[evlatests] Settling Times for Referenced Pointing

[Juergen Ott]

- for OTF interferometry, speed will not be critical but a better 
control of the antennas will be (the new ACUs should be a huge improvement).

- apart from the observation technique, there are, potentially, also 
transients that flare on minute timescales. Faster slewing would be very 
helpful to catch these sources if EVLA scheduling allows such quick 
response times.

Juergen

On Nov/22/10 10:50 AM, Bryan Butler wrote:
> my argument was just a generic one for typical observing, since bob
> asked if anybody knew and i thought i could make a stab at it, and his
> argument was a more generic one.
>
> if you want to argue that the special cases below drive a change to the
> servo systems (that will probably cost us more than a simple
> modernization [though, truth be told, i don't *know* that it will cost
> more - if it doesn't, then we all win!]), then you have to argue that
> they either occupy a significant fraction of the observing time (because
> if they don't, then we can just give them more overall time to observe
> to get the same result), *or* that they are special enough that they
> warrant an upgrade in any case, that is worth the cost.
>
> 	-bryan
>
>
> Claire Chandler wrote, On 11/22/10 06:43 AM:
>> 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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