[asac]reminder: science examples for calibration review

[Christine Wilson]

Hi, everyone,

Just a reminder that the calibration group needs examples of ALMA Science 
as part of meeting one of their first milestones (see my original email 
below). I've also attached the email from Stephane discussing the 
particular aspects of calibration that could use examples, but I'm sure 
ANY science examples that illustrate calibration requirements would be 
very welcome. 

Please try to take some time before our telecon next Wednesday to try to 
come up with a specific science example that could be useful to the 
calibration group. This is one area where having input from as diverse a 
group of science interests as possible is very important. Please send 
your examples to me and to John Richer  <jsr at mrao.cam.ac.uk>

Thanks,
Chris

---------- Forwarded message ----------
Date: Wed, 13 Nov 2002 12:15:44 -0500 (EST)
From: Christine Wilson <wilson at physics.mcmaster.ca>
To: asac at polaris.cv.nrao.edu
Subject: [asac]science examples for calibration review

Hi, everyone,

As we discussed in our telecon last week, one of the first tasks of the 
calibration group is to review the calibration requirements for ALMA (see 
text of draft Level 2 milestone below). This review is to include science 
examples and this is an area where the ASAC can provide useful input to 
the process.

I ask you all to try to come up with one example of science that will be 
done by ALMA. For examples of the types of things, you can have a look at 
the science examples in the stringency report from our last face-to-face 
meeting.

Stephane will circulate a summary of the key calibration issues by email 
in the next day or so. If you can design a science example to exercise 
one of these specific issues (i.e. 1% calibration accuracy, total power
stability, etc.), that would be excellent. But even if your favorite 
science example doesn't seem particularly difficult for ALMA to achieve, 
please pass it on, as I think having a good range of science examples 
will be important for this exercise.

Please send your contributions to John Richer and me by 
Monday, December 2. 

Thanks,
Chris

Level 2 milestone: 
     Review of calibration requirements with science examples complete. 
     This includes: phase, amplitude, bandpass, polarization, antenna 
     location, illumination offset, pointing, focus, delay, opacity, 
     and decorrelation correction.   How and where and at what frequency 
     opacity is required to be measured at the site.

  
_______________________________________________
Asac mailing list
Asac at listmgr.cv.nrao.edu
http://listmgr.cv.nrao.edu/mailman/listinfo/asac
-------------- next part --------------

The ALMA project is currently encountering some difficulties in developping and
realizing devices to be used for the calibration of the various instrumental parameters.
Among the possible devices which have not been sufficiently developped is an accurate
coherent source (made of a phase-locked photonic emitter located in the subreflector) which,
if sufficiently accurate (in amplitude, phase and polarisation as function time 
and frequency) could have been used to calibrate the instrumental parameters.

Current projections from the engineers quote amplitude variations as function of
frequency of several 10 %, unclear polarisation properties, and significant difficulties
to achieve high amplitude stability versus time. 

In our present understanding, it is unlikely that such a device can be used successfully
to achive high accuracy calibration of either the bandpass or the polarisation. 

To help investigating the problem, it is important to re-examine what are the REAL 
specifications for the calibration of ALMA, and to put also in perspective what would be
goals to be achieved if possible. 

	Specifications are the performance ALMA should deliver in all (average) circumstances.
In terms of instrumentation, instrumentation out of specification should be refused. Thus,
we cannot impose unachievable specifications.

	On the other hand, "Goals" indicate what level of performance might be reached on
special circumstances, involving either dedicated observing modes, or fortunate 
coincidences (such as exceptionally good weather, or strong astronomical calibration source
available).

	
The "old" ALMA project Book mentionned the following for Calibration

 Table 3.1 ALMA Calibration Requirements.
 Pointing                          0.6" absolute
 Primary Beam                      2-3%
 Baseline Determination            0.1 mm
 Flux Calibration                  1% absolute flux accuracy goal
 Phase Calibration                 0.15 radian at 230 GHz
 Bandpass Calibration              10000:1 to 100000:1 
 Polarization Calibration          10000:1 
 Single Antenna Calibration        Employed

This is a mixture of Scientific specifications (e.g. 0.15 radian at 230 GHz), goals
(e.g. 1%), Engineering specifications (e.g. 0.6") and "out of the blue" numbers (e.g.
Bandpass Calibration 100 000:1)

We now have to set Engineering specifications on some key items in ALMA. For example,
the receiver stability has to be specified. It can be derived from several of the above
numbers. Single-Dish observations require short term (1 second) stability of order 10^-4, 
but this number is Frequency dependent. 1 % amplitude calibration accuracy requires 
the receivers to be significantly more stable than 1 % (say 0.1-0.2 %) but on timescales 
much longer (say 10 minutes).

The most difficult issues to be translated in Engineering specifications are the Bandpass
and the Polarisation Calibration. To help us in doing so, we would like to re-assess the
actual astronomer needs. To do so, we ask you to send us a few Science Examples (2 or 3
would be sufficient) which would represent
	- a "normal" experiment, which would be used to derive ALMA Specifications
	- a "demanding" experiment, which would be used to derive ALMA Goals
focussed on Bandpass and Polarisation issues.

The Science Example should be short (1 to 2 lines of description will in general be
sufficient). It should contain the expected observing frequency, the typical linewidth,
and the expected signal intensity (in Flux density, Brightness or Line-to-continuum
ratio). 

For Polarisation, the expected percentage of polarisation and requested precision
on the position angle of linear polarisation should be mentionned. Also, it may turn
out to be difficult to equip ALMA with adequate devices (most likely quarter wave
plates) for polarisation measurements at all frequencies. Your help is requested 
to define which frequency(ies) is(are) the most important.

-----------------------------------------------------------------------------------------

Here is an example (for Bandpass). 

- "normal" experiment:
	Detection of narrow absorption lines in front of a quasar.
	Observing Frequency: any from 90 GHz to 700 GHz
	Linewidth: 1 km/s
	Line-to-continuum ratio: 0.01
  Implications on Bandpass accuracy
	1000:1 on a 1 to 3 MHz window

- "demanding" experiment:
	Observation of a narrow absorption lines in front of a quasar to measure accurate
	isotopic rations.
	Observing Frequency: 300 GHz
	Linewidth: 1 km/s
	Line-to-continuum ratio: 0.01
  Implications on Bandpass accuracy
	10000:1 on a 3 MHz window (to get a few % accuracy on the line intensity)


- "demanding" experiment:
	Detection of a broad, weak emission line from CO around a bright quasar
	Linewidth: 600 km/s
	Line-to-continuum ratio: 0.01
  Implications on Bandpass accuracy
	1000:1 on a 600 MHz scale