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
-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.