SIS

If we use SIS mixers, these will have to go in the main Dewar and will presumably be based on the ALMA band 5 mixers. Sensitivity is then excellent and stability almost certainly acceptable given a suitable switching scheme. One can argue that no significant development effort on the mixers is required. The standard IF choice is not ideal (1 to 9 GHz would be better), but we could live with it. For example the LO could be at about 180 GHz so that the upper-sideband IF range of 4 to 12 GHz would correspond to line offsets of  0.7 to 8.7 GHz. The lower sideband would not be used and would have to be rejected at about the 25 dB level. The mixers would provide a certain amount of sideband rejection and this could be enhanced by having a waveguide filter at the input to the mixer, since the operational frequency is fixed. Although there will naturally be strong resistance to giving up one of the astronomical ``slots" (or making the Dewar larger and more complicated), this option is sufficiently attractive that it should probably be kept open for the present. A straw-man design for it could be worked up and costed but no development work seems to be needed now. We should also consider here the possibility of using the astronomical band-5 receiver to do the radiometry. Given the high sensitivity it might be possible to obtain sufficient accuracy from the shape of the line plus perhaps frequency switching, in which case it should not be necessary to compromise the astronomical performance of the receiver by adding additional switching components inside the Dewar. Another option would be to insert a 45-degree polarising grid into the beam when selecting this mode. This would make it possible to use the two polarisation channels as a cross-correlation receiver. This should also provide a way of doing sideband separation. This would of course mean that correction would not be available when using this receiver for astronomy. (Under good conditions, however, it might be possible to do the water vapour measurements with the band-7 receiver using the 325 GHz water line.) Some additional electronics for generating the LO and processing the IF would need to be added. Extra optics would be needed to do the single-dish pointing corrections and these would have to be inserted into the beam to select this mode. An important additional consideration is that using an SIS mixer should give sufficient sensitivity to provide a correction for the water vapour emission when making total power observations with another receiver. One can see that this should be possible from the fact that, for 1mm of precipitable water, the extra emission $\Delta T_b$ for a given $\Delta w_v$ is several times stronger between 181 and 185 GHz than it is at say 345 GHz. Again these options seem sufficiently attractive that they should be explored in more detail. The interactions with the rest of the system are nevertheless a substantial negative factor. If nothing else we would be compelled to have band 5 available on all antennas from day 1, which may not coincide with the astronomical priority.