA, and change their values
– within seconds I have the new mirror.
But it gets better! If the secondary fixture
is linked to the mirror it holds and I
change the size of the mirror, the fixture
updates too! And, so do all the mass properties!
Many manufacturers, anxious to
Image 4
make more sales, are now offering free 3D
models of their parts. You can log on to
McMaster-Carr’s web site, as one example,
and download free 3D solid models
of many parts for use in virtually any
CAD program. I rarely design commercially
available parts any more. I simply
download the geometry from the appropriate
Internet site. The knob shown in
Image 4 is in my designs for collimation.
It’s a solid model and a download that I
can purchase, or not.
If this trend continues, you can
expect eyepiece manufacturers to offer
simple 3D CAD solid models of their eyepieces
along with their mass properties.
They do not appear to be available yet, so
when it came time to see how a 17-mm
Wide-Angle Vixen Lanthanum affected
the balance of a scope, I simply reverse
engineered a real one myself. I only had to
60 Astronomy TECHNOLOGY TODAY