UAT08.01 Scavenger Hunt #3: Exploring ALFALFA Grid Fields
The purpose of this scavenger hunt is to give you experience with investigating
fields of interest.
To participate in the scavenger hunt,
you need to keep track of how you found your answers; Your method is
as important as the answers!
- Be sure you have answered all the questions in Hunt #0,
Hunt #1 and Hunt #2
before you tackle this set. You should use the tools you learned about in those
exercises to explore the fields selected here.
- A bit about ALFALFA grids:
After a region of the sky is mapped in both passes by ALFALFA and all the
data are reduced to Level I (bandpass subtraction, calibration and
flagging performed), all the observations are combined to generate
a "grid" field. Each grid field covers 2.4 x 2.4 degrees of sky. Grid
fields are centered every 20 minutes in R.A. and every 2 degrees in
declination, so they overlap a bit. The amoung of overlap
is not the same for all grids. Why not?
- Spectral line grids are 3-dimensional, with velocity/frequency/channel as the
3rd axis. To reduce memory requirements, we generate 4 spectral subgrids
for each grid field, each containing 1024 spectral channels and allowing for
for some overlap; the beginning and ending portions of the 100 MHz band are
discarded. The subgrids are then labelled a,b,c,d:
- "a" covers about -2000 to 3300 km/s
- "b" covers about 2500 to 7900
- "c" covers about 7200 to 12800
- "d" covers about 12100 to 17900
By how many channels do the grids overlap?
- Explore the 1404+09 grid field that will be used to demonstrate the grids and catalogs that
comprise the ALFALFA data products. Use the various tools you've explored in our
previous hunts to find what to expect when you work with this field.
- More to come............
This page created by and for the members of the
ALFALFA Survey Undergraduate team
Last modified: Tue Jan 8 16:53:31 EST 2008
by Martha