ALFA beam orientation for ALFALFA
All rumors to the contrary aside, understanding the ALFA beam configuration on the sky is not
a trivial matter. In fact, the precursor run for A1946 got off to a rocky start because of
problems in the CIMA code for coordinate conversion. However, some of us (RG, MPH and GLH)
are old enough to have experienced first hand the tried and true method of using drift scans to measure
beam parameters. That experience proved to be useful for this exercise, even though no ink
pen chart recorder was available. (And, no, EGGs, RG
did not resort to using a ruler on the screen to measure, despite his natural instincts.)
The principal trick here has to do with Arecibo as an Az-ZA (alt-az in other conventions)
telescope. The orientation of the beams on the array is fixed of course, but the illumination
of the beams on the sky depends on offsets in Az and ZA which then reflect to different points
in the sky north and south of transit. Think of it this way: when you observe in the north, the dome
is in the south, and a feed at higher ZA looks further north; one further west, looks further east.
So all the angles get flipped.
You too can figure this all out, but in the interest of time, here is the bottom line:
- A table of the beam offsets, with examples for equally
spaced beams as adopted for ALFALFA.
- Plots of the beam assignments for ROTANG=0 and
the adopted ALFALFA one, giving beam tracks that are equally spaced in declination.
Useful other links
This page created and maintained by
Martha Haynes and the members of the
Cornell ExtraGalactic Group
Last modified: Fri Oct 29 18:33:13 EDT 2004 by mph