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 SG comments are interspersed in the original butler review, and are denoted by 'begin SG' and 'end SG'. Summary: The title for this memo is somewhat misleading, as it treats issues which would not normally be thought of as "amplitude calibration" (in the traditional way). But, formally, these issues do impact the ability to calibrate the amplitudes - it's just that folks may be surprised to open up the memo and find, e.g., a discussion on focus. Nearly all aspects of calibration are discussed and treated (except polarization, which is specifically excluded), and as such I think it can serve as a valuable basis for further discussion on the complete calibration scheme for ALMA. But the memo suffers from a number of flaws which prohibit it from being accepted as is as a recipe for the calibration scheme for ALMA. In addition, there are a number of places where assumptions or statements are made with no discussion or corroborative evidence ("proof by assertion"). Two examples: 'Moreover, polarized emission from the asteroid edges will be a larger fraction than for the small planets or giant satellites.' - this statement is not discussed or defended, and is not strictly correct; begin SG: - OK, to be discussed further end SG: and 'Since the accuracy of opacity correction allows to take a source 10 deg away,...' - again, not defended or discussed, and I'm not sure that it is correct either. begin SG: - I guess you missed the discussion because you skipped over Section 6. It is discussed extensively in 6.1.2, page 16. end SG: These kinds of statements are sprinkled throughout, making the document mostly a collection of qualitative arguments lacking rigour. Again, it's a nice framework for discussion and further refinement, but cannot be taken as is. To me, the 2 most important points of the memo are: the importance of the sideband gain ratio calibration (I think this needs more work); and the suggestion to relax the 1% accuracy spec to 3% in the submm (this needs more discussion, but is certainly reasonable to consider at least). 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: '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: 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: Here is the first elaboration of what the authors perceive as the necessary 'calibrations' - what will be discussed further in the memo. I agree with most of it, and will make some comments later on specific disagreements, but let me make one comment here. 'decorrelation estimates' are included in item 6. If we have data that is significantly decorrelated on the timescale of fast switching or WVR calibration, then we are in real trouble. That is the point of having those calibrations - to avoid the decorrelation, i.e., make it possible to track accurately the phase variations on short timescales and correct them in the data. p.4: I find the description of an observing session extremely confusing. What is 'nf'? 'na'? etc... begin SG: - Arbitrary repeat counts. We can add a short explanatory sentence. end SG: p.5&6: There are several problems with the discussion on the emission from planets, satellites, and asteroids: - The uncertainties on Uranus and Neptune are better than 5%, if you believe Gene Serabyn's most recent results. begin SG: - The text already says Uranus is known to better than 5 %. For Neptune, it is yet unclear: it may be true way off the lines of CO, some of which are as strong as 30 %... We all agree that so far Uranus is the best "ABSOLUTE" calibrator (if not the only one...). end SG: - Titan's mm/submm continuum is *not* known to 5% accuracy. ==> Rafael - Polarization at mm/submm wavelengths is *not* negligible for Titan (in the long-mm, surface emission contributes a significant amount, in the submm, there may be polarization from large haze/cloud particles). ==> Rafael - Modelling the planets and satellites as a smooth isothermal dielectric sphere is a mistake. There is no reason to do this, either - especially the isothermal part, but also the smooth part (and, in addition, the spatially homogenous assumption which is implicit here). If such assumptions are made, there is no way we will reach 1%. We should use proper models which have temperatures calculated properly, and use all available information on surface and subsurface properties for planets, satellites, and asteroids. begin SG: Absolutely correct. The more complete the model, the better. The smooth isothermal dielectric sphere is a first order approximation only. end SG: - The polarized emission from the asteroid 'edges' is no more than that from the satellites or solid body planets, at least for the larger (spheroidal) ones [which are the only ones useful in this context]. The physical mechanism is the same, being caused by the different Fresnel transmittivities in the 2 different linear polarizations as the emission passes through the surface-to-atmosphere interface. The only difference is in the 'order' (or regularity, if you will) of the polarized response. Asteroids will have a somewhat more disordered polarization response because their topography/roughness is a larger fraction of their size. However, for the larger asteroids (> 150 km diameter or so - the only ones useful in this respect), this should be a small effect, and good shape models can be used to ameliorate the problem. See Lagerros' papers on this. ==> Rafael - There is a major problem with using the giant planet satellites in this way - we do not understand physically why the brightness temperature is depressed as it is at mm wavelengths, so we have trouble modelling it successfully. The authors quote Muhleman & Berge (1991), but leave out this important finding from that paper - I quote from it: "Much work remains to be done to explain the anomalous behavior of the Galilean Satellites in both microwave emission and radar backscattering." == Rafael - 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: p.7: In discussion on quasars as primary calibrators: 'At the Chajnantor site, given the variety of hour angle and declination, one of them will be available at anytime for bandpass calibration.' We may find that we want the bandpass calibrator near the source of interest, not just anywhere in the sky. This is certainly the case at the VLA. It will depend on the stability of the bandpass with time, temperature, antenna motion, etc... begin SG: - That is a key issue and a key design problem. We have to build the instrument so that the bandpass IS stable in time. Otherwise, there is little hope to be able to calibrate it out in any way. It would be interesting for ALMA to understand the main cause of the VLA bandpass instability. The fact that a nearby calibrator gives better result on the VLA suggests some possible major origins - delay change due to insufficiently accurate cable-length compensation - differential delay changes due to different mechanical behavior of the antennas - receiver gain changes as a function of antenna elevation end SG: 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: section 4: I find this section nearly useless, since the phase fluctuation statistics from the STI were not used. I'm not sure how the authors attempted to represent the correlation between the various parameters, since it is not discussed, but I view it with skepticism. A proper treatment of this should use mostly the phase from the STI and opacity from the 225 GHz tipper (scaled properly with frequency) [a possible addition is the change of temperature with time, i.e., dT/dt, since during dawn or dusk the temperature gradient will be large and you probably won't want to observe in the submm then because of antenna thermal deformations]. begin SG: - Accounting for the correlation (or lack of) between phase noise and transparency should be done at some point. However, for the amplitude calibration, the first order effect remains the atmospheric absorption, and as such Section 4 is not "useless". end SG: p.12,13: Why use 'several "simple" approximations" to Tsys? Why not simply calculate it, or use what has been published before in ALMA memos? begin SG: - Because it is useless: the "exact" Tsys depends too much on the exact observing frequency and atmospheric conditions. A representative, easily verifiable, approximation is better for our purpose. end SG: section 6: I did not go through these sections much at all because memos 422 and 423 supersede this section. 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: section 7.2: This is an important point, and one that has not received enough attention, I think. Is this calibratable a priori? How strong a function of frequency within a given band is the effect? We need some interaction with the engineers on this topic. 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: section 7.5: It is appropriate to bring this up, and, again, we need more discussion on this (especially in combination with the coherent device in the subreflector). I'm not sure I agree with the statement that it will take 100 times longer to calibrate it than the bandpass or sideband gain ratio. I'm also not sure what they mean by a 'half-wavelength modulation scheme to reduce any standing wave pattern'. Maybe this is well known in some circles, but I am not sure what they are referring to. begin SG: - Spending half of the time with the nominal focus and half of it with the focus displaced by a quarter wavelength would add the baseline ripples in opposition, thereby minimizing them. This is a standard procedure on single-dish telescopes. The whole point of Section 7.5 is to point out that it may be better to "suppress" the ripples as much as possible than to expect to be able to calibrate them, given the long integration times required for this. end SG: section 8: I don't know why they use a 'semi-optimized five point method' when scanning circles or continuous triangles have been shown to be more efficient (see e.g., Steve Scott's OVRO memo on how they do it there). begin SG: - it does not change significantly the time estimate end SG: 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: 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: The calculation shown in Figure 3 is a nice one, but despite all of this observers may wish to determine pointing *at the observing frequency*. Theoretically, it only needs to be determined at one frequency and then have (presumably) well-known collimation offsets applied for the other frequencies. In practice, this does not work as well as one would hope, and our experience at the VLA is that if you want to do the highest dynamic range/fidelity/sensitivity mapping observations you want to determine the pointing at the observing frequency rather than determining it at, say, 3.5 cm and then applying the collimation offsets. begin SG: - I agree: collimation offsets may change because of thermal deformations of the main dish which affect differently each frequency... It will depend on the antenna quality, and cannot be decided before we test them... end SG: I like very much the discussion on looking at a number of nearby sources to get a 'local pointing model'. This is a good concept and one we should adopt as the working model for pointing calibration, I think. 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: The statement 'Frequencies around 90-100 GHz are optimal.' does not make any sense - the focus needs to be determined for each band (on each antenna) independently. begin SG: - This is like collimation offsets: focus offsets should be constant to first order, but only to first order. end SG: section 10: I think the arguments here are superseded by memos 403 & 404 for fast switching. In fact, I'm not sure why this section is included at all, except in a 'completeness' sense. begin SG: - Memo 372 is older than 403 or 404... Also, we thought that the required integration time for atmospheric transparency calibration was an important issue. Observing strategies will depend very much on whether this time is short or not. end SG: section 11.1: 'Since the accuracy of opacity correction allows to take a source 10 deg away, we can use a Q7 quasar of typical flux S0 = 1.5 Jy...". This isn't right, since your flux density calibrator can't be just any old Q7 quasar, but has to rather be one of a small subset, *unless* you plan on monitoring every Q7 quasar regularly (and by this, it means every day, since flux density variations of several percent or larger occur daily). begin SG: - No. It can be any Q7 quasar, but you have to bootstrap the flux of this quasar to some SECONDARY calibrator for each observation. end SG: 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: section 11.4: 'Using a source model and the actual layout...'. Which source model? begin SG: - Table 9 was build using a 1" disk, but the order of magnitude will remain the same whatever the exact source model and the exact layout of the array are for the same angular size. end SG: I disagree with 'It is thus sufficient to design the largest configuration with 1 short baseline to provide the same sensitivity.' begin SG: - The 4 km array at 850 GHz has 2016 x 0.00016 = 0.32 "effective" baselines for a 1" source. So just 1 baseline in the 14 km array will provide a better result... end SG: section 12.1: As I pointed out above, I don't think it should be necessary to calibrate delays and focus for every observation. SG: - This is absolutely required, see before. Additionally, as pointed out above, I don't see the need to separate the bandpass calibration into fine and large scale. begin SG: - If we had enough S/N at the observing frequency, there would be no need. It is only one way to beat this S/N limitation end SG: section 12.4: 'Monitoring of these secondary calibrators should be done regularly to provide sufficient reference sources.' This is a huge time commitment. To get 1% accuracy, you need to monitor them every day, at all frequencies. If they are suggesting to monitor every Q4 & Q7 quasar, this is *alot* of time (1 Q4 quasar per 16 square degrees means several thousand of them, and even at 1 second per observation, this is many hours!). They must be advocating some subset of these quasars, but even if that subset is only 100 quasars (of order 1 per 100 square deg), then this is still a serious time commitment. begin SG: - "secondary" calibrators do not mean the same thing for you and us. For us, - a PRIMARY (flux) calibrator is a source of ABSOLUTELY known flux - a SECONDARY (flux) calibrator is a source whose flux is regularly measured against one (or more) PRIMARY calibrator - an AMPLITUDE calibrator is an intermediate object whose flux must be determined against a PRIMARY or a SECONDARY calibrator at the time of observations. The total number of SECONDARY calibrators should be the smallest number which allows to always have one visible at any given time within the elevation constraints. end SG: section 13: The recommendation to relax the submm spec to 3% is an important one, and should be considered seriously. I guess the ASAC should be queried on this. SG: - Done already I agree with the directions for future research here, but would add development of the dual-load calibration system. SG: - Agreed too.