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Дата изменения: Wed Mar 20 17:44:08 2002
Дата индексирования: Sat Dec 22 17:16:04 2007
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Drift Map

This is the optimum mapping routine (in terms of giving the flattest baselines) for the Arecibo/Gregorian system. The intent of the routine is for the user to map an area slightly larger than the area in which he/she is interested. Baseline subtraction then occurs via fitting a baseline between the opposite ends of each scan (Figure 1).

Example showing 'typical' mapping techniques for baseline removal.

A number of decisions have to be made when running this routine:

• Number of Strips: This is the total number of strips to be covered for the map. The map size (in declination), is then (Number of strips $-$ 1) $\times$ (Dec step) + (Beam size). If you wish to do a complete map, you must have (Number of strips) = 2$\times$INT[(Dec Offset)/(Dec Step)+0.5] +1

• Time per strip: This is the ``drift duration" for each strip (in seconds), and it sets the map size in the RA direction. For computing this time, just note that the sky will drift by at 15 arcseconds/sidereal seconds. There are 1.0027379 sidereal seconds per solar second.

  Each strip ill be centered such that its center will be the RA given by your source list, while the center of the central strip will be centered on your listed Declination.

• RA Offset time: This procedure differentiates between solar and sidereal seconds. The integration times of the correlator, and drift times for the strip are measured in solar (or UTC) seconds. The 1-second ticks from our atomic clock drive this. Setting this option to ``solar" will give a slightly funny RA offset, but the routine will do a better job than the ``sidereal'' setting in aligning the map relative to the correlator integrations (as long as driftTime is exactly divisible by corRecSecs).

  Note that the zenith angle position of the telescope for each strip is set by the starting RA ([object RA] + [RA Scan Length]/2) and the current LST.

• Dec Offset: This is the starting offset from the map center for the declination strips, in arcminutes. A complete map is then 2$\times$(Dec offset) wide. If you wish to do a complete map, you must have (Number of strips) = 2$\times$INT[(Dec Offset)/(Dec Step)+0.5] +1 the declination of your first strip will be ([object Dec] + [Dec Offset]).

• Dec Step: This is the amount by which the declination is stepped at the end of a strip in arcmin (or series of strips, if the number of strips per step is larger than one). If you wish to do a complete map, you must have (Number of strips) = 2$\times$INT[(Dec Offset)/(Dec Step)+0.5] +1

• Strips per Step: This setting allows you to integrate multiple strips before stepping in declination.

• Doppler Update; This setting allows you to update the Doppler parameters either once at the beginning of your map or at the beginning of each strip.

• Cal & Cal Time: These are simply the noise diode options described in Section 1.6, with the additional option of setting the integration time for the calibration scan. The noise diodes are fired at the end of each strip.

• Settle time: This setting tells the routine to delay for an extra x seconds at the start of each strip before taking data. This gives any oscillations in the azimuth arm extra time to settle down.

• Doppler Correction: This is described in Section 1.8.