[mmaimcal] Sensitivities continued

[Al Wootten]

My table, revised after this appended conversation with Stephane, is at:
http://www.cv.nrao.edu/~awootten/mmaimcal/sens.txt
Comments/?

Al
From: Al Wootten <awootten at NRAO.EDU>
To: "Stephane Guilloteau" <guillote at iram.fr>
Subject: Re: Sensitivities
Date: Wed, 12 Jul 2000 11:11:10 -0400 (EDT)

Stephane Guilloteau writes:
 > Hello Al,
 > 
 > I agree with your approach. However looking at the table, I spotted a couple
 > of inconsistencies
 > and have a few additional questions
 > 
 > 1) What is the meaning of the various columns
 > 
 > I guess the column just before "Ant eff" is the Tsys, and the previous ones
 > must be contribution from
 > receiver, sky, and ground, or something like that ?
Yes, the titles of the columns didn't come out on the spreadsheet when
printed.
 > 
 > 2) You quote a Trec of 37 K at 230 GHz and 44 K at 140 GHz. I get Trec = 66
 > at 230 GHz...
You are correct.  In fact, the sensitivity corresponds to Trec=70; I'm not
sure why the table as printed comes up with 37K.  I had to edit the table
to get the numbers under the correct columns; I imagine I messed this up.
I corrected the WWW copy now.
 > 
 > 3) You use Trec = 14 K at 35 GHz. My impression is that no HEMT amplifiers
 > have such a low noise
 > at these frequencies. Furthermore, optical losses will most likely be larger
 > at 35 GHz than at higher frequencies.
 > I think we should use the 10 h.nu/k for that band.
Roger.  Yes, without a real receiver design we don't have a good estimate
for this band, for the bandwidth we want.  I called Marian Pospieszalski
about this.  He thinks that over the 30-45 GHz band with cold horn, window
and room temp. mixer, we should achieve 14 K or so.  So OK, let us add a
few K for mirrors and perhaps something else.  10 hnu/k gives Trx=21K or so
and the continuum sensitivity climbs to .02 mJy/min, with the line (1 km/s)
sensitivity increasing to 5.2 mJy/min.  This is Tsys=38 K or so, and
Carlstrom reported numbers as low as 34K for his BIMA/OVRO experiment over 
narrower bands in the recent ApJ paper, so I think we are quite conservative
with this.
 > 
 > 4) You use the 25 % percentile for 345 GHz. From the scientific demand, it
 > may be required much more
 > frequently. I would tend to use the 50 % percentile for this band.
That is fine with me--just requires another asterisk!  I think that for
the purposes of the brochure, I will leave it as is, as the submm/mm
distinction is an easy one to explain in limited space.
 > 
 > Otherwise, I think your numbers are quite close the "rule of thumb" I give
 > for non expert:
 >     Tsys = 35 + 0.45 (nu/100 GHz)    for nu < 350 GHz
 > 
 >     Tsys = 1000 - 1500 K for the 650 and 850 GHz windows
 > 
 > 5) I don't quite understand what are the two rms sensitivity for 1 km/s
 > resolution. What integration time
 > do you assume ?
All integration times are for 60 sec.  The two sensitivities are really for
1 km/s and 25 km/s.  Again, I ruined the table in my attempt to get the
column headings over the correct columns.  Thanks for pointing this out.
I have fixed these various things on the WWW copy.
 > 
 > 
 > Concerning sensitivities to be presented, it would also be good to give some
 > brightness sensitivities
 > for "typical" angular resolutions. That may be more easily compared to
 > candidate objects.
I agree.  However, I don't plan on putting that in the brochure revision,
owing to space limits.  
 > 
 > A memo summarizing all the assumptions and giving both "rule of thumbs" and
 > more detailed curves
 > (e.g. for 25 %, 50 %, 75 % quartiles) as function of frequency, to be
 > co-stamped by both of us ("ALMA approved" !)
A little circle with a capital A enclosed...
 > would be a useful reference. I am ready to help in writing that, either by
 > making a first draft, commenting
 > one, making figures or whatever may suit you.
We have a mechanism in place to update memos.  I'd suggest a revision of
the memo which Bryan and I put out last fall, with these revisions and your
imprimature.  This would then be memo no. 276.1 ( we may run to several
more decimals before this project is done!) and avoid the confusion of trying
to find the most recent memo among the increasing mass of them.
 > 
 > By the way, I have been using the "old" ATM code from J.Cernicharo to
 > compute atmospheric properties.
 > It's OK at mm wavelengths, but has the wrong dependence for the 650 and 850
 > GHz windows. Do you
 > have Liebe's code in some computer form ? I would need that to compute
 > "decent" numbers in the sub-mm
 > regime.
I do have it, but the version I have is a mess, a holdover from the days
when we used it to fit radiosonde data for the GBT and the South Baldy site.
I'll see if Bryan has a better code.  We should all use the same thing, and
at least for ALMA/US, this is Bryan's sinecure.  I will get back to you
on this point.

Thanks for the material on the compact array.

Cieux clairs,
Al