SMA Technical Memo #113

Baselines to JNLT and Keck

August 6, 1997

Eric Keto

The question has come up as to whether it would be useful to provide submillimeter receivers for the JNLT and the Keck telescopes. Both telescopes along with the JCMT are 10 m class instruments each with about a third of the total collecting area of the eight 6 m SMA antennas. The capacity of the SMA correlator after the expansion for the ASIAA antennas is probably 60 baselines. Twelve antennas, 8 from the SMA plus the JCMT, CSO, JNLT, and Keck would require 66 baselines. So it is possible to take nearly full advantage of the additional antennas if the necessary receivers and infrastructure are provided. (See note below for a clarification on the use of 12 antennas with the 8 antenna correlator.)

However, in terms of imaging, the JNLT and the Keck do not provide significantly greater resolution than will be obtained using the JCMT and CSO. Referring to the map below, we see that the longest baseline between the JNLT and the SMA (pad 19) is about 660 m. Between the Keck and pad 19, the baseline is 810 m. These baselines are not much longer than the longest baseline with the JCMT, 640 m, and with the CSO, 780 m. In comparison, the longest baseline in the 8 antenna SMA array is 507 m. With the exception of the last figure, all the baseline lengths are very approximate, but sufficient for the purpose of this memo.

The longest baseline with the proposed Gemini telescope will be about 1400 m

NOTE: The correlator is designed with a sufficient number of connections, fan out, so that when the SMA is expanded from 6 to 8 antennas it will be possible for example to combine signals from antenna 1 with antennas 7 and 8, in addition to antennas 2 through 6, by means of signal repeaters. To go beyond 8 antennas will require dropping one of the receivers, in effect using the correlator capacity of the second receiver to handle the additional baselines. It is believed that this is possible because the LO's and phase rotators are independent for each baseline whether it comes from a second receiver or additional antennas. It is unlikely that any of the other telescopes, the JCMT, CSO, JNLT or Keck will be easily able to handle dual receivers. So the limitation to one receiver is perhaps not so severe. The SMA technical memo #81 describes how to wire the correlators for up to 10 antennas. In addition, it is possible that there are technical drawings by Jim Levine on this topic.

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