There are a set of fifteen pre-designed patterns available for NICMOS observations. Users may define their own pattern specifications as well in APT, during Phase II development. The pre-defined patterns include four dither patterns, four chop patterns, five dither-chop patterns, and two map patterns. For each of these, the observer will be able to specify the number of positions desired (1 to 50), the dither size (0 to 40 arcsec), the chop size (0 to 1440 arcsec, also used for mapping), and the orientation of the pattern with respect to either the detector or the sky. The
POS-TARG special requirement is still available for offsetting the telescope and creating custom-design patterns as well, but there are a number of advantages to using the pre-designed patterns:
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All the observations pertaining to an exposure specification line in a pattern result in one association and are simultaneously calibrated and combined in the data calibration pipeline, including background calibration, cosmic ray removal, and flat fielding. Observations obtained with POS-TARG do not result in associations, and will have to be combined manually by the observer.
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Multiple exposures may be obtained at each position by the use of the Phase II exposure level parameter for Iterations. This may be useful for cosmic ray removal. In addition, exposures in different filters at each pattern position can be obtained by linking together exposure lines as a pattern group.
The fifteen NICMOS pre-designed patterns are listed in Table D.1, together with applicable parameters, such as the allowed values for the number of steps in the pattern, the dither size, or the chop size. In addition, the figure number where the pattern is graphically shown is given in the last column of
Table D.1. Offset sizes and number of steps in a pattern affect the amount of overhead time required to perform an observation (see
Chapter 10). The effects of dithering or chopping on an astronomical image are shown in a set of examples in the next section.
The pattern parameter syntax requires additional input on orientation. Specifically, the pattern must be defined in either the
POS-TARG (camera) frame or the
CELESTIAL (sky) frame. Dithering to remove detector characteristics should always be performed in the
POS-TARG frame of reference. A pattern orientation angle may be specified as well. In the
POS-TARG frame, this is the angle of the motion of the target from the first point of the pattern to the second, counterclockwise from the x detector axis (the directions are defined in
Figure 6.1). In the
CELESTIAL frame, the angle is measured from North through East.
Note that some of the pattern names in Table D.1 are doubled except for an additional -SKY-. The chop can be specified either as
POS-TARG or
CELESTIAL (default—see below for details).
The pattern syntax attempts to resolve the confusing dichotomy in the old pattern implementation, as to whether the pattern moves the telescope or the target. It does this by providing the two reference frames described above.
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Patterns done in the POS-TARG reference frame will move the target, just as the “POS-TARG” special requirement does.
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The telescope is slewed in small angle maneuvers (SAMs) so that the target moves within the detector frame of reference as specified by the pattern. When NICMOS images are displayed with IRAF, the
POS-TARG x- and y-axis are as shown in
Figure 6.1.
The dither patterns are recommended for measuring the background adjacent to point sources (longward of 1.7 microns), and for the reduction of sensitivity variations and bad pixel effects. The four types of canned dither routines are
NIC-XSTRIP-DITH,
NIC-YSTRIP-DITH,
NIC-SPIRAL-DITH, and
NIC-SQUARE-WAVE-DITH. Most of the names are self-explanatory: the
NIC-SPIRAL-DITH pattern produces a spiral around the first pointing; the
NIC-SQUARE-WAVE-DITH pattern covers extended regions by moving along a square-wave shape; the
NIC-XSTRIP-DITH and
NIC-YSTRIP-DITH patterns move the target along the x and y directions of the detector, respectively. The difference between the
NIC-XSTRIP-DITH and the
NIC-YSTRIP-DITH patterns is that the first moves by default along the grism dispersion (orient default = 0°), while the second moves orthogonal to the grism dispersion axis (orient default = 90°). These patterns are illustrated in
Figure D.3, and the direction of the x- and y-axis are the same as in
Figure 6.1.
Note that there is an additional parameter for dithering patterns, to center the pattern on the target. The default is to start the dithering at the target position.
The chop patterns are recommended for measuring the background adjacent to extended targets. For each chop pattern, half of the exposures are taken on the target (position 1). There are two basic patterns,
NIC-ONE-CHOP and
NIC-TWO-CHOP. The
NIC-ONE-CHOP pattern produces one image of the target and one image of the background. The
NIC-TWO-CHOP pattern produces two images of the target and two background images, with the background fields positioned on opposite sides of the target. These patterns may be repeated by specifying the number of points in the primary pattern. For example, calling the
NIC-TWO-CHOP pattern in an exposure with number of Iterations = 1 will produce four images, one on the target, one off to one side (+x detector direction), one back on the target, and one off to the other side (–x detector direction). If the number of Iterations = 2, the observer gets eight images, two images at each position of the pattern. If the primary pattern has number of Points = 2, the pattern will repeat (1,2,3,4,1,2,3,4), and the observer will get eight images. Chop patterns are illustrated in
Figure D.4, and the direction of the x- and y-axis are the same as in
Figure 6.1.
Because chopping is best done to empty regions of the sky, we provide a set of chopping patterns that are in the
CELESTIAL coordinate system, as well as the standard set (that are in the
POS-TARG frame). These have the word
SKY in their name, and must have a pattern orientation angle (degrees E from N for the first motion of the pattern) supplied. These should be used when the region around the target contains some objects that should be avoided when measuring the background.
SKY patterns are illustrated in
Figure D.6, and the direction of the x- and y-axis are the same as in
Figure 6.1.
The combined patterns permit dithering interleaved with chops to measure the background. They are recommended for simultaneous minimization of detector artifacts and background subtraction, for observations beyond 1.7 microns. Three types of combined patterns are implemented:
NIC-SPIRAL-DITH-CHOP,
NIC-XSTRIP-DITH-CHOP, and
NIC-YSTRIP-DITH-CHOP. Their characteristics are analogous to the dither patterns
NIC-SPIRAL-DITH,
NIC-XSTRIP-DITH, and
NIC-YSTRIP-DITH, respectively, with the addition that each dither step is coupled with a background image obtained by chopping. These combined patterns are shown in
Figure D.5, and the direction of the x- and y-axis are the same as in
Figure 6.1.
In a manner similar to the regular chop patterns, the combined patterns have “SKY” versions implemented in the
CELESTIAL frame. The chop patterns require an pattern orientation angle which is defaulted to 0.0 (North). The angle is measured from North through East. These are illustrated in
Figure D.6.
There are two MAP sequences. These allow the telescope to be pointed at a regular grid of points, doing a series of exposures at each point. These are done in the
CELESTIAL frame, so a pattern orientation angle must be supplied, and the telescope motion on the sky is specified (rather than the target motion relative to the detector, see note above). The
NIC-SPIRAL-MAP sequence is basically the
NIC-SPIRAL-DITH sequence in the
CELESTIAL frame, and automatically maps the (square or rectangular) region around the target. The
NIC-MAP sequence defines an arbitrary parallelogram on the sky. The observer may specify the number of points in each of two directions, and the position angle (E of N) of each direction.
As with the dithering patterns, the observer has the option of specifying whether the target is centered in the pattern or not. The target will be centered in the
NIC-SPIRAL-MAP pattern if there are 9, 25, 49,... points in the pattern, but will not necessarily be centered otherwise. The observer can specify if the target should be centered along one axis or the other, or both, of the parallelogram defined by the sequence. These are illustrated in
Figure D.6.
On occasion, it may be advantageous to specify a POS-TARG on the exposure line to move the target to a different position than the aperture reference point. For this situation, the
POS-TARG offset is always performed first to change the telescope pointing. For example, a user wants to position a target in each of the four quadrants in Camera 2. The user specifies the NIC2-FIX aperture for which the aperture reference point is at the center of the array (128×128 pixels) and specifies a
POS-TARG –4.8,–4.8. A four point dither pattern using the
NIC-SPIRAL-DITH pattern with point spacing = 9.6 arcseconds and pattern orient = 0 would achieve the desired results. (See the following example.) The target will be in the lower left quadrant of the array for the first position of the pattern, the lower right for the second position, the upper right for the third position, and in the upper left quadrant for the fourth position of the pattern.
Predefined convenience patterns are recommended for NICMOS observations. These predefined patterns can be selected using the APT pattern editor. Observers can specify their own pattern by using a generic pattern form. Patterns are not supported by
calnicb which combines dithered observations into a mosaic. The IRAF/STSDAS task
drizzle and
MultiDrizzle (Koekemoer
et al., 2002 HST Calibration Workshop, p337) can also be used to combine images into a mosaic. See also the new
Multidrizzle Handbook (Fruchter et al 2009) for examples on how to combine NICMOS data with Multidrizzle.
