Using Arecibo and using ALFA for ALFALFA

1.   Which of the ALFA beams are "uphill" when we observe a source at a Declination of +03^o24'24"?
   
   
   
2.   We do not normally observe in the afternoon. Why not?
   
   
   
3.   The A2010 observing mode is called "Almost Fixed Azimuth": we actually move the telescope slightly, to account for Earth's precession.
   • What does J2000 refer to?
   • Why does making this adjustment matter to us?
   • How does the precession adjustment vary with (1) T = time since 2000, (2) RA,Dec?

   
   
   
4.   What is the LST time at 21^h00^m AST today?
   
   
   
5. The angle to which ALFA must be rotated so that its beams sweep out tracks equally spaced in declination is close to the negative of the parallactic angle plus some offset.
   •   What is the "parallactic angle"?
   •   What is the parallactic angle of a source on the meridian?
   •   To what angle must ALFA be rotated for the beams tracks to be equally spaced in declination for a source with a declination of +13 degrees when it is at azimuth 300 degrees?   Hint:   See the routine "optangtab.pro" in the eggidl/gen directory
   •   How does the spacing of the ALFA beams change with telescope azimuth?
   •   What rotation angle do we use for ALFALFA?

   
   
   
6.   What declination will correspond to each of the seven ALFA beams when we observe ALFALFA drift 14p2 at a Declination of +03^o24'24". Be sure to indicate numerically each beam according to ALFA convention.
   
   
   
7.   The ALFALFA observing setup uses the WAPP spectrometers which deliver spectra which cover 100 MHz total, centered on 1385 MHz. Each spectrum has 4096 "channels" (or "pixels" in frequency space, or "array elements", however you want to think of them).
   • What channel corresponds to 1385.0 MHz?
   • To what velocity does a frequency of 1385.0 MHz correspond for the 21 cm HI line?
   • What RFI occurs occasionally at 1381 MHz?
   
   
   
8. Run through the exercises on the Learning to use the ALFALFA digital archive page