Arecibo Observatory

ALFALFA 2 pass strategy

The spacing of the sky tracks of the ALFA beams in the 19 deg rotated configuration is about 2.1 arcminutes. For a HPBW of 3.5 arcminutes, we would ideally space the beams closer than 2.1 arcmin to get complete resolution.

  • For the second pass, we want the beams to produce drift scan tracks that are halfway between the ones of the first pass, producing a final spacing of 1.05 arcminutes.

  • Since Beam 0 is more sensitive (higher gain) than the outer beams, we center it for the 2nd pass 7.3 arcmin from its 1st pass location.

A big problem with observing at L-band these days is the presence within our observing band from 1335-1435 MHz of man-made radio frequence interference RFI. Radio frequency allocations are controlled by law and the Observatory is supposed to be afforded some protection, but in reality, Puerto Rico is a highly populated island and no where is protected from satellite transmissions. Some RFI, such as the FAA radar at the San Juan airport, is well-known and predictable. Other RFI is transient, appearing briefly or sporadically. We like to say that "ALFALFA would be easy if it were not for RFI".

The strategy we adopt for dealing with RFI is to make sure that the 2nd pass occurs 3-9 months after the first pass. When we set our observing frequency range, we do not account for the Earth's motion around the Sun. During the second pass of part of the sky several months later, cosmic signals should have shifted in frequency according to the Earth's motion, whereas terrestial ones, moving with the Earth, will remain fixed.

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Last modified: Sun Oct 7 11:44:33 EST 2007 by martha