Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.naic.edu/alfa/ealfa/meeting2/handouts/minchin.txt Дата изменения: Sat May 13 00:11:08 2006 Дата индексирования: Sun Dec 23 02:11:28 2007 Кодировка:

Dear All,
An alternative strategy for ALFALFA would be to survey only half
the area but to cover it twice. I have been doing some work on this idea,
based on using only the top 4 strips from the strawman proposal (+22, +26,
+30, +34) for an area of 2748 sq. deg. This leads to a reduction in
volume of approximately 30% at most masses.

However, HI surveys can never reach zero distance - there is always a
minimum distance which a source must be at to have a believable mass
(thanks to Masters, Haynes & Giovanelli 2004 for reminding me of this).
The attached figures show the volume covered by the wide-area ALFALFA and
the reduced-area ALFALFA with minimum distances of 15, 10 and 5 Mpc. As
the reduced-area proposal sees galaxies of a given mass to a greater
distance, it actually covers a larger volume than the wide-area proposal
at the lowest mass levels (the lowest 0.5 dex detectable at that
distance).

The reduced-area survey would also have the advantage of covering the sky
twice at independent times. This would greatly enhance interference
rejection and would mean that, by interleaving the two scans, we could get
actual Nyquist sampling rather than almost Nyquist sampling.

There are, of course, other considerations, such as selecting the strips
so as to get the best-possible early science. I selected the north-most
strips as a) this is the smallest-area scenario, so any other selection of
strips will do better in terms of volume and b) this provides the smallest
overlap with the HIPASS coverage.

Robert Minchin
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