[RadioTelescope] Stepping rates
andrew at physics.uwa.edu.au
Mon Oct 11 12:45:58 WST 2010
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.
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.
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.
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...)
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