[evlatests] Settling Times for Referenced Pointing

[Claire Chandler]

I don't think the motivation for faster slew times comes from increasing 
the time on source for the "typical" observation described below.  As 
Bryan notes, slew time is a small factor in the overall sensitivity for 
such observations.  Rather, it comes from wanting to do (a) OTF mapping, 
(b) large surveys for which you only want to spend a minute per source 
(similar to the flux density run from which this discussion began; the 
larger bandwidths of the EVLA will make this a more common observing mode 
in the future), and (c) "fast switching" at high frequencies for which we 
know the observing efficiency can be 50% or worse.

Claire

On Mon, 22 Nov 2010, Bryan Butler wrote:

>
> 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.
>>
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