dither<N> / ap_dither<N> Scripts


This page is part of the ING manual WHT-INGRID-2: INGRID IMPB Software Operations Manual


 
Usage: dither2 int_time num_coavgs tcs_x_offset tcs_y_offset ["title"]
dither3 int_time num_coavgs tcs_x_offset tcs_y_offset ["title"]
dither4 int_time num_coavgs tcs_x_offset tcs_y_offset ["title"]
dither5 int_time num_coavgs tcs_x_offset tcs_y_offset ["title"]
dither8 int_time num_coavgs tcs_x_offset tcs_y_offset ["title"]
dither9 int_time num_coavgs tcs_x_offset tcs_y_offset ["title"]

ap_dither2 int_time num_coavgs tcs_x_offset tcs_y_offset ["title"]
ap_dither3 int_time num_coavgs tcs_x_offset tcs_y_offset ["title"]
ap_dither4 int_time num_coavgs tcs_x_offset tcs_y_offset ["title"]
ap_dither5 int_time num_coavgs tcs_x_offset tcs_y_offset ["title"]
ap_dither8 int_time num_coavgs tcs_x_offset tcs_y_offset ["title"]
ap_dither9 int_time num_coavgs tcs_x_offset tcs_y_offset ["title"]
 
Category: Observing System Scripts
Description: These commands build on the services provided by the Instrument Support Platform (ISP) to provide a means of performing a sequence of dithered observations ie a series of telescope moves and data acquisition cycles, starting and ending at a fixed point.

Scripts have been provided to implement the following patterns:
 

ap_dither2
dither2		 ap_dither3
dither3		 ap_dither4
dither4		 ap_dither5
dither5		 ap_dither8
dither8		 ap_dither9
dither9
   1                         2                         1                4       2                5       1       8       7       2       9       8   
o 1 o 1    2       o       6       3       1       7   
                     2                         3       2                3       3                4       3       4       5       4       5       6   

The numbers in the table above indicate the positions at which the data acquisition cycles occur. As can be seen the normal movement is anti-clockwise.  In all cases the telescope starts and ends at the centre of the pattern.

Internally, the script performs data acquisition by means of the ISP run command.  Thus, the observational data files which are generated have run numbers allocated in the normal way. The number of reads in the MNDR cycle may be modified using the numreads commands.

Telescope movement is achieved in one of two possible ways:

  • the ap_dither <N> scripts send commands to the telescope to define the points of the dither pattern as a series of telescope apertures. Movement is then achieved by invoking each TCS aperture in turn.

    •  
  • the dither<N> scripts move the telescope directly as required, using the TCS SLOWOFF command. NB In the previous scripts release (V1.10) the TCS offset command  was used, but the current TCS implementation (W14-1-10) cannot work with this reliably.
There is little to choose between these two methods. In the case of the ap_dither<N> scripts a slight time penalty is incurred in setting up the apertures before invoking them. Another subtle difference is the way in which the telescope position information is represented in the FITS headers (in all cases sufficient information is provided to determine the position on the sky).  See the WHT TCS documentation for further details.

These scripts generate output files whose OBJECT header items have been constructed by prefixing the supplied title argument with a string indicating the index position within the dither sequence. Because of this the supplied title argument must be shorter than with other data acquisition commands such as run. For ap_dither<N> scripts the prefix is of the form"AD-<n>/<m>: " where <n> and <m> specify the index number of the current run and the total number of runs respectively.  For dither<N> scripts the prefix is "D-<n>/<m>: ". 

The geometry of the dither pattern is defined by the tcs_x_offset and tcs_y_offset arguments. It is possible to supply negative values, in which case the sequence of moves will be reflected  about the corresponding axis.
 
Arguments:  int_time is the MNDR integration time specified in seconds to the nearest millisecond. The minimum integration time is determined by the speed with which the SDSU Controller can scan the Hawaii detector array (typically about 1.2 seconds).

num_coavgs specifies the number of coaverages in the observation sequence. num_coavgs should be a positive integer in the range 1 to 99. 

tcs_x_offset  specifies the geometry of the dither pattern in the telescope x axis. The offset is specified in arcseconds in terms of the distance from the centre of the pattern to the outlying points.

tcs_y_offset  specifies the geometry of the dither pattern in the telescope y axis. The offset is specified in arcseconds in terms of the distance from the centre of the pattern to the outlying points.

title is optional, but if specified provides additional information which is recorded in the FITS headers. The title string should be less than 11 characters and should be supplied in double quotes if more than one word.
 
Examples: dither5 10 2 3 9 test1 - perform a 5 point dithered observation  using the telescope offset command. The geometry of the dither pattern in x/y axes will be 3 x 9. At each point where the telescope stops a run will be taken with 10 second integration time and two coaverages. 

ap_dither9 8 1 5 -5  - perform a 9 point dithered observation using telescope apertures. The geometry of the dither pattern in x/y axes will be 5 x 5. Because the y offset has been specified negatively the normal sequence of movement will be reflected negatively about the y-axis. At each point where the telescope stops a run will be taken with 8 second integration time and no coaveraging.