This specifies the type (and shape) of a pattern (generally an instrument-specific name; see “Convenience Patterns” on page 139), and, depending on the type, may constrain or completely determine the values of the other parameters. Many pattern types have been defined for the different instruments, as given below. Pattern parameters that are determined by the pattern type, or have default values, need not (or cannot) be changed in the form.
Some pattern types are intended for use with a combination of two patterns (Primary_Pattern and
Sub-Pattern). With such pattern types the
Sub-Pattern is mandatory. The
Pattern_Type may specify that a parameter of the
Sub-Pattern is constrained or determined by the value of the corresponding parameter of the
Primary_Pattern. Whenever two patterns are used, the
Pattern_Type of the
Sub-Pattern defaults to the pattern type of the
Primary_Pattern.
A Pattern_Type defined for a particular instrument is legal only if the first primary exposure in the pattern uses that instrument. Generic pattern types are valid with any instrument. Patterns specific to individual instruments are described in
Section 8.4 on page 139. The following are generic pattern types.
This specifies a box pattern; the Number_Of_Points =
4 and all other parameters are open.
DITHER means small-scale motions used to improve spatial resolution and data quality.
MOSAIC means movement of the aperture to sample different parts of an object or regions of sky.
BACKGROUND means a movement away from a source to sample the sky background, ordinarily used with infrared instruments.
OTHER means that the reason for the pattern should be provided in the
Comments field. Unless a default is determined via the
Pattern_Type, this parameter is required, although you may edit it from the default value to another valid value.
This specifies the spacing between adjacent points in a pattern, in arcsec. For parallelogram patterns Point_Spacing will specify the length of the segment between the first two pattern points. The value of
Point_Spacing ranges from 0 to 1440 arcsec. Unless a default is determined via the
Pattern_Type, this parameter is required.
Unless PCS MODE Gyro is specified on the visit, all patterns must be contained within a circle of radius 24 arcmin (1440 arcsec), with the first pattern point at the center.
This field has two legal values, POS-TARG and
CELESTIAL. It specifies whether the pattern is being done in the
POS TARG (spacecraft) frame or the celestial frame (by specifying offsets from the original target position on the sky). For patterns with a
Pattern_Purpose of
DITHER,
POS-TARG is the only legal value.
This field specifies the orientation of the first segment of the pattern with respect to the chosen frame, in degrees. If Coordinate_Frame:
POS-TARG, this angle will be measured from the POS TARG +X axis toward the +Y axis. If
Coordinate_Frame:
CELESTIAL, it will be measured North through East, and will specify the direction of an offset from the target on the sky.
A value of 0 will cause the first pattern segment to be oriented along the POS TARG +X axis if Coordinate_Frame:
POS-TARG, and along the North vector on the sky if
Coordinate_Frame:
CELESTIAL.
This indicates whether the pattern should be centered relative to the pointing (hereafter referred to as the “default pointing”) that the exposures would have had in the absence of a pattern. For a primary pattern (Primary_Pattern), the default pointing is the target position, unless the
POS TARG Special Requirement (see
POSition TARGet <X-value>,<Y-value>) is used to shift the target relative to the aperture. For a secondary pattern (
Sub-Pattern), the default pointing is the pointing offset from the target determined by the given pattern point in
Primary_Pattern.
Legal values for Center_Pattern are
YES (check in box provided) and
NO (no check). The default is
NO, unless overridden via the
Pattern_Type. If
Center_Pattern:
NO, the first pattern point is placed at the default pointing. If
Center_Pattern:
YES, the first pattern point is offset so that the default pointing is placed at the geometric center of the pattern.
If applicable, the Sub-Pattern information must be provided (Type, Shape, Number of Points, etc.).
