Compensating for mechanical play

A few questions:

1. what will the thermal environment be for the entire system and will it be controllable? Perhaps in a very controlled environment, one could calibrate the entire system over its dynamic range, avoiding backlash by slewing only certain motions, with the understanding that this may not be stable over the long term.

1. can you optically track (which would obviate need for extreme tolerances and precision)? Even if the target is not always visible for tracking, certainly the background should be useful for that and using an ephemeris for reference one should certainly be able to precisely move the mount.

I have a similar stalled project (which includes a homebrew peltier-cooled imager in development) so I too am interested in this discussion.

Regards,

Michael

You probably look at alt.astronomy or subscribe to snipped-for-privacy@yahoogroups.com or snipped-for-privacy@mailman.rahul.net

All off these groups are dealing with the problem you are describing. Once you have solved all of the problems that you have identified you will start to face a secondary set of problems starting with mechanical bending of the telescope tube itself depending on the pointing angle. Image rotation over time. Cyclic errors from gears compounded by high gear ratios. Structure temperature changes over the course of an evenings viewing induces positioning errors. High resolution shaft encoders are only useful if comparable system accuracy.

A lot depends on what you ultimately want to do with the mount. Simple goto systems work quite well for going out and viewing through the eyepiece. Astrophotography is a lot bigger problem for control systems. Current systems are diverting light from the main tube or using a separate imager in a secondary scope tube mounted to servo the telescope to the sky as a way to compensate for system errors. Imaging now is often done with lots of short images rotated and "stacked" together with some very good software written for just this purpose.

Start simple, get sky movement compensation working first and add more accuracy and control later. This project can be a lot of work. A trip to a starfest will be time well spent talking to those that have done it and looking at some of the solutions they have implemented.

Dark Skies

Walter Banks

Andrew Smallshaw wrote:

In article , Andrew Smallshaw writes

You may already know about this but a very well known name for software/hardware in driving amateur scopes (and also for interesting telescope mounts) is Mel Bartels. The home page (link to the software/hardware is at the top) is...

There is a yahoo group at...

I recommend it as a starting point...

HTH

The temperature range is whatever the weather throws at it: here in the UK that means between 0-20C 90% of the time. 99% of nights would be in a range that extends another 5C either side. Controlling the temperature isn't an option - heating introduces all kinds of optical effects due to turbulent air. It has to work at ambient temperature. Equally humidity can't be controlled although that's probably less of a concern, everything must be able to tolerate a coating of dew.

Optical tracking is certainly an option, but something I'd like to keep in reserve if at all possible. In general you cannot use the same imaging device for both tracking and imaging, so two images are needed. There are two main ways of doing it: one is a seperate guide scope that piggy backs on top of the main which could affect the balance (unless we were clever) and generally make everything both bigger and heavier, potentially making accuracy even more difficult. The other is to split the image using e.g. a half-silvered mirror. This is a much smaller option but wastes light, meaning that in practice a given setup can only see brighter objects. One of these approaches may turn out to be necessary in practice, but I'm trying to design them out of the equation at this stage.

I believe I've already worked around some of the problems you've described, albeit only in theory. Your point about the gears is noted, and in part forms the basis for my original question. Why do you say my shaft encoders will only be as good as the rest of the drive system? I'm proposing to put them at the very end of the drive train so that any errors can be compensated for, possibly attached to the load bearing axes themselves. Do you see anything wrong with that approach?

Hmm, thermal effects on the structure itself... hadn't thought of that one. Something new to address...

Compensating for sky movement should hopefully be straightforward as this is an equatorial mount so it's single axis, constant rate stuff - I've devised a method of getting fairly good polar alignment for a fixed installation so anything more advanced shouldn't be necessary. Having regular progress indication in the RA drive would be useful though so that any errors can be detected in short order, say on the order of every few arcminutes rotation. Of course, this will all be in one direction only at that point so no play to worry about there. At the end of the day, you get reasonable results from small scopes and clockwork drive, so I'm not overly concerned by it. Allowing for the initial positioning is the thing, even if I don't go for full goto at the outset.

Op Thu, 6 Sep 2007 17:25:56 +0000 (UTC) schreef Andrew Smallshaw:

Some books have been written about this: Michael A. Covington, How to Use a Computerized Telescope. M. Trueblood and R. Genet, Telescope Control.

Some of the higher resolution encoders use an analog interpolation method, so you would need to watch the LSB noise. If you used a seek-then-lock scheme, rather than a servo method, you would tolerate much lower spec encoders. A really smart system would track the LSB's as it moved, and be able to interpolate to the final setting.

Stepper motors (esp with microstepping) would seem the best for the subsequent slow rotation tracking, and they work nicely with any encoder - interpolation.

You can get ICs with Microstep built in:

-jg

Thanks for your comments. I think at least this part of the problem looks feasible enough. Incidentally, for the low speed tracking I'm thinking of simply tracking the time between encoder increments. It appears that the steppers are going to have considerably higher resolution than the encoders so it is a question of getting them ticking at the right rate. Using a 4096-count encoder, the lowest precision I could get way with, I'd expect an increment just under every 21 seconds. If it gets up to 22 or whatever, I know to increase the step speed a little. In this way I hope to ensure that the tracking is much more accurate than the resolution of the encoders.

As a side note, someone asked yesterday about whether I could control the temperature in any way. I had vision of a space heater or something like that warming the entire building which I dismissed out of hand. I've since realised I can of course pop the encoders in their own enclosures and heat them to a relatively constant 30 or 35C which should aid accuracy.

I'm a little puzzled you're planning on using steppers at all. Constant-speed, enormously slow rotation is just about the last thing I'd consider steppers for.

The canonical approach has always been to have four or five axes of rotation in a telescope mount: one or two fixed ones only needed during installation, which point the third exactly at the celestial pole (parallel to earth's axis of rotation), and two more to point the scope to a direction relative to that axis. Then you equip that third axis with a special motor and gears that does one turn in 24 hours.

The main advantage of this approach is that moving and fixed axes are separate, so their treatment can be optimized independently: a linear motor for celestial rotation, and a pair of servos (steppers or other) for pointing at a star. A minor advantage is that the servos can be fed declination and right-ascension almost unchanged.

I had assumed that the O.P. (from the language in his posting) had an unusual application in mind such as moving target tracking (asteroids, UFOs, etc.) rather than just backyard astronomy.

Regards,

Michael

I'm inclined towards steppers simply for resolution and predictable handling at low speed. There isn't just the tracking to worry about where the steppers are merely ticking over, there is also the panning from one point in the sky to another at maybe 100 times that rate. I wouldn't trust conventional motors to run accurately at 1% of their maximum speed whatever method is used to control them.

Later in your post you hint at separate motors for tracking and panning on RA. I briefly considered that but dismissed it as needlessly complex. The only real avantage to that that I could see is that one setting would be fed RA directly. Converting between various forms, ie, RA to RA offset by sidereal time, is one thing computers are good at.

You're correct that there are more axes involved than I have declared here, but I didn't consider them relevant to this discussion since any adjustment would involve a spanner rather than a computer or anything electrical for that matter. I'm planning on a few degrees of manual adjustment in azimuth for fine tuning precise polar alignment. The altitude element will be designed into the mount (largely non-adjustable and specifically designed for my latitude), fine tuning being done by re-levelling the mount at its base.

But doing that you're setting aside the important fact that your stepper doesn't move remotely like your target does. A stepper jumps, a star moves perfectly smoothly. Not to mention that all that jumping makes the subject problem of mechanical play worse. It'll shake loose each and every thing that can be shaken. You don't want shaking.

Ultimately the problem is that a stepper doesn't do low speed at all. It stops or it moves fast (over a small distance).

Which is why it's better to leave these two jobs to two different types of drive. Steppers for fast-or-stop pointing, linear for slow-and-steady tracking.

You'll probably end up investing more complexity into controlling your setup than you saved by a supposedly simpler setup.

That was why I suggested Micro-stepping controllers. That gives the best of both worlds, you get the nice step-prediction and much lower sensor cost of a ateper, but the microstep pushes the step-shock effects into the angular noise.

-jg

I'm not sure I follow the logic. You know the gearing, and the step-sizes, so you should already know more precisely than the sensor, the angular effects. In fact, you could work backwards, and use the precision of the stepper, to calibrate the Sensor edges to fractions of a LSB. The sensor is really just a way to re-sync the stepper after power off, and get reasonable first seek handling.

yes, but the sensor temperature is not really the weakpoint.

-jg