[RadioTelescope] Stepping rates

Harry McNally harrymc at decisions-and-designs.com.au
Mon Oct 11 13:34:32 WST 2010

Hi Andrew

Andrew Williams wrote:
> On 11/10/2010 8:40 AM, Harry McNally wrote:
>> Hello Telescopists
>> Has anyone derived the range of stepping rates required in azimuth and
>> elevation ?
> The formal treatment gets really messy, because there's a singularity in
> the az/el coordinate system at the zenith - you need to slew infinitely
> fast in azimuth to track a source that goes directly overhead.

Yes. Remembered that from your previous email. I'm pondering more what the
maximum step rate is that the motors can deliver (torque-wise) although I
presume that's pretty high when the dish is accelerated up to that limit.

> In practice, the dish is small so the beamsize is huge - a few degrees
> across - and the chances of a target tracking precisely overhead is
> small. Slew rates only need to be fast enough to be convenient for
> users, and since (with a small dish) you'll be staying on any one target
> to integrate over quite long times (tens of minutes?), it doesn't matter
> if it takes a minute or two to go between targets.

Ok. The problem is I don't know the gearbox reductions for az and el to motor
steps yet so that will have to be figured out if there isn't a drawing with
the data.

> Obviously the faster it slews the better, but I wouldn't plan on trying
> for worst-case point-to-point slew times any better than tens of
> seconds. As an example, the Parkes 64m dish takes about quarter of an
> hour to do a complete rotation in azimuth.

Impressive :)

> The worst-case slew time is a complete (360 degree) rotation in azimuth.
> You might think it would only be 180 degrees, but you need to allow for
> cable-wrap constraints - you can't keep spinning the dish round and
> round like in 'The Exorcist'...
> (BTW, I'd plan on cable-wrap limits in hardware, not software,
> especially if you're going to allow a bit more than 360 degrees...)

Ok. There are no limit switches that I can recall on the mount.


> Andrew

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