Memo Review Reply

Memo: 372 - An Amplitude Calibration Strategy for ALMA
      Moreno & Guilloteau, 2002May10

Reviewer: Larry D'Addario
Date Received: 2002Aug22

Reply from: Stephane Guilloteau
Date Received: 2002Sep20


SG comments are interspersed in the original d'addario review, and
are denoted by 'S.G.:'.

> The memo is quite lengthy, and it contains a great deal of useful
> material in the form of calculations and anecdotal data.
> Unfortunately it is rather confused and disorganized, and therefore it
> cannot form the basis of any "strategy" for the ALMA telescope.
>
> Throughout the memo, the authors fail to make some important
> distinctions, of which the two most crucial are
>
> 1.  Interferometric vs. single-dish techniques; and
> 2.  Calibration of the instrument vs. measurement of natural variables
> (primarily the atmosphere).
>
> These failures cause extensive confusion and prevent useful
> conclusions and the development of practical methods.  Regarding
> point 2, perhaps the confusion is intentional in view of such
> statements as, "Atmospheric correction... is usually performed with
> receiver gain calibration, because the two problems do not easily
> separate" (page 3).  But this is not at all true.

S.G.:
 You are correct, the confusion is somewhat intentional. The point is 
 that the astronomer is NOT interested in getting 0.1 % receiver gain 
 calibration accuracy if it is then followed by an atmospheric 
 calibration which is accurate to only 10 %. If one can devise a 
 calibration scheme which gives an overall accuracy of e.g. 2 %, but 
 does not separate the relative contribution of the receiver and 
 atmosphere, this is nevertheless much better.  

> While section 5 gives formulas and numerical results for
> interferometric sensitivity, the only gain calibration methods
> discussed (section 6) are single dish techniques.

S.G.:
 This is because we consider ABSOLUTE calibration techniques. 
 Interferometric calibration has great difficulties in getting an 
 absolute calibration because the correction for decorrelation is a 
 challenge.

> Although the memo is about "amplitude calibration," some consideration
> is given to phase calibration as well (section 10 and elsewhere).
> That discussion contains several misconceptions, including that "point
> sources are required" (p 25) and "the basic...cycle uses two sky
> frequencies, one for observation and one for calibration" (p 4).
> Neither of these statements is correct.  The source can be somewhat
> extended if the brightness distribution is known.  

S.G.:
 It does not even need to be known (self-calibration works...), but the 
 signal to noise of the calibration (or the required integration time) 
 then depends on the source structure. Let us get the first order 
 correct before looking into refinements.

> The two-frequency
> technique produces no information about the instrumental phase, which
> must be measured with exactly the same setup as the target source
> observations.

S.G.:
 There is here a hope (to be substantiated by proper design) that this 
 instrumental "phase" (actually, there is both a phase and a delay) 
 remains constant in time, and needs to be calibrated only once per 
 project. This is accounted for in the (admittedly cryptic) overall 
 calibration cycle Section 2, Page 4. 

> No consideration at all is given to self-calibration techniques, even
> though they will be of critical importance for imaging with ALMA.

S.G.:
 Self calibration does not bring you absolute calibration. You loose 
 three major absolute numbers in self-calibration: the position (two 
 coordinates) and the total flux.

> Various secondary effects are considered, and each is described as
> requiring a separate "calibration."  These include sideband ratio,
> delay, bandpass, pointing, and focus.  In each case, the discussion in
> the memo contains misconceptions and confusion.  Sideband ratio is
> unimportant for continuum observations, provided only that it is the
> same for the target and calibrator.  For line observations, very large
> suppression of the undesired sideband occurs in the correlator, so
> explicit knowledge of the gain ratio of the front end is not
> necessary.  "Bandpass" is described as requiring separate "fine,"
> "coarse," and "standing wave" calibrations, yet these distinctions are
> of little practical value; it is more useful to distinguish the stable
> (predictable) and variable parts of the frequency response. 

S.G.:
 This is not so clear cut. It would require another 10-20 pages to 
 detail what is involved in the Bandpass calibration. Work is in 
 progress on that... 

> For both
> pointing and focus, it is assumed that calibration can be done in a
> different band from the observation, yet total offsets (from encoder
> readings) are substantially different among bands; not all of the
> pointing and focus parameters can be scaled between frequencies.

S.G.:
 Experience from the IRAM PdB shows this is not totally hopeless 
 either. If we do not use such dual-frequency techniques, calibration 
 time becomes prohibitive (to the point of making things impossible) at 
 sub-mm wavelengths. To first orders the offsets should be stable, but 
 it remains to be verified to what accuracy they are.

> Most of my comments here apply to interferometric observations.  To
> the extent that the target sources will also be observed in total
> power or by single-dish spectroscopy, different techniques may be
> needed.  Such observations are sensitive to the total system noise,
> most of which is uncorrelated between antennas and invisible to
> interferometers.  The semi-transparent vane may be useful for this
> purpose, even though it is unable to provide the direct measurement of
> electronics gain that is desired for interferometry.  

S.G.:
 The electronics gain is not desired. It may be needed as an 
 intermediate stage in the calibration, but only the conversion from 
 Correlator counts to Jy (corrected for atmospheric extinction) is 
 ultimately required.

> Knowledge of the
> sideband ratio may also be needed, but only for the case of spectral
> line measurements in single dish mode along with gain determination
> using continuum calibrators in total power mode; a better approach is
> to use interferometry for the calibrator observation.  In any case, a
> sensible discussion of calibration strategies must clearly separate
> the cases of zero-baseline and non-zero-baseline observations.

S.G.:
 It will be needed. The astronomers repeatedly insisted on the 
 necessity to add single-dish flux to get high fidelity images. It will 
 also be needed to provide accurate continuum measurement. With 
 bandwidth of 8 GHz, separated by 12 GHz, differential opacity 
 correction between the calibrator and the source will play a role in 
 not corrected for.  And it depends on the sideband gain ratio.

> Table 12 (p 33) provides suggested "repeat rates" for various
> calibrations, yet none of these is justified.  They depend strongly on
> the stability of the instrument, which is largely unknown at present.

S.G.:
 The repeat rates are justified in the document. Most of them derive 
 from the atmospheric transparency. These are MINIMUM repeat rates 
 based on known specifications (e.g. pathlength stability of the 
 antennas) or known properties (atmospheric transparency behaviour). 
 They assume the electronic is more stable than any of these (which is 
 a specification also).