What does AR(angle range) mean ?

  Last updated 2003/11/08 03:35 UTC
Updated 2002/09/21 03:30 UTC
1st appeared 2002/01/05 12:15 UTC

  Continued form the top page....

   I have been read many times that the processing time required for each WU is highly dependent on the value of AR(angle range) of the WU. If the value is very low, that is less than 0.1 degree (called as VLAR), some of the SETI@home client (windows command line or so) takes much more time than regular ones (with ARs nearly equal to 0.4 .. 0.5).

  I  have not been interested in this subject because I use linux version of the SETI@home client so I am not sure of this phenomenon.  However, recently I joined an activity of collecting statistical data of WU-processing time, and I have an interest on AR.
   Although I could not find any SETI@home original article that describe the AR directly, I believe that I reached the correct understanding of the meaning of AR after reading several web sites. I think the AR( angle range ) stands for

Angle(in degrees) that the focal point of the radio telescope scans in the sky during the recording period of each Work Unit.

  Ex1:
If the telescope had fixed direction with regard to the earth in the recording period, the focal point moves as the earth rotates, and the data of 107 seconds length in each WU has about a
AR = 0.45 [deg] = 360 [deg/day] / 24[H/day] / 3600[s/H] * 107[s]

This calculation is too simplified, I know. It doesn't say anything about the declination of the telescope against zenith nor the latitude of Arecibo. If the telescope scans the sky staight up, 360[deg/day] should be 360[deg/day]* cos(18°20'), and it yields about AR=0.43 deg.

  Ex2:
If the telescope had been tracking some stellar objects, such as stars or galaxies, the focal point will not move on the sky map, and AR is zero. It is one of the instances of VLAR WU.

   BTW, I found that most of the WUs I've worked on had ARs around 0.4 .. 0.5. They seem to be taken in situation with Ex1. In the case of VLAR WU, searching gaussian is meaningless because the focal point has moved through little or no angle in the sky map. Why meanigless ?   Please check out this article on gaussians . The "12s period" used to explain gaussian pattern seems to be the value of Ex1 (fixed-direction situation).

  I believe the explanation above is basically correct because my understanding coincides with the description in the FAQs(#q125) in SETI@home, especially where "the time resolution of 1 beam is determined entirely by the slew rate of the telescope".