Dither Catalogs: Important Information
Paddle Tool
The MMIRS home directory /home/mmirs/cats contains generic 3x3, 4x4, and 5x5 dither catalogs that can be used in most situations. You can also make your own dither catalogs. This is what you need to know.

Dither catalags are ASCII text starbase tables with two columns: azoff and eloff, in units of arcseconds.

Do not be confused by the names. Azoff and eloff are offsets in the x- and y-directions, respectively, in instrument pixel coordinates. That is, azoff and eloff do not correspond to 'azimuth' and 'elevation,' nor the cardinal directions 'east' and 'north,' unless the telescope instrument rotator is set to track north.

The format of a starbase table is a header line, a dash separator line, followed by lines with offsets in units of arcseconds. Everything must be separated by tabs. For example, consider the table:

azoff eloff
- -
-60 60
60 -60
-60 -60

Loading this dither catalog, setting first=1 and last=4, and pressing 'Go' will cause MMIRS to take 4 images at the indicated offsets from the telescope catalog position. To take a second series of 4 images with the same dither catalog, but not centered on the catalog position, use the paddle tool to offset in RA/DEC coordinates (not INST coordinates) and then repeat the observation.

To re-iterate, dither offsets are applied in (relative) instrument coordinates, and do not alter the angle of the instrument rotator. The operator normally sets the instrument rotator so that it tracks North = up. Thus in normal operation the example dither catalog offsets are purely north-east-south-west. If you wish to map at a different angle, simply tell the telescope operator to have the rotator track at your desired angle (measured in degrees east of north=0).

The filter and exposure time used for a dither sequence are the filter and exposure time set on the MICE window. Alternatively, you can add columns filts and exptime to set the values yourself. For example:

azoff eloff filts exptime
- - - -
0 0 H 15
420 0 H 15
0 0 J 10
420 0 J 10

This dither catalog will cause MMIRS to take a 15 sec H-band image ON and OFF source, followed by a 10 sec J-band image ON and OFF source.

To repeat a dither sequence on a slightly offset position, use the paddle tool to offset your pointing in RA/DEC coordinates (not INST coordinates), and then run the dither sequence again.

Comments or Questions? Contact wbrown@cfa.harvard.edu