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
NRAO Algorithms Working Group Meeting Report - Myers
Follow-up to last week's discussions
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
" 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
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
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."
Dusty and Molecular Universe 27-29 October 2004, Paris
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.
- 30 Nov 2004