If you're really stuck, ask me and I'll send you plenty of more
specific links and references that I've accumulated over the
years.
My best suggestion for inspiring ideas / dialog is to take a
look at the various "ATM" sites (amateur telescope
makers / making) out there. There's a LOT of
good information on all kinds of homebrew motorized
drive mechanisms for various kinds of mounts / platforms
out there. There are all kinds of options from the simple
to the byzantine and the elegant to the rube goldberg
"wow I can't believe that works".
There's nothing inherently wrong with steppers; being
a fairly old time amateur astronomer myself, I realize
that the two objections are (a) complexity -- needing
a less common motor, and digital/analog/power drive
controller board electronics to drive it at the proper rates,
and (b) smoothness -- if you're tracking something at high
magnification, or doing long exposure astrophotography,
then the jitter due to the jerky steps / pulses can cause
irritating vibration / dancing / blurring of the object
even though the jitter motion may be quite small.
That being said, almost all the modern scope designs
for modestly big instruments including ones that
are excellent at astrophotography do use steppers and
feedback control system technology to control the
motors. In fact many scopes use ALTITUDE-AZIMUTH
mounts and it's ONLY the smart positioning/tracking
systems that keep them tracking anything at all. The
cost / size / mechanical inconvenience of an massive
equatorial mount is FAR worse a burden than something
effectively as cheap/simple as an alt/az 'dobsonian' like
mount added with the cost of the relatively more
complex stepper / computerized tracking & motion control
system and position sensing systems etc.
Also you can do things like micro-step steppers even
to the point of using them as somewhat linear thrust
motors in the space between one step and the next
so in effect it' acting more like a DC motor (which it
is, really, just one with several individual phases of
stators to control) than a digital 'step' motor.
You can get things called slow / synchronous motors that
are AC sine wave motors that are powered by household
power, and they run synchronously to the AC power
frequency which is only 60 or 50 Hz. Given that and very
modest up/down gearing, you can end up with very slow
speeds indeed, but you're limited to places where you have
sources of sine wave AC power to run them.
If you pick the appropriate DC motor you can run it
quite slowly, you just need to find one that's got plenty
of torque and can run with up to 12 hours * many days
per year of service duty in a near stalled condition with
very little energizing current if it's being used at much less
than it's usual rated speed. Generally that'll be a very
over-rated motor used at a fraction of its capacity.
There are also lots of ways to 'cheat' and get an excellently
smooth tracking with relatively cheap parts. One being
using an 'equatorial platform' which is basically just
a board / 'ramp' that the telescope sits on that happens
to point north and be tilted at the local angle of latitude.
Think a couple of doors screwed together by one set of
hinges, one laying flat, the other tilting open at whatever
angle is needed, and a brace set between them to keep
them open at the right angle. Just a crude description;
there are lots of designs for building them out of
anything from plywood to concrete etc. etc. check the
ATM articles.
Given such an adjustable equatorial platform you're
already north aligned and equatorially tilted, then you
set an alt-az mounted scope on it e.g. like a dobsonian.
Then use a manual ALTITUDE adjustment on the scope
since ALTITUDE (now == declination given the
e.q. platform) never needs to change once you're pointed
at the object of interest. So then the only axis that
needs mechanical tracking is the AZIMUTH
(== right ascention now given the eq. mount). This
can be done ever so smoothly, cheaply with a
tangent arm drive. Get a long threaded rod with modestly
fine threads per inch of its length.
Fix one end of the threaded rod to your favorite motor,
DC, AC, synchronous, whatever. The motor and rod are
fixed to the platform and do not move with the scope.
Put a "nut" aka "follower" on the fixed position rotating
threaded rod, and hold the nut / follower from rotating
and have it 'push' tangentially against a rotation circle
attached to the azimuth (aka R.A.) axis of the scope.
As the motor turns, the nut/follower 'unscrews' along
the length of the threaded rod and pushes the scope
along. Disadvante being it can only track for a limited
period of time since the follower is moving in a line and
the scope is rotating, so the line is basically a chord of
the rotation circle of the scope and once the nut is at
maximum extension you have to mechanically 'reset'
the scope by rotating the tube mount relative to the base
so that the 'retracted' position now again points at the
object of interest and you're ready to track for another
length of time until you reset again. With even a modestly
slow motor and modestly resolute threaded rod you can
devise these to track for hours at a time which is usually
quite sufficient even for photography, though it's unsuited
for something that should function from dusk to dawn
entirely automatically. Of course you can add
altitude / declination motor drive if you want too, but
that's quite optional.
Another variant is to make the motor and threaded rod
so they move linearly on a slide as the rod screws against
a fixed nut/follower; in that case you use the end of
the screw rod itself to push tangentially against a
fin/driving point on the rotation axis of the scope so that
the tip of the screw pushes along the rotation of the
scope. Again, smooth, simple, and the drawback is
that it'll work over a certain angle of arc driving duration
before needing to be reset to bring the rod back to
the retracted position, bear the fin against the rod, and
rotate the scope relative to the fin for another period
(minutes, hours, depends on your mechanical design)
of driving.
Another simple / cheap option is again basically like
a dobsonian idea; instead of using a very very fine
toothed right ascention gear and a fine gear to rotate
against it, just abandon the gears and use a drive
circle attached to the rotation axis of the scope that's
not a gear at all but is just a very large diameter CIRCLE
that's FRICTION DRIVEN by the rotation of a small
motor mounted next to it. If you use something like a
small circular table top, circular piece of plastic
countertop, circle-cut plywood, etc. it's hardly difficult or
expensive to get a drive circle that's perhaps two, three,
four feet in diameter. Bicycle wheel, same idea.
It's not particularly in the way, heavy, or inconvenient
if your scope tube is even longer (e.g. 12" F/5 = 5' long
tube or 8" F/8 tube etc.), and the drive circle can even
be disassembled into two or four pieces if need be for
transport / storage compactness.
So given something like a 3' e.g. diameter drive circle,
that's 113" of circumference over 360 degrees of arc.
Drive against it with something like a 1" diameter
friction wheel and that's a 1:36 rotation reduction right
there from the gear ratios. So for the big wheel to
turn at 1/1440 RPM (1 turn/day) the small wheel
would turn at a rate of 1/40th RPM which is a lot more
easily achievable with common gear box reduced motors
due to the large drive circle to small drive rotor ratio,
and it'll help minimize the effect any irregularities in drive.
The friction drive against the drive circle is of course
easy to mechanically or electrically (solenoid/spring)
disengage for manual repositioning of the scope, and it's
advantageous since there is no precise alignment or
gearing of the scope to the drive motor, so no worries
about stripped gears or precise assembly.
) is how to figure out how slow a dc motor can safely go given the data provided by manufacturers such as mclennan.co.uk and pittmannet.com.
