Memo Review Reply

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

Reviewer: Bryan Butler  (unsolicited)
Date Received: 2002Aug28

Reply from: Stephane Guilloteau
Date Received: 2002Sep20

Reply to Reply from: Bryan Butler
Date Received: 2002Oct10


SG comments are interspersed in the original butler review, and
are denoted by 'begin SG' and 'end SG'.

new BJB comments are denoted by 'BJB>' at the beginning of a line.

Reply:

Let me now make more specific comments on the memo:

p.2: 'However, they have not yet been measured or modeled with better
   than 5% accuracy.'  This is not right - Gene Serabyn has numbers
   on Uranus and Neptune that he thinks are accurate to a few %
   and perhaps better (see his DPS paper from the Pasadena DPS, or his
   contribution the the IAU Morocco site testing meeting if there is 
   nothing more recent on this from him).

begin SG:
- by the time the memo was written, these were not published results.
  Also, to be accepted as accurate, the values would need some 
  confirmation (measurement, or independent modeling from another 
  group...).  What is the difference between 5 % and "a few %" ?
end SG:

BJB> it doesn't really matter that the results weren't available
BJB> when the memo was written.  if we are going to use this memo
BJB> as a basis for a calibration plan, then we need to know that
BJB> this information is available now.  i see no reason to require
BJB> independent confirmation, but agree that the results need to
BJB> be more formally published.
BJB>
BJB> the difference between 5% and 'a few%' is large (roughly a 
BJB> factor of 2, quite obviously).  just as the difference between 
BJB> 'a few%' and 1% is large.

   'In most cases, going from 5% to 1% typically implies 25 times
   longer calibration times.'  This is only true if you are thermally
   limited in your errors at both 5% and 1%.  

begin SG:
- This is the reason of the "In most cases". Calibration is often 
  thermally limited, as the many examples in the document show.
end SG:

BJB> not absolute calibration.  it is almost always limited by
BJB> systematics and uncertainty in flux density scale (or,
BJB> equivalently, uncertainty in load temperatures or some other
BJB> required quantity if true absolute temperature calibration
BJB> is being attempted).

p.3: 'For a single-dish telescope, the last term plays no role.' 
   This is not true for large single dishes.  The LMT, for instance,
   will have to worry about this.  For a single dish the effect is
   much smaller, of course, but it is just decorrelation on a size
   scale equal to the diameter of the antenna.  The effect may be
   very small, and might be negligible for 12-m antennas on the
   Chajnantor site, but it is not correct to say that it 'plays no
   role.'

begin SG:
- Decorrelation due to electronic phase noise plays no role in 
  single-dish
- Atmospheric effects related to pathlength fluctuations within the 
  antenna diameter are usually called "anomalous refraction", and as 
  such I agree they do play a role in single-dish data. It is just 
  usually affected to the pointing error budget, and should not be 
  counted twice.
end SG:

BJB> it is true that decorrelation due to electronic phase noise
BJB> plays no role, but this is not what i was commenting on.
BJB>
BJB> if you want to think of the atmospheric fluctuations across
BJB> a single dish in terms of zernike polynomials, then only
BJB> the second order term (the 'tip-tilt') should really be referred
BJB> to as "anomalous refraction".  higher order terms produce true
BJB> decorrelation, i.e., the amplitude of the signal is really
BJB> reduced.

      - Another problem with using the large icy satellites is that
        they are not distributed on the sky very uniformly.  Asteroids
        do not suffer from this problem, of course.
   I think that because of these arguments, asteroids might be much
   more useful than the authors conclude.  They are relatively bright,
   relatively small, have relatively easily modeled emission (including 
   the light curves - see recent papers/theses by Lagerros and Muller),
   and are well distributed across the sky (one is observable at most
   times).

begin SG:
- The last (and most convincing) Lagerros papers were not out at the 
  time we wrote the memo. Lagerros claims an accuracy "within 5 %" for 
  Ceres, Pallas and Vesta, and far less good for the others 
  (10 -- 15 %)), and at IR wavelengths only. But all these asteroids 
  have low emissivity at mm wavelengths which are not yet fully 
  understood (Redman et al 1998). This is exactly like the "anomalous"
  behavior of Galilean Satellites at mm wavelengths. Our understanding 
  is limited here.
end SG:

BJB> again, we need to know about these papers now, if we are to
BJB> base a calibration scheme on this type of observation.
BJB>
BJB> i admit that there are other problems related to asteroids, but
BJB> still submit that they are better.

   In addition, I think that the quasars might actually be useful
   at the long-mm wavelengths (we use 3C286 at the VLA at 7mm,
   and the accuracy is limited mostly by the uncertainty on the
   brightness temperature of Mars).

begin SG:
- 3C286 can be an excellent SECONDARY calibrator, since its flux 
  appears stable in time.  But it is impossible to predict its flux 
  from theory: so it cannot be a PRIMARY calibrator.  Also no theory 
  ever tells you it will remain stable...
end SG:

BJB> but empirically we know that it is stable.  we don't need
BJB> theory for that.  now, it *is* true that as you go to the mm,
BJB> these sources become more dominated by the core, and the
BJB> cores are known to fluctuate (as opposed to the lobes, which,
BJB> on good theoretical grounds, do not).
BJB>
BJB> the semantics of whether you call it a SECONDARY or PRIMARY
BJB> calibrator seem completely meaningless to me.  in the baars et al.
BJB> scale, 3C286 is a SECONDARY calibrator, yet we refer to it as a
BJB> PRIMARY calibrator at the VLA, and, in practice, it *is* a PRIMARY
BJB> calibrator.  in fact, by the argument that you have to be able
BJB> to predict the flux density from theory, there are no *true*
BJB> PRIMARY calibrators.  yes, we have theories for the emission from
BJB> planetary atmospheres and surfaces, but the theories are completely
BJB> useless without the observational data - at least at the 1%
BJB> prediction level.  similarly, all other sources do not have flux
BJB> density which is completely calculable from first principles to
BJB> the 1% level - not Cas A, not Cygnus A, not Taurus A, not the
BJB> planetary nebulae (NGC 7027, e.g.), not DR21, etc...  they are
BJB> *all* determined observationally, *not* by theory.

section 7.1: The discussion here is correct, but the authors seem to
   be operating under the assumption that every observing program will
   need to calibrate delays.  I disagree.  The delay only needs to be 
   determined once (each time an antenna is moved and reconnected) per 
   antenna.  It may need to be done once per feed/Rx system, but even
   that remains to be seen (there may simply be a stable difference
   between each band).

begin SG:
- That is overly optimistic. In a system with cable length compensation 
  active, tracking the instrumental delay when the receiver is retuned 
  is not a trivial task.  Delays can change for many reasons: thermal 
  dilatation is one of the most obvious, but switching an attenuator 
  can have an effect also. Continuous changes can be tracked by a 
  round-trip phase correction, but jumps cannot. Re-calibrating the 
  delay is an important step for final accuracy.
end SG:

BJB> i would argue that this remains to be seen.

sections 7.3 and 7.4: I do not understand the distinction between
   'fine scale' and 'large scale' bandpass.  The bandpass is the 
   bandpass.  It might be different on source (could be narrow) and 
   secondary calibrator [*not* the 'bandpass calibrator'] (always wide 
   bandwidth), so will have to be measured in 2 correlator 
   modes/frequencies, but I fail to see the reason to separate it into
   fine scale and large scale bandpass.  The discussion of the 
   coherent source in the subreflector to calibrate the bandpass is
   good, and we need to visit that topic in more detail.

begin SG:
- The bandpass is the product of the contributions from many 
  components. To mention only 3: atmosphere, receiver, bandpass filter. 
  The bandpass filter response does not depend on the receiver. The 
  receiver bandpass response should have no narrow feature. Hence it 
  may be interesting to calibrate the overall bandpass in two steps. 
  Section 7.3 and 7.4 just attempts to quantify the required 
  calibration time in such a mode.
end SG:

BJB> i still see no compelling reason to do it in 2 steps.  again,
BJB> the bandpass (at the backend) is the bandpass.

   I would argue against using 'major 
   satellites' for pointing.  The variable emission from the primary 
   (variable because it's moving around in the beam) will probably 
   confuse things more than we want.  

begin SG:
- In interferometry, this is far less of a nuisance than in single-dish
end SG:

BJB> but still a nuisance, and an unneccessary one.  i maintain
BJB> that using major satellites for pointing is a bad idea.  and
BJB> they are not needed, so why bother with them?

   Asteroids, on the other hand, might be quite useful for this.  

begin SG:
- Yes, if only the ephemerides are known to 0.1" accuracy. I believe it 
  is not the case, and the current ephemerides only have 1" accuracy, 
  in which case they are useless for pointing.
end SG:

BJB> i thought it was closer to 0.1", at least for the larger ones.
BJB> i have a query in to don yeomans and myles standish on this.

section 9: The discussion here is good and I think correct, but I would
   make a similar argument here as for the delay determination.  The
   focus needs to be determined only once per antenna per receiver 
   cartridge, and perhaps as a function of elevation (although the 
   model for this deflection should be pretty good and might be good
   enough to use from scratch - only experience will tell).  Thermal
   deformations can probably be modeled as well.  The focus only needs 
   to be redetermined if something changes mechanically on the antenna.
   It does not need to be determined by every observing program.

begin SG:
- That is not true: the antenna is always changing due to thermal 
  deformations.  The whole point of Section 9 is to show that, with the 
  current antenna specification, the focus may need to be re-determined 
  every 10 minutes at high frequencies. This is a serious issue.
end SG:

BJB> yes, it is always changing, but it should be changing *in a
BJB> predictable way* (mostly), so that you don't need to re-determine 
BJB> the focus, merely use the current measurements on the antenna, along
BJB> with frequency, elevation, etc... information, to look up the
BJB> proper focus.  this of course will require testing, measurement,
BJB> experience, and the creation of the look-up table, but i think
BJB> it might work.

   I think that
   given our current thinking on breaking observing runs into small
   chunks, the use of quasars for absolute flux density won't work.
   If we had long runs (similar to what is currently done at mm arrays 
   [or cm arrays, for that matter]), then you could catch one of the 
   small number of prime flux density calibrators at some point in your
   run, but given small chunks, this becomes unrealistic.
   I think we'll want primary calibrators closer than 10-15 deg from
   secondary calibrators.  But, this might be a problem with short
   observing blocks.

begin SG:
- Yes, and the conclusion is that "SMALL CHUNKS ARE UNREALISTIC". 
  Although not explicitely written in the memo conclusions, this is an 
  important one which derives from the calibration time estimate given 
  in the Tables.
end SG:

BJB> this is an important point that we should discuss with the
BJB> SSR and computing groups.  they are already building in very
BJB> short blocks (i think of order 30 mins) in the system.