I propose a simpler variant of original idea of Gibson & Welch, which = could lead to a much simpler implementation of an accurate absolute = calibration. Gibson & Welch performed absolute flux measurement using a complex = instrumental setup which uses a standard gain horn on the side of one of = the BIMA dishes, connected through a waveguide switch to the BIMA = receiver.=20 The success of this method, as implemented, makes use of all or several = of the following properties, depending exactly on the implementation = details: =20 1) The accurate a-priori knowledge of the gain of the horn, which is = well characterized by Schelkunhoff's gain formula 2) An accurate measurement of the waveguide losses using a network = analyzer 3) The common receiver gain shared by the large antenna and the = standard gain horn after the waveguide switch 4) The proximity of the gain horn and the large dish, which ensures = that similar atmospheric conditions are applicable to both signals, = provided the switching between the gain horn and the large dish is fast = enough 5) An accurate calibration of the antenna temperature scale of the = large antenna 6) An accurate calibration of the antenna temperature scale of the = standard gain horn 7) An accurate correction for atmospheric extinction 8) An accurate correction of any gain-elevation dependence of the large = antenna. 9) A negligible decorrelation on at least 3 baselines on the timescale = required to detect with sufficient signal to noise the astronomical = calibrator The experiment was done at 28.5 GHz, where the BIMA antennas have no = gain-elevation dependence, thus satisfying point (8), and where the = opacity is small and can be measured with sufficient precision using a = tipping curve, as for (7). Decorrelation (either instrumental or = electronic) is also negligible at that frequency, as required in (9). Welch proposes a similar scheme for the ALMA. However, because of the = widely different frequencies and atmospheric opacities, this scheme = cannot easily be transferred at all ALMA bands. While it is conceivable = that the scheme could be simply applied at Band 3 and below, waveguide = switches may not be available at all for Band 7 and above. It is worth = reviewing why all properties 1 to 8 are required in the original method. The absolute calibration actually produces two different results: a) The absolute gain of the large antenna,=20 and b) The absolute flux of the astronomical calibrator. Result (a) requires a comparison of the (absolute) gain of standard gain = horn (Req 1), corrected for the waveguide losses (Req 2), to that of the = large antenna. Since the same receiver (Req 3) is used alternatively, at = a relatively high switching rate, the ratio of cross-correlation = measurements gives the ratio of the voltage gains of the large antenna = and standard gain horn, provided the atmospheric effects remain the same = (Req 4). Atmospheric decorrelation is assumed to be statistically = identical on both measurements in this case (electronic decorrelation = being identical, if any).=20 Once Result (a) is obtained, Result (b) can be established in several = different ways. - In Method I, the large antenna is subsequently used in Single-Dish = mode to measure the flux of one calibrator. This makes use of Req 5, 7 = and 8. At first view, it has the advantage of avoiding any potential = problem with decorrelation, but this is not true: in obtaining Result = (a), the gain of the large antenna is affected by decorrelation linked = to some of the antenna mechanics... In addition, it requires good = antenna temperature calibration... - In Method II, the standard gain horn is used directly to measure the = flux of one calibrator. This cannot be done by Single-Dish mode, and = thus requires to isolate the interferometric horn gain (using closure = relations on 3 baselines) and make sure decorrelation effects are = negligible (Req 9). The gain horn antenna temperature may be easier to = measure than that of the large antenna, since a load covering the horn = is quite feasible (note that in their original experiment, Gibson and = Welch actually used the waveguide switch also to calibrate the antenna = temperature scale of the large antenna). Correction for atmospheric = extinction remains required. Thus, in addition of Req 1,2, this method = makes use of Req 6, 7 and 9. Method II can be used to establish the absolute flux of a strong = astronomical calibrator independently of whether Result (a) has been = established or not. From the description of Method I, one can sees that = the process can be reverted if needed, i.e. one can derive Result (a) = using=20 Result (b) and the properties 5,7 and 8. Hence, providing decorrelation can be adequately controlled, it is = possible to avoid the delicate switching mechanism between the gain horn = and the antenna and yet provide an accurate absolute calibration. All = absolute calibration methods require proper correction for the = atmospheric extinction. They also all rely on an intermediate antenna = temperature scale which is determined using calibrated loads.=20 Applicability to ALMA: ---------------------- ALMA antenna / Gain horn ratio : 3 to 10^4 (assumption) ACA antenna / Gain horn ratio : 10^4 (assumption)=20 Sensitivity issues (Single Baseline, one minute integration, typical = weather) Frequency \ ALMA-ALMA \ ALMA-Horn \ ACA-Horn \ URANUS \ ACA SNR \ = Required time 90 GHz \ 2.5 mJy \ 0.43 Jy \ 0.74 \ 6.61 \ 9 \ = 125 min 230 GHz \ 6.3 mJy \ 1.10 Jy \ 1.91 Jy \ 29.9 \ 15.6 \ = 41 min 270 GHz \ 8.3 mJy \ 1.45 Jy \ 2.50 Jy \ 38.3 \ 15.3 \ = 43 min 345 GHz \ 12.4 mJy \ 2.14 Jy \ 3.72 Jy \ 55.4 \ 14.9 \ = 45 min 670 GHz \ 64.7 mJy \ 11.2 Jy \ 19.4 Jy \ 136 \ 7.0 \ = 204 min 860 GHz \ 87.5 mJy \ 15.2 Jy \ 26.2 Jy \ 175 \ 6.7 \ = 223 min The required integration time is inversely proportional to the number of = baselines which can be used to derive the horn gain. In a proper = geometric setup (horn surrounded by a number of antennas), it could = easily be divided by 3 to 6 without resorting to baselines affected by = significant decorrelation. At the longest wavelengths, using brighter sources like Mars would = increase the S/N by a factor 4 or so. However this is obtained by using larger sources. The angular = resolution of the ACA is around 9"/100 GHz (that of the ALMA compact = configuration is 3"/100 GHz). URANUS is about 3", so at the highest = frequencies, some source model may be required, although a very limited = one (elliptical uniform disk) is probably sufficient at this level.=20 The strongest quasar are only 20-30 Jy, so using planets is required for = sufficient S/N. Proposed Technique: ------------------- It is proposed to use a comparison method in which the gain ratio of the = horn and a nearby antenna is measured interferometrically. The proximity = of the two receiving systems will ensure that the same atmospheric = correction is applied to both, and that the atmospheric decorrelation = effects are similar. This setup does not guarantee that electronic = decorrelation is identical, but this should be easier to control. The technical setup would essentially be an antenna mount and a special = receiver (and receiver cabin) but devoid of any reflector. Each receiver = band would be connected to a standard gain horn, rather than to the = common optics in the other antennas. Large loads (hot and ambiant, = perhaps rather than hot and cold) would be inserted in front of the = horns to define the antenna temperature. The receiver cabin should have = a special enclosure and receiver fixation system so that the horns can = look at the sky without blockage. This "calibration antenna" could be = correlated with any other antenna, or either ACA or ALMA, as part of the = 16 (ACA) or 64 (ALMA) antennas that the correlator could process. = Because of the size of the strongest sources, as well as of the adverse = effects of decorrelation, it would be advantageous to do so with ACA = antennas which always sit in a compact configuration.=20 Using this "calibration antenna" would only occur occasionally. However, = for practical purpose, it would be advantageous that this antenna could = be remotely connected to the correlator at any time, because it would = guarantee use of the best weather circumstances to perform the absolute = calibration. This an be done either as always considering it part of the = array (thereby sacrifying one antenna), or by having an electronic = switch build in the system. Both LOs and sampled data should be = controlled by this switch. An hybrid solution in which the LO is always = available, but the sampled data can be analyzed or not is acceptable. Such a dedicated antenna would probably cost less than a fully equipped = ALMA antenna. The antenna cost would be minimal (200-300 k$ perhaps): = since the mount of one of the prototypes could be re-used for this = purpose, only refurbishing of the cabin being required. But the cost of = the receiver could be substantially higher than those of the series. A = modified cryostat would be required, plus dedicated horns, significant = studies and well designed loads. To that, the cost of the automatic = switch should be incorporated if feasible. Although this may look costly, it is perhaps not much more expensive = than trying to implement switches in one of the antennas to connect = horns located at the periphery of the main dish, since the antenna mount = is essentially available at no cost. Moreover, it avoids potential = problems with the modification of the collision diameter of one antenna. = And a cost of 5 M$ is less than 1 % of the cost of the project.=20 I suggest we evaluate the feasibility of such this approach.