I’ve mentioned the “spring” and “fall” skies of ALFALFA at least once already, so I thought it was time to explain what I meant. And since a picture is worth a thousand words, here’s a helpful graphic put together by Brian Kent:
The map above is a projection of the sky with coordinate lines of right ascension and declination. The blue areas represent the planned survey coverage of ALFALFA. The red shows the coverage of the Sloan Digital Sky Survey (SDSS), an optical survey that has photometry and spectroscopic redshifts for hundreds of thousands of galaxies. The SDSS footprint (sky coverage) is shown for comparison purposes. Where there is overlap, we’ll be able to use the two datasets in a complementary manner.
The middle chunk in the picture is our “spring” sky. This extends from 7hours:30 minutes to 14h:30m in right ascension (R.A) and from 0 to 36 degrees in declination. (R.A. is the equivalent of longitude for stellar coordinates and runs from 0 to 24 hours. Declination is the equivalent of latitude and uses degrees.) We refer to this region as our “spring” sky because this part of the sky is visible at night during spring months. Of course, I use the word “spring” very generally, as we are currently observing the spring sky of our survey, and January and February do not qualify as spring (especially in Ithaca). The other two chunks you see are actually contiguous and constitute our “fall” sky. The declination range is the same as for the spring sky and the R.A. ranges from 22 to 3 hours. Once again, the “fall” refers to the fact that this part of the sky is visible during fall months and is a loosely used term. (November and December? Not really fall in my opinion.)
The declination limits on the survey are placed mostly by the telescope itself; Arecibo can only look so far away from zenith (straight overhead). Arecibo is located at about 18 degrees latitude, so we are looking about 18 degrees north and south of zenith. The R.A. ranges are designed both to include interesting local structures (discussed here) and to avoid the Galactic Plane, as there are lots of other astronomers who study our own Galaxy and use Arecibo to observe that part of the sky.