Format of input parameters
- The RA and DEC portions of the coordinate should be separated by a comma. Coordinates can by in the form hh:mm:ss,dd:mm:ss or in decimal hours and degrees (the coordinates are nominally in the J2000 frame). The calculator attempts to recognise other forms as well. Alternatively the name of a source can be given (the NED name resolver is used to convert this to a position).
- The observing epoch gives the time of the
observation. This is needed to compute the Doppler correction needed to
account for the observatory's motion relative to the rest frame. The
observing epoch can be given in any of the forms:
yymmmdd yymmmdd:hh:mm:ss.s ccyy-mm-ddThh:mm:ss.s yymmmdd.dd dd/mm/yy dd-mmm-ccyywhere dd, mm and yy give 2-digit day-of-month, month and year respectively, mmm gives 3-character month (e.g. feb for February), and ccyy gives the 4-digit year (eg 2004). For example 04nov22 is 00:00 UT on 22 November 2004.
General note on velocitiesBecause the ATCA does not Doppler track, and because it does not correct for the systemic velocity of a source, the observer has to enter the actual sky frequency of the spectral line of interest. In converting to sky frequency, the observing frequency has to consider
- The known systemic velocity of the source. There are two conventions in use for defining the velocity used - the so-called radio and optical conventions. There are also two commonly used rest frames - the solary system barycentre, and the so-called local standard of rest (LSR).
- The additional velocity of the observatory relative to the rest frame.
Note, however, that the ATCA does not Doppler track. Account of the Doppler correction is done in the off-line software.
Velocity conventionsThe relativistic expression relating frequency to radial velocity is
For high redshift objects, it is common to give the redshift of the object
Rest framesIn defining a velocity, a rest frame must also be given. Because of the Earth's diurnal and annual motion (spin on its axis, and rotation around the Sun), the Earth's surface is not a good rest frame. The diurnal and annual motion have maximum velocity components of approximately 0.5 km/s and 30 km/s respectively. Two commonly used rest frames are the solar system barycentre (the dynamical centre of the solar system) and the local standard of rest (LSR) (which accounts for motion of the solar system relative to a collection of local stars). The LSR is generally used as the rest frame for Galactic astronomy, whereas the barycentric frame (often misnamed heliocentric frame - which is slightly different) is generally used for extragalactic work. An extra complexity is that there are a number of definitions of the LSR. The definition used is that the solar system barycentre is moving at 20 km/s in the direction of (RA,DEC) = 18 hours,+30 degrees (B1900) (this is called ``kinematic'' definition of the LSR).
Original: Bob Sault (22-Nov-2004)
Modified: Bob Sault (26-Nov-2004)