Contact Information

Agenda/Minutes for meeting Tuesday, 30 November at 4:00 pm EST.

Date: 30 November 2004

Time: 4:00 pm EST ( 2:00 pm Socorro, 2:00 pm Tucson)

Phone: Phone: (434)296-7082 (CV SoundStation Premier Conference phone 3rd floor). No Video planned.

Past minutes, etc on MMA Imaging and Calibration Division Page


2 December ALMA Board Telecon -- ASAC Charges to be discussed.
6 December Computing Group face-to-face, Bavaria
14 December ASAC Telecon
5-7 Jan 2005 UNSC URSI Boulder meeting Commission J Agenda
11 Jan 2005 ALMA Town Meeting, AAS San Diego; Agenda
27 Jan 2005 ALMA Board Telecon . Dates for 2005
24 Feb 2005 ALMA Board Telecon. Rebaselining to be discussed.
5-6 April 2005 ALMA Board Face-to-face meeting, Pasadena, CA.




EU Workshop topics 2005: In conjunction with RADIONET, the ESAC has discussed the following meetings:

3 workshops/small meetings

*Suzanne Aalto's modeling workshop in Sweden. Data cube to best possible model of the astronomical object.

*S-Z effect in Paris (P. Cox, Paris) 1st two weeks of April. Invitations out.

*Spectral Line Surveys esp wrt Pre-biological Molecules (David Field, Denmark)

One Giant Meeting

ALMA Global Sci Meeting, in Madrid (R. Bachiller)

My Tuesday lunch Presentation on DUSTY04

NRAO Algorithms Working Group Meeting Report - Myers

Follow-up to last week's discussions

ATF Interferometry:

Since last week it has been reported that the bandwidth available for Prototype System Interferometry using the evaluation receivers will only have 800 MHz of bandwidth available. The evaluation receiver specification reported in late 2003 to the Science IPT suggested considerably higher bandwidths. Sensitivity is compromised; some system tests will not be possible. It is urgent that the preproduction receivers be used at the ATF as soon as practicable.

Holdaway submitted a Justification for quick multi-frequency work at the ATF Interferometer

"Many tests which we would want to carry out with the ATF test interferometer could be done with just one frequency band (90 GHz). However, ALMA is really counting on a great deal of agility in the antennas and the on line software to switch quickly from one band to another (ie, for fast switching, or for observing at a high frequency band and doing some other sort of calibration, pointing or focus, at 90 GHz). If there are problems with the antennas or the software switching quickly from one band to another, it would be good to understand the problem sooner rather than later. Hence, I recommend that we have access to multi-band observations at some point at the ATF.

"A reduced bandwidth of 800 MHz would probably make any 230 GHz work impossible [AW: The 230 GHz spec is for a 2 GHz IF bandwidth], and would certainly change the scope of the 90 GHz work.[AW: The 90 GHz spec is for an 8 GHz bandwidth]"

Darrel checked with Perfetto: " The 3mm evaluation receiver at the ATF has the full 8 GHz bandwidth. The 1mm evaluation rx at the ATF only has 2 GHz bandwidth.

" Antonio does however warn me that the ripples across the 8 GHz passband of the 3mm receiver are quite bad, so the effective bandwidth may be a little reduced. Once we have a correlator that can renormalize things that shouldn't be too bad.

"This is just for the frontend: of course the relevant downconverters and samplers for the test correlator are needed to support the full bandwidth."

Chilean Integration:

Holdaway reports that "preliminary calculations by Emerson and him suggest that installing a correlator at the OSF for holography is not a good idea -- there is not enough sensitivity to do much useful -- I am trying to quantify that, as it relies on a hand-waving argument so far, and in a few hours (unfortunately, the end point of that FEW HOURS is likely AFTER the Thanksgiving holiday) I will have some quick and dirty AIPS++/glish holography simulation code.

"I think Rick Perley has been optimistis on holography in the past largely because he has been doing it at 22 and 43 GHz where the VLA surface is poor, and you actually have a lot of scattering and you have some real power in the sidelobes, so you can actually detect what is going on (ie, the strength of 3C273 coming in through the sidelobes is bigger than the noise).

"In our case, at 90 GHz, we won't have that luxery -- though as I said, we are working on that.

Wootten discussed this with Laing on 11/29/04; he has also done some rough calculations which suggest that although very large scale deformations might be detected by OSF holography, detailed holography with a goal toward panel setting would probably have to await higher sensitivity observations at the ATF.

The PMCS folks told me today, on Commissioning and Science Verification, that most of what is currently in the Science schedule should in fact be covered in the SE&I schedule and should be removed from the Science schedule. The only items that would be left in the Science schedule would be Plans development and formal commissioning tasks.

I objected strongly to this. SE&I schedule shows the prototype correlator moving to the OSF in summer 2007 and being used for interferometric holography there without involvement from the science IPT, for example.

The Project Engineer's Plan is available.

Baseline Ripple and quantization noise

Wootten is working on action item to develop a spec for this.

New discussion topics

Total Power spectral line calibration

Ron Maddalena reports on calibration with the GBT: "Bob Garwood, Jim Braatz, and Joe McMullin forwarded some e-mails that showed you had an interest in how Tsys is used by the calibration routines for GBT data. You might not have seen my discussions with Bob, Jim, and Joe or some of the draft memos that are being distributed. I also don't know how many others might be interested in the subject so feel free to distribute this to the interested parties.

"I'll use DISH as a way to describe where we have been and where we need to go. Essentially, d.calib in DISH determines and applies an average Tsys over the full spectrum, regardless of the bandwidth. For most types of observations, d.calib does:

[(SIG-REF)/REF] * Tsys_REF

d.calib assumes the observer is firing the noise diode throughout the observations. It determines Tsys_REF from:

Tsys_REF = MEAN [ Tcal(f) * REF / (REF_CALON - REF_CALOFF) ]

where Tcal(f) is some estimate of the noise diode's temperature as a function of frequency across the band

"Since we know that Tsys can vary greatly in a few 10's of MHz, the simplistic assumptions of DISH lead to the miscalibration of spectral lines, especially if there are multiple lines within a band. It's also very obvious one cannot calculate Tsys on a channel-by-channel basis since the radiometer noise in the determined Tsys would be greater than in (SIG-REF)/REF. Thus, the Tsys one needs to use has to be either averaged over some range of frequency, averaged over time, or some combination of both. The need for a well-determined, non-mean Tsys that varies across a wide band was first suggested at least three years ago in the calibration memo that was used by those developing d.calib.

"The first attempts to implement this failed for a few of reasons. First, the Tcal values provided by the engineers vary from one frequency to another by 10% due to the inaccuracies of the methods they employ in the measurements of Tcal. This leads to structures in the bandpass that are not real. I also made a mistake in the estimate of the number of channels one has to average over to keep the statistical noise in Tsys below that in (S-R)/R. This led to nosier data than if one applied a mean Tsys. The DISH 'boxcar' routine for a proper averaging of Tsys was very, very slow and switching to the use of 'mean' gave DISH a heft performance boast. Finally, at that time we were using a first-cut calibration for our NOD observing technique. The preliminary algorithm led to baseline shapes that were falsely attributed to using a non-mean Tsys. Since time was tight and we wanted to move on, the temporary 'solution' was to just have d.calib do the above 'mean' calculation. Unfortunately, the splintering of the Aips++ project last year meant that the temporary solution persists.

"I'm in the process of writing a calibration manifesto that goes into all the grizzly details. I'm about 2/3rds done and already the beast is longer than the text part of my thesis. A good approximation for the number of scans or number of channels one needs to average over is:

N_scans * N_channels >> ( Tsys / Tcal ) ^ 2

Typically, Tsys~10*Tcal, so N_scans * N_channels must be >> 100. Let's say 500 channels or 500 scans or some combination of these. Essentially, one must average Tsys over a range of channels that is smaller than the frequency structure of Tsys and, at the same time, average Tsys over a range of time that is smaller than the time for Tsys to change appreciably. To me, there's no one combination of time or frequency averaging that will work for all receivers, observing techniques, or observed object. Rather, it has to be the observer that has to make that decision.

"This still doesn't address the issue of the engineer's Tcal values and it's influence on the frequency structure of the determined Tsys. I'm working on an astronomical method which provides Tcal values with high frequency resolution and a relative accuracy of less than 1% and an absolute accuracy of about 5%. Take a look at for what my thoughts were a couple of years ago.

"I've recently extended the technique of the above memo to include the determination of Tsys with high accuracy and frequency resolution. I'm hoping this technique will help with the removal of some of the residual baseline structure in wide bandpass observations. If you think of SIG as being: Tatmosphere + Tline(f) + Tcmb + ... +Tcontinuum + Treceiver(f), only Treceiver has any substantial frequency shape across a typical band. REF would be similar except Tcontinuum or Tatmosphere might be different. In the numerator of (SIG-REF)/REF, the contributions of everything cancel out except for: Delta_Tatmos + Delta_Tcont + Tline(f). But, Treceiver(f) remains in the denominator. Whenever Delta_Tatmos or Delta_Tcont aren't zero, (S-R)/R will contain traces of Treceiver(f). The larger the deltas, the more you'll see Treceiver(f). But, if one had a great model of Tsys(f), then Tsys(f) could cancel the frequency structure of the denominator when you take the product: [(S-R)/R]*Tsys. I should know by the end of March or so how well this technique works. But I'm hopeful since it's very close to what is behind the scenes in the 'template' fitting algorithm of Solomon, Vanden Bout and Maddalena that got us a z=2.6 detection of HCN with the GBT.

"There's more nuances to the topic than I can present in this e-mail. But, I hope I've provided you with the direction we will be heading for future calibration algorithms."

Science Corner:

Upcoming Meetings

Dusty and Molecular Universe 27-29 October 2004, Paris Presentations Online

2005 IEEE International Conference on Acoustics, Speech, and Signal Processing March 19-23 2005, Philadelphia See Radio Astronomy Session

IAU Symposium 227 Massive Star Birth: A Crossroads of Astrophysics May 16-20 2005, Acireale, Italy

Astrochemistry throughout the Universe: Recent Successes and Current Challenges 2005 August 29 - September 2; Asilomar, California

Protostars and Planets V 24 - 28 October 2005 Hilton Waikoloa Village, The Big Island, Hawaii

URSI General Assembly 23-29 October 2005; New Delhi, India; "Mm/submm Techniques and Science" session 25-26 Oct.

-- AlWootten - 30 Nov 2004
Topic revision: r5 - 2006-11-01, AlWootten
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