Steering Headers

Headers which direct a recipe are called steering headers. These can include timing information, such as when a flat should be made; qualifiers about the frame such as whether it is of a target or sky. They use header information and membership index within a group. The names of steering headers should be longer than eight characters to avoid confusion with FITS keywords.

Most steering headers are per frame and are accessed by ORAC::Frame::uhdr() (see Frame headers in SUN/233). Steering headers which must persist between frames, such as those which define groups of related raw or processed frames like differenced chopped pairs, must be stored in the group internal headers, accessed by ORAC::Group::uhdr().

While programmers can create their own steering headers as needed, it is silly to invent new names for ones which already exist and are accessed by many primitives, which the programmers would then not be able to use directly.

Here is a list of the common steering headers used in the imaging recipes. Not all headers are pertinent to all recipes. The headers used exclusively by specialist recipes, such as for Fabry-Perot or nod-chop in a scan mode, are not listed below. The abbreviations for data type are B: boolean (1 for true, 0 for false), I: integer, F: floating point, and S: string. Note that some of the timing headers may in fact switch on for the same frame.

Name	Type	Meaning
CREATE_CATALOGUE	B	Whether or not to create an object catalogue.
CYCLE_NUMBER	I	Number of the cycle, a cycle being a set of frames to complete a pass through the recipe. The first cycle is 0.
DEC_OFFSET_DIFFERENCE	F	The declination displacement between a nodded pair of frames.
DIFFERENCE_PAIR	B	Whether or not to subtract pairs. It is normally true every second frame.
DO_APHOT	B	Whether or not perform aperture photometry. Photometry is performed once the mosaic is made.
EXTENDED_ROW	I	The row number of the frame for EXTENDED_n x m recipes.
FLAT_DIVIDE	B	Whether or not to apply a flat field.
JITTER_FIRST	B	This selects the ordering of polarimetry frames. If true the jittering occurs at all positions before the waveplate is turned. If false, all waveplate angles are observed at a given offset.
JITTER_NUMBER	I	The number of frames in the jitter.
MAKE_FLAT	B	Whether or not to make a flat field.
MAKE_GRAND_MOSAIC	B	Whether or not register the frames and make the full mosaic for EXTENDED_n x m recipes.
MAKE_MOSAIC	B	Whether or not register the frames and make the mosaic. For EXTENDED_n x m recipes it is the time to make a row mosaic.
MASK_OBJECTS	B	Whether or not to mask the objects. Masking occurs when all the jittered frames in a cycle are available.
PAIR_ORDER	B	Pair subtraction order, true means take second from the first, and false means take the first from the second.
POL_CYCLE_NUMBER	B	Number of the polarimetry cycle, a cycle being a set of frames to complete a pass through the recipe for all waveplate angles. The first cycle is 0.
RA_OFFSET_DIFFERENCE	F	The right-ascension displacement between a nodded pair of frames.
REFERENCE_FRAME	B	A true value specifies the reference frame for normalisation of the masked frames. It is true for the first frame and false for all subsequent frames in the observation.
REFERENCE_LEVEL	F	The reference modal level, used when combining masked frames to form a flat.
REFERENCE_SKY	F	The reference sky level, used for sky subtraction.
REGISTER_IMAGES	B	Whether or not to register and resample the polarimetric e- and o-beam mosaics.
SUBTRACT_SKY	B	Whether or not it is time to subtract the sky. Sky subtraction normally occurs once all the frames in a cycle are available.
TARGET_OR_SKY	S	This is "target" for a target frame, and "sky" for a sky calibration frame.

Name	Type	Meaning
TARGET_NUMBER	I	When TARGET_OR_SKY is "target", this counts the target frames, starting from zero. It is used for interpolation between sky measurements.
USE_VARIANCE	B	Whether or not variance processing is to occur.
WAVEPLATE_FLAT	B	Whether or not to make a flat for each polarimeter waveplate angle. For non-polarimetric data, the value is immaterial. For polarimetric data, false means combine all waveplate angles to make the flat (there should be an equal number of each angle).