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Дата изменения: Fri Feb 14 18:19:43 2003
Дата индексирования: Sat Dec 22 18:38:40 2007
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Поисковые слова: ceres

RA/Dec & Dec/RA maps

These maps allow you to drive the telescope in RA or Dec while stepping in Dec or RA, respectively. When using these routines, you must carefully consider what method you will use for baseline subtraction. The unique geometry of the Arecibo system produces a wide variety of standing waves in a typical baseline. These waves change shape considerably as the telescope is moved in azimuth and zenith angle. As a result, baseline stability offered by drift scans is considerably better than when the maps are made by driving the telescope. This means that if you can make your desired map using the drift map routine, then, although it will take longer, it is in your best interest to do so.

Many of the options available for these procedures are the same as for the drift map routine, which is described above. The differences are:

• First Strip: This option allows you to pick up a map in the middle, instead of forcing a full map coverage each time. Here you would enter the number of the strip at which you wish to begin observing.

• RA (Dec) Offset & Strip Time: These parameters not only define the size of your map in the RA (Dec) direction in arcmin, but also, when combined with the strip time, set the drive rate on the sky; i.e., the RA drive rate is set to 2$\times$60$\times$(RA offset)/(Strip time) [in arcsec/sec]. The type of arcsecond (little circle or great circle) is set by the RA offset type (see below), or a Dec drive rate of 2$\times$60$\times$(Dec offset)/(Strip time) for the Dec/RA map option.

  You should adjust the correlator dump time to sample the data at whatever rate you want (normally less than or equal to half a beam), where faster sampling can allow for better RFI reduction, but the over sampling can result in extemely large data files.

• Dec (RA) Offset & Dec (RA) Step: These options set the map size (in arcmin) in the Dec (RA) direction to be 2$\times$(Dec Offset) (or 2$\times$(RA Offset) if you are driving the telescope in the declination direction). Additionally, when combined with the Dec (RA) Step option, these options set the number of strips in each map to be (Number of Strips) = 2$\times$INT[(Dec offset)/(Dec Step)+0.5] + 1 (or (Number of Strips)= 2$\times$INT[(RA offset)/(RA Step)+0.5] + 1 for maps driving in Dec). If you only want one strip centered on the Dec (RA) listed in your source list, you should set the Dec (RA) step size and offset to zero.

• Drive Direction: This option specifies whether you wish to drive the telescope, taking data alternating in direction between scans, or only in one direction. The second option is more time consuming as the telescope must slew across the mapped region before beginning the next strip.

• RA Offsets in great/little circles: This option allows you to set whether the RA offsets are in great or little circles of arc. (Recall that a little circle is the standard astronomical definition for RA. Great circle will cover 1/cos(Dec) greater length on the sky.)