Telescopes are something I know a little about, What you don't tell us and what matters quite a lot is the mechanical gearing. How are the motors connected to the mount?
If this is an equatorial mount what are there three motors? and why care much about if declination moves smoothing as it is only for slewing to a new target. But if this is Az, El then yes I see why three motors, You have Az, El and Feild rotation and all most move smoothly and in exact synchonized motion. But an equatorial drive needs only one moer to move smooth. As for PWM. PWM is nothing at all to do with stepper motors. You can forget about it. Unless you decide to not use steppers. The way to work this is to choose a gear ratio, typically a very large reduction, andchoose a step size that is literally "invisable" given you angular resolution. Of course angular resolution depends on aperture or maybe the pixel size and focal length if this is purly a photographic 'scope. The steps need to be smaller then 1/2 the angular resolution and conservative designers might go even smaller, Hence the use of really big worm/wheel reduction systems Example. (1) You have a 180 tooth gear and a 1.8 degree step size. That works out to 36 arc seconds per step. This might be good enough for a small scope used in poor seeing condidtions. (2) 360 tooth gear and 1/8th microstepping now you are at roughly 2 arcseconds per step. This is much better How fast do you need to step? Assume the Earth turns once per day ;-) That is 1,300,000 arc seconds per 86,000 seconds. Or about 15 arcseconds per second. So the motor in case number two steps only about 7 or 8 times per second. So it looks like you could do 1/16th step or even more. But you trade off slewing speed if the microsteps are too small. So you want them to be invisible at your optical resolution but no smaller. This depends on the aperture of the scope and the camera's angular pixel size But is this az, el or equatorial? And what is the gearing? I just made up those examples from thin air. One thing you really want is FEEDBACK from a guide star or even from the main camera. A guid camera can detect feild drife and change the speed of the motor compensate. Drift is caused by (1) changes in atmospheric refraction as the target moves and (2) the telescope mount flexing as the scope moves. Some sophisticated software tries to model this and calculate the effect but a close loop is best and now days cheap to do. Last time I did this we used NTP to keep the controller's clock synchronized to actual time. We used a local GPS receiver to create a local NTP server Yes, one could use LinuxCNC to drive the motors it might be easier to just use a microcontroller. On Thu, Jul 9, 2020 at 11:26 AM R C <cjv...@gmail.com> wrote: > Hello, > > this is (probably) off topic, been seen that happen. If it is please > ignore it. > > > I am building a "motorized" telescope mount (dobsonian) with what is > called an equatorial platform, it has 3 axis which I am going to drive > with stepper motors. > > > The stepper motors I use with a stepper driver, those common DM542 ones, > the stepper motors themselves are 2A and 1.8 degrees per step. > > > What I want to accomplish with the equatorial platform) (it compensates > for the rotation of the earth) is that, the start and end position > accuracy is not that important, smooth and constant/consistent movement > is. for the azimuth/altitude precision is not a really big deal, but > you'd want to move these 2 axis somewhat swift. > > > So there are a few factors to decide. > > > I probably want micro stepping, what settings on the driver for pulses > per rev, is best to use (or is that just trial and error?) > > > As with PWM itself, I am probably just not too familiar with it. From > what I understand, the voltage I use for the motors determines how fast > I can go (I am going to use a 48V switching power supply). > > > as for PWM, I can of course change the length of the pulse itself > and, independently, change the time between two pulses. What is the > relation ship there? WHat does a longer width of the pulse itself do? > and what exactly does a longer gap between the pulses do (of course the > wider the gap between two pulses the slower the motor turns). > > > for, especially, the equatorial platform, I want to avoid "jerking" it, > meaning starting and stopping the stepper motor as little as possible > and just go at a 'slow' constant speed. > > > > sorry if totally of topic.... > > > thanks, > > > Ron > > > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users > -- Chris Albertson Redondo Beach, California _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
