Return how long a source takes to drift across a given great-circle distance.

Arguments:

Declination dec

The declination at which to compute the drift-time.

PointingOffset angle

The desired great-circle angular distance on the sky.

TimeScale scale

The type of elapsed time wanted, from:

• UTC - Universal Coordinated Time.
• LST - Local Sidereal Time.

Example 1:

The following example shows how to have a source drift from a given angle on one side of the telescope's bore-sight, to the same angle on the other side. It starts by tracking jupiter, then moves the telescope to a position that is 5 degrees in front of jupiter's path. It then lets the telescope drift for the amount of time that it takes jupiter to go from 5 degrees ahead, to being at the pointing center of the telescope, then waits again, this time until jupiter has again moved 5 degrees away from the source.

  track jupiter
  until $acquired(source)
  radec_offset ra=-$sky_to_ra($declination(jupiter), 5)
  until $acquired(source)
  halt
  until $elapsed($sky_to_time($declination(jupiter), 5, utc))
  print "Jupiter is now drifting through the bore-sight of the telescope."
  until $elapsed($sky_to_time($declination(jupiter), 5, utc))

Context:

From the rotating reference of the earth, the sky appears to rotate about around line between the north and south celestial poles. Sources near the celestial poles describe small circles, which cross very small angular distances on the sky, whereas sources on the celestial equator describe a large arc that crosses the width of the sky. The time taken to cross a given great-circle angle on the sky thus depends sensitively on the declination of the source.

The sky_to_time() function returns the time that it takes a source at a given declination to drift across a given angular distance on the sky. First it calculates the hour-angle offset that corresponds to the specified angular offset, using the following equation:

      cos(ha_angle) = 1 + (cos(sky_angle) - 1.0) / cos(dec)**2)

It then converts this to a time, by dividing by the number of radians in a circle, and then multiplying by the number of hours in a day. Finally, since this calculates a sidereal time interval, if a UTC time is requested, then it multiplies this by the number of UT seconds per mean sidereal second.

Note that the maximum sky distance that can be achieved at a particular declination, is the great-circle distance between hour-angles that are 180 degrees apart. Thus the maximum sky angle that can be accomodated at declination, dec, is given by:

     max_sky_angle = 2*(90-dec)

If larger angles than this are requested, then the sky_to_time() function returns a time of 12 hours.