[alma-config] Near-in sidelobes

[John Conway]

On Thu, 8 Jun 2000, [Leonia Kogan] wrote:

> John,
> 
> Your comparison of the two arrays (spiral and donut) sidelobes based on
> the the one dimension slice of the beam patterns. At least this is the
> case shown in
> http://www.oso.chalmers.se/~jconway/ALMA/SIMULATIONS/SIM3/.
> 
> It is quite possible that the two dimension beams will give another result.
> 
> 
> Leonia
> 
> 


For long tracks on equatorial sources (+20 to -40 dec)
the maximum  sidelobes (dominated by near-in ones) for all
arrays will be in the  N-S direction, which is  what I've plotted on the
above web page,

Think of the long track
beam as the superposition of the streched (and slightly rotated) zenith
snapshot beam. At ALMA most sources come up the Eastern horizon
and go over the top and set in the west. Each snaphsop beam in that
sequence is like the zenith snapshot beam but streched in the E-W
direction. The result is that systematic near-in sidelobes in the E-W 
direction are reduced somewhat compared to the snapshot case, however
the N-S are hardly reduced at all.  Your optimisation on sidelobes
give a xenith snaphsot with approx 4% near-in (in a ring) and 4% far-out
sidelobes.  The far -out sidelobes are reduced a lot by synthesis
the E-W near-in sidelobes somewaht and the N-S near-in ones hardly at all,
hence you end up with a peak sidelobe after systhenis which is only
slightly less than the peak sidelobe in the snapshot (approx 4%).


For soirals of course, eve after doing some optimisation in
snapshot mode you have >4% far sielobes and hardly any near-in
ones. Going to a systhesis the far-sidelobes are reduced a lot
and the result is a beam with peak sidelobe overall which 
are less than the dual ring array,

So in terms of raw-sidelobe level the spirals are likley to 
better for synthesis and donuts for snaphsots. But of course
that not the full story because I don't believe 
that all sidelobes are equal for imaging - but thats
a whole other story.

  John.