Receivers

Along with the telescopes, the receiver packages largely determine the capabilities of ALMA. The Joint Receiver Development Group (JRDG) has raised a number of questions and requested clarification from the ASAC. These may be broken down into questions concerning the frequency bands and their priority, the total power stability, the Water Vapor Radiometer (WVR) specifications (dealt with in a separate section), polarization requirements, calibration accuracy, and receiver configurations (principally single sideband versus double sideband operation). Recommendations for each of these areas are outlined below. Frequency Bands. The ASAC concurs that the four bands to be initially installed on the array should be (in order of increasing frequency) Band 3 (86-116 GHz), Band 6 (211-275 GHz), Band 7 (275-370 GHz), and Band 9 (602-720 GHz). The ASAC reiterates that the frequency coverage should be as complete as possible, but we respond to the request for prioritization of the bands as follows.

• First Priority: Bands 3, 6, 7, and 9
• Second Priority: Bands 1, 4, and 2 (see below)
• Third Priority: Bands 5, 8, and 10

We strongly urge that the JRDG study the possibility of extending the lower frequency range of Band 3 to include the SiO maser transition near 86 GHz. If this is possible, Band 2 would drop to third priority. The frequency intervals of the other bands are reasonable. Band 10 is scientifically quite interesting. It is in the third priority because the technology of THz SIS heterodyne receivers is in an early state, and it will be difficult to make ALMA work at its highest operating frequency. Some delay in the installation of this band will enable the most sensitive receivers to be installed and the telescope performance to be optimized. Note that Band 1 is in the second priority list, and it must be considered in receiver layout. If it will not be in the main Dewar, then designs for optics that allow a second Dewar, possibly also containing the WVR, should be developed. It is not necessary for the WVR and Band 1 receivers to operate simultaneously. Total Power Stability. For On-The-Fly (OTF) mapping capabilities, the requisite total power stability is of order 10^-4 in one second (see Section 6, Appendix D). Stabilizing the gain to this level can be accomplished by selecting components with low temperature coefficients and by regulating their temperature to $\Delta T \leq 10^{-2}$K. Regulating the rest of the electronics in the laboratory to that level will be difficult, and it might be best to use a (temperature regulated) total power detector on the front end for the continuum total power measurements, rather than trying to use the correlator as the continuum detector. The ASAC recommends that this level of gain stability be a goal, rather than a hard specification, pending further study. The over-riding concern is the receiver sensitivity, and better performance should not be sacrificed for stability at this stringent level. However, this level of stability may allow considerable simplication (avoiding nutating subreflectors, see section 6), and we encourage the JRDG to study the issue and report back to the ASAC on the prospects for achieving this level of stability and on possible tradeoffs in doing so. WVR Specs. These are discussed at length elsewhere (Section 7). The main point here is that this system must be incorporated into the overall design and receiver specifications. Polarization. Polarization work will be an important part of ALMA research (Appendix C). Strong efforts should be made to have the polarized single-dish beams as stable as possible; consequently, the ASAC recommends that careful consideration be given to placing the 345 GHz receiver on-axis. For linear polarization work the basis state of feeds would ideally be circular polarization. If circular feeds impose important limitations on tuning range or increase significantly the noise temperature, a system for rapid, accurate calibration of linear feeds should be implemented. Obtaining zero and short spacing polarization data is essential. A nutating subreflector has a limited angular throw and introduces varying angles with respect to the optical axis of the primary mirror. The OTF technique proposed for total power observations would be ideal for polarization if the requisite gain stability can be achieved. Finally, the different polarization properties of the two prototype antennas and other polarization properties of the test interferometer and single-dish techniques should be carefully measured as they may be a consideration in procurement decisions (see Section 6). Calibration Accuracy. The ALMA calibration specification of 1% for absolute intensity is adequate scientifically, perhaps even a bit aggressive. A cold calibration load in the primary Dewar is probably unnecessary. Receiver Modes. The superb quality of the Chajnantor site and the non-ideal nature of any optical system means that the theoretical improvement in single sideband (SSB) versus double sideband (DSB) receivers may be difficult to realize in practice. DSB receivers are far easier and cheaper to fabricate, especially at submillimeter frequencies, and the ASAC recommends that a careful design study be undertaken that assesses the likely performance loss for DSB operation. If the loss is sufficiently small, considerable cost savings and ease of operation can be realized by adopting DSB systems. The ASAC would like to revisit this question once the SSB versus DSB study is completed. It is very likely that ALMA will become operational with both SSB and DSB receivers. This change in operational characteristics has important implications for the ALMA correlator, and the ASAC also recommends that the initial and subsequent ALMA correlators be designed with both modes of operation in mind. The operating system and software environment may also be affected. Summary. The ASAC confirms that Bands 3, 6, 7, and 9 have the top priority and should be installed first. While complete frequency coverage is important, we have divided the other bands into second and third priorities. We recommend study of extending the lower end of Band 3 to include 86 GHz. In addition, the JRDG should consider placing the Band 7 receiver on-axis. Designs that accomodate the Band 1 receiver are essential. The ``relaxed'' WVR constraints may allow the Band 1 and WVR receivers to share a Dewar, and the JRDG should consider such designs. Finally, the ASAC requests a presentation at our next meeting of a detailed plan for the mass production, integration and testing of the ALMA Phase II receivers.