[evlatests] Settling Times for Referenced Pointing

[Bryan Butler]

a BOTEC (Back Of The Envelope Calculation).

assume that for "typical" observing, you're slewing back and forth 
between source and calibrator every 10 minutes (this is a rough average 
between high frequency which needs it more often, and low frequency 
which can be less often).

these sources are typically fairly close together (5-ish degrees), so 
say 20 seconds slew and settle for each, and you have two of them per 
cal-source cycle, so 40 seconds of every 10 minutes.

if you cut that in half you have 20 seconds of every 10 minutes, or 
roughly 3% more time on source.  but sensitivity goes like sqrt{t}, so 
it's really like a 1.5% increase in sensitivity.

that would be equivalent to about a .7 K improvement in Tsys for a 50 K 
system (receiver + atmosphere + spillover + ...).

it's all cost-benefit, of course - there is a clear gain, but is it 
worth the cost?  it's clear we need to modernize the system, but whether 
it is worth making it faster (in slew and settle) depends on how much it 
would cost...

	-bryan


Bob Hayward wrote, On 11/19/10 14:31 PM:
>
> Does anybody know how much of the time the telescopes actually spend
> slewing rather than tracking? My uneducated guess would say 10-20% of
> the time, depending on the observing program. If you could speed the
> drives up by a factor of two, you could get 5-10% more time on the sky
> looking at your favorite sources. That is the same gain in sensitivity
> you would get by reducing the receiver temperature performance of every
> one of the front-ends by 1 or 2 degrees Kelvin (which is an unlikely
> scenario as they're already as good as current technology allows). Put
> another way, I think it is equivalent to the sensitivity improvement you
> would get by adding an extra 25m antenna to the array (i.e., a full time
> 28th dish). So if you want to improve the sensitivity of the array,
> upgrading the drives might be a relatively cheap way to do it.
>