Celestial coordinates, time and using Arecibo for ALFALFA
The Arecibo Observatory is located at (long, lat) = (66d45m09s W, +18d20m39s N).
The ALFALFA observing strategy is simple: We fix the telescope's azimuth arm along
the meridian with the dome set to the proper zenith angle corresponding to the
declination of the drift we want to observe in a particular observing session,
start the data taking and let the sky drift by. This is called "fixed azimuth drift
scanning".
In reality, we move the telescope by a small amount so that the drifts along
which we acquire data in a given observing session follow tracks in constant
J2000 declination. Otherwise, over the 6-7 year period of the survey, the Earth's
precession would cause tracks at different declinations to wind up not being
parallel in J2000 declination; the amount of deviation would depend on the
right ascension. A minor effect, but why not avoid it to start with?
Work the answers out so that you understand where you got them. In some cases
you can find the answers by searching on the web. That's not a healthy way to learn -
so don't do it!
Things to do:
- Make sure you understand the concepts of:
- Right ascension and declination
- Azimuth, altitude and zenith angle
- Sidereal time and solar time
- Hour angle
- Celestial equator, ecliptic, annual motion of the Sun
- Precession, J2000 coordinate system
- Arecibo is located at +18 degrees 21 min North latitude.
- What is the declination of a star that passes through the zenith at
Arecibo?
- What is the noontime altitude of the Sun as viewed from Arecibo
on Dec 21st?
- What is the noontime altitude of the Sun as viewed from Arecibo
on June 21st?
- If we observe on March 21st, what will be the right ascension of a
source detected in ALFALFA Beam 0 at midnight? (Give the simple answer.)
- Suppose the date is February 14 and we plan to observe the ALFALFA declination strip at
+33d24'06".
- At what azimuth should we position the azimuth arm so that the
dome is in the right place?
- What will be the zenith angle of the dome during this drift?
- ADVANCED For ALFALFA, we align the beams of the ALFA
so that their tracks in the sky are equally spaced in declination. In order to do that, we
have to rotate it according to the "parallactic angle", the angle between the direction to the
pole and the direction to the zenith for a given point at a given time. Check out these references
so that you understand what parallactic angle is:
- ADVANCED Because ALFALFA will take 6-7
years to complete, we have to worry about the precession of the Earth on its axis.
- How long is the precession period?
- In fact, we see the difference at some right ascensions after only a few years,
Check out the ALFALFA precession discussion
Of course, the purpose of these exercises is to get you to understand how
to work through all the answers without the need for a lot of calculation. If
you can't figure out how to answer these questions simply, then you're probably
on the wrong track. So, ask! Questions we can answer are free.
Last modified: Sat Oct 6 10:52:47 EST 2007 by martha