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.