Round stars at last!
Prelim images from testing tonight and a few nights back.
Hopefully in a day or so I will compile an expanded thread to explain a recent re-tuning of my
R.A. stepper motor on my homemade Bartel stepper drive,
Edit: as promised:
As all Bartel-o-philes will know, the Bartels stepper drive is a beautiful thing to work with.
When everything is tweaked and set just right it can provide a
very efficient dual axis drive for a GEM, Dob or EQ platform.
It can even be retrofitted to commercial fork SCTs or EQ-GEMs.
Bare bones it can just be a dual axis goto, but is capable of many more add on extra
features such as fine focus, camera rotator, digital setting circles, dome rotation etc.
And it is cheap. Software is free and open source. ASCOM integrates as well,
such as POTH, LX200 commands etc.
My homemade setup has worked beautifully for years.
I added mouse encoders and remote focus.
Dome rotation is underway, but needs some maintenance on the 13 year old foam dome first.
The guts of it:
My RA and DEC steppers are tuned to provide the smoothest torque but at the
lowest current and vibration that the minimum torque required can still work at.
That includes the torque needed for high speed slews.
This involves some experimenting with two values in a config file that the Scope.exe program
runs in DOS. MsPause and MsDelayX. Different steppers need custom values, no value will work for all steppers.
The two values are microstepping parameters. Each step of the stepper motor
is further divided into 20 or up to 40 microsteps.
If these microsteps are smooth enough then that should allow the overall rotational smoothness
to be pretty good as well. PWM microstep values are expressed in values up to 100.
If the PWM going to two windings of a steppper are 100 and 0, the rotor will be right over one 'step' position.
If two windings are 100, 100 then the microstep is halfway between steps.
This isn't always the case though.
I used a good set of microstepping parameters for years but recent mods to the coupling connecting
the RA stepper to its geartrain and final worm gear made these parameters useless.
I needed better numbers.
So I started from scratch and attached the laser to the stepper to check my microstep spacings again.
Years ago ,for my initial microsteps, I projected the laser only about 3/4 of the way across the dome, about 2.5m.
For my newer set I projected it out through the dome door and on to a whiteboard chart about 5m away.
This allowed very fine adjustment of the microstep values.
The old ones were terrible. After a few afternoons refining them then checking the scope at night on stars
I had better figures and lower vibration as a result.
But this needed different values for MsDelayX and MsPause. And so you seem to go round in circles.
It is a good idea to set these first for lowest vibration (and lowest viable current/ torque) then set the PWMs next
with the laser.
I did this and viola, way better smoothness and when checked on the sky, nice round stars.
Almost no vibration gets through the coupling now to the OTA.
With finer spider vanes too, vibration has to be kept low.
It also has to be checked with the OTA in all attitudes, what might be fine for polar region 35 degrees alt, might be terrible
for facing eastward close to zenith for example. I found that out the hard way.
For the record, RA stepper is an AstroSyn 10v, current measured at 55mA while microstepping, between 20-30mA while
half step slewing.
MsDelayX = 6
MsPause =265
Microsteps=20
PWMs/tick~ 85 on a Pentium 120 laptop (circa 97)
Some pics:
3. laser mounted on the RA stepper
4. chart at 5m away showing refinement of PWMs
5. PWMs adjusted 'on the fly' in Mel's brilliant software
6. RA stepper / gearbox / worm, note rubber mounting of stepper.
7. The old girl. That GEM was made in early 80's and has undergone many mods.
Steve
