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Infrared imaging very commonly uses "pair subtraction,"
in which two images of identical exposure times and coadds and
taken close in time to each other are subtracted. The image pair
sometimes consists of a target field and a widely separated "sky
field." Generally you would use snapi
and the nod parameters to control this type of data-taking. Recall
that snapi takes an exposure,
then moves the telescope by the nod parameters, takes another
"sky" exposure, and then returns the telescope to its
original position.
Moving the target all the way off the detector, however, cuts
your on-source integration time in half. Many projects use smaller
moves, in which the target is moved from one part of the detector
to another. This technique is called "dithering," and
is most often controlled by one of various procs which use the
goi command:
| Procedure
| Description
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| box5 |
Moves the telescope in a 5-position box pattern using the nod distances.
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| box9 |
Moves the telescope in a 3x3 box pattern using the nod distances.
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| bxy5 x y
| A 5-position pattern specified by x and y arcsecs in NIRC coordinates.
|
| bxy5q x y
| Same as bxy5, but does not display differences.
|
| bxy9 x y
| A 3x3 pattern specified by x and y arcsecs in NIRC coordinates.
|
| bxy9q x y
| Same as bxy9, but does not display differences.
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| mosaic nx ny dx dy
| An nx by ny mosaic of dx and dy increments in NIRC coordinates
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| thinxy x y n
| A 5-position pattern in either x or y arcsecs (the other direction should be set to zero), useful for when the guide star is on the edge of the field. n is the number of times to repeat the 5 positions.
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| thinxyq x y n
| Same as thinxy, but does not display differences.
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| Spectral Dithers
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| s5 n
| A 5-position pattern along the slit, repeated n times.
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| sp2 n
| Switch between 2 positions along the slit, separated by 12.8 arcsecs. Repeat n times.
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| sp55 n
| A 5-position dither along the slit, repeated n times. The sequence is (+10, -5, +5, -10, 0) arcsecs.
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| sp56 n
| A 5-position dither along the slit, repeated n times. The sequence is (+12, -6, +6, -12, and 0) arcsecs from the initial position.
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| sp74 n
| A 7-position dither along the slit, repeated n times. The separation is 4 arcsec.
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