[RadioTelescope] Stepping rates

[Andrew Williams]

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...)

Andrew