I read over the antenna specifications. I have a list of typos, etc, which I will convey as an annotated pdf file. Here are some comments I've concocted and will pass along, for your information/amplification. -Al Page 27. Additional comments on over the top motion. Over the top motion of the antenna may offer several benefits but by far the most useful is that it offers a simple and effective way to measure the offset of the elevation and azimuth axes of the telescope. This offset acts like a baseline error and therefore may affect ALMA astronomical imaging in the interferometric mode. Over the top motion may increase the antenna cost, however, and constrain antenna construction so a consideration of the ability to measure the offset in other ways should enter a decision on whether to include over the top motion as a specification on production antennas. At IRAM, observations to determine the baseline are made in good conditions (and short baselines to minimize atmospheric phase terms). Observations over a wide range of elevations and dozens of sources are made. The phases phi_ij are fit for baseline i and antenna j as: phi_ij = C_i + X_i*cos(H_j) + Y_i*sin(H_j) + Z_i*sin(D_j) + E_i*cos(El_j) X_i, Y_i, Z_I (equatorial coordinates of the baseline errors), and axes offset E_i are computed to minimize the weighted sum of square phase residuals, where H_j, D_j, El_j are respectively the hour angle, declination and elevation of measurement number j. E_i is measured for the baseline, not the individual antenna, but it is relevant to baselines. At SMA, which does not go over the top, the value of E_i is about 0.1mm or less for most antennas. While it is not necessary for the antenna to go over the top to measure the offset, it makes the measurement much much easier--measure phase, go over the top, measure again and the axis offset of an atenna is determined. Without over the top, one needs as much range of elevation as possible. An error analysis of the solution above is being performed for an ALMA implementation. For ALMA, the baseline errors must be less than 65 microns ('Calibration of ALMA' document). The errors in determination of the offset of the axes must then be much less than this, and the offset should remain stable in time. The degree of stability and the length of time are issues to address. The baseline plan with ALMA is that from the most compact configuration, four atennas are moved every four days. This continues to the largest configuration and back again until the antennas return to their original location in the tightest configuration. At present this cycle takes several months. so the stability of the measurement of the axis offset should be such that offset errors do not contribute to baseline errors over this time period. Conservatively, the axis offset should not vary by more than 20 microns over nine months, if the antennas do not go 'over the top' and hence the offset is not easily measured during the period of antennas being on long baselines. During this period, each antenna may be moved many times. I have discussed the axis offset measurements with Rick Perley, who was visiting from the VLA. As one may recall, those antennas do go over the top, though they don't have a lot in common with the ALMA antennas (they do weigh a similar amount). Years of measurements of the compact configuration baselines (the VLA was conceived to cover cm wavelengths only, so the current operation at 7mm is quite a stretch over what was planned) have given measurements with typical rms of a fraction of a millimeter--about 0.5mm if I recall properly. I asked Rick if there was any data on the stability of the axis offset determined from those numbers. He said that there was evidence that either the model that they were using was inadequate or that there was some variation, but not so large as to be something he could definitely attribute to variation of axis offset, which he thought was definitely stable to something better than a millimeter over many years. The science ipt is investigating the accuracy to which similar constraints may be placed on the axis offset in ALMA. Antenna IPT could investigate whether a metrology solution for measuring the axis offset is practical. Page 27: Additional comments, from Mark Holdaway, on accelerations. "In this message, I look into the possible effects of reducing the maximum acceleration in AZ from 24 deg/sec^2 to 18 deg/s^2. First off, I don't think that this reduction will affect fast switching capability, so I concentrate on OTF (On-The-Fly). My model for the antenna motion and OTF capability is derived from top secret VERTEX documents which I should not have seen -- but these are old documents, based on VERTEX simulations and not on actual measured profile motions, so nobody will have to kill me for looking at these docs. Anyway, I created a mathematical model for antenna motion, loosely based on the Vertex simulations, which does a simple parabolic acceleration turn-around at the end of a constant velocity scan across the source. The *jerk* (ie, time derivative of the acceleration) is the dominant way that vibrational modes will be excited in the antenna (if these vibrational modes are excited, then we could get into situations where the antenna is momentarilly bent up and isn't pointing where we think it should be), I built in the constraint that the JERK always be less than 100 deg/s^3 -- this number coming from looking at the graphical Vertex simulation results. OK, this possible antenna motion profile fits within the constraints of what should be possible -- but will not be optimal, but may be close to optimal. Remember, for OTF total power work (where we need to really zip fast to reduce atmospheric fluctuations), we don't need to have the 0.7" repeatable pointing, we just need to know where the antenna was pointed after the fact, and not be so far off from the desired path so as to result in unsampled parts of the source. I, The Programmer, have the power to set the maximum AZ acceleration to WHATEVER I WANT, be it 24 or 18 (or even 12) deg/s^2. The max AZ velocity is 6 deg/s. The antenna specification for OTF motion states 0.5 deg/s is the top velocity on the sky. With a decreased antenna acceleration, we won't be able to turn around as quickly for a source at very high elevation angles. (At high elevations, we need to move faster and faster in AZ to make the 0.5 deg/s on the sky; the time on source per OTF scan will be the same, but as we are going faster, it will take longer to turn around and the duty cycle will decrease.) How much of an impact is this likely to be? Consider a source 0.5 deg across, and we observe it by zipping across it at 0.5 deg/s ON THE SKY in a constant elevation path. What is the duty cycle (t_on/t_total) for the 12deg/s^2, 18deg/s^2 and 24 deg/s^2 maximum accelarations, as a function of elevation? # results for sourcesize = 0.5 deg and velocity = 0.5 deg/s # el duty12 duty18 duty24 #[deg] 20 0.8135 0.8135 0.8135 25 0.8108 0.8108 0.8108 30 0.8072 0.8072 0.8072 35 0.8029 0.8029 0.8029 40 0.7975 0.7975 0.7975 45 0.7910 0.7910 0.7910 50 0.7830 0.7830 0.7830 55 0.7732 0.7732 0.7732 60 0.7609 0.7609 0.7609 65 0.7453 0.7453 0.7453 70 0.7232 0.7247 0.7247 75 0.6641 0.6960 0.6960 80 0.5701 0.6522 0.6522 85 0.3996 0.4996 0.5706 So, it is only above 80 deg elevation that there is a difference between 18 deg/sec and 24 deg/sec. In other words, the JERK constraint is usually limiting the acceleration -- only at the highest elevations when we are zipping like crazy does the decreased acceleration limit us. Smaller (or larger) sources will have different values of the duty cycles, but the duty cycle will not diverge between the 18 and 24 deg/s^2 cases until we go above 80 deg. If a slower velocity is used, we won't see an effect until we go to even HIGHER elevations. If a faster velocity is used (ie, above the 0.5 deg/s spec), then we will see the divergence at lower elevation angles. WARNING: My antenna motion profiles are not magic. There may be other ones out there that might be good and would go a bit faster on the turnaround, and would come up against the accleration limit at slower antenna velocities (ie, lower elevations). I don't see any problem with going to 18 deg/s^2 in AZ, I don't see any problem with going to 9 deg/s^2 in EL, and I have no official position on 12 deg/s^2 in AZ. -Mark Holdaway" Page 33: It would certainly be very benefical to ALMA if the delivered antennas met the twenty micro surface accuracy goal which the prototype antennas delivered (well, we'll see what they actually delivered). Antenna efficiency at 850 GHz would be increased by 33% as the surface is improved from 25 microns to 20 microns. This is substantial and if it comes at small cost, as it did with the prototype antennas (to be proven).