The Orac-dr infrastructure translates instrument metadata stored in FITS headers into internal
headers. An internal header offers a common name, meaning, and units independent of the
instrument, thereby makes most recipe code portable between instruments, and increases code reuse,
a major theme of Orac-dr. The internal headers are accessed through the ORAC::Frame::uhdr() and
ORAC::Group::uhdr() methods (see Frame headers in SUN/233).
Below is a list of these translated internal headers and their meanings and units. Here are some notes pertaining to the table.
ORAC_ prefix, for example,
ORAC_DEC_SCALE.
| Name | Type | Meaning |
| AIRMASS_START | F | Airmass at the start of the observation |
| AIRMASS_END | F | Airmass at the end of the observation |
| DEC_BASE | F | Declination (J2000) at reference position (and offset 0,0) [deg] |
| DEC_SCALE | F | Pixel scale along declination axis [arcsec] |
| DEC_TELESCOPE_OFFSET | F | Telescope declination offset with respect to the DEC_BASE position [arcsec] |
| DETECTOR_BIAS | F | Detector bias (only used by IRCAM) [V] |
| DETECTOR_MODE | S | Such as STARE, NDSTARE, CHOP |
| EQUINOX | F | Equinox of co-ordinates (fix at 2000.0) [y] |
| EXPOSURE_TIME | F | Exposure time for each co-add [s] |
| FILTER | S | Filter name |
| GAIN | F | Conversion factor [electrons/data number] |
| INSTRUMENT | S | Name of instrument such as IRCAM, UFTI, UIST Michelle, ISAAC but its use is deprecated |
| NUMBER_OF_EXPOSURES | I | Number of exposures in the integration |
| NUMBER_OF_OFFSETS | I | Number of jitter offset positions |
| NUMBER_OF_READS | I | Number of reads per exposure |
| OBJECT | S | Name of the object, preferably adhering to IAU standard |
| OBSERVATION_MODE | S | Operating mode of multi-mode instrument,
|
| OBSERVATION_NUMBER | I | Observation number (starting at 1 each night) |
| OBSERVATION_TYPE | S |
|
| RA_BASE | F | Right Ascension (J2000) at reference position (and offset 0,0) [h] |
| RA_SCALE | F | Pixel scale along right-ascension axis [arcsec] |
| RA_TELESCOPE_OFFSET | F | Telescope right-ascension offset with respect to the RA_BASE position [arcsec] |
| READMODE | S | Such as STARE, NDSTARE, CHOP |
| ROTATION | F | Angle of the Declination axis with respect to the frame’s axis measured counter clockwise. |
| SPEED_GAIN | S | Speed and type of readout, e.g. Normal, Fast, Higain for UFTI; Standard, Fast, Deepwell for IRCAM. |
| TELESCOPE | S | The PAL palObs telescope name |
| STANDARD | B | Whether or not the observation is of a standard |
| UTDATE | S | UT date of the observation in yyyymmdd format |
| UTEND | F | UT of the end of the observation [h] |
| UTSTART | F | UT of the start of the observation [h] |
| WAVEPLATE_ANGLE | F | Polarimetry waveplate position angle [deg] |
| X_LOWER_BOUND | I | Start column of array readout |
| X_REFERENCE_PIXEL | F | The pixel index of the reference position for RA_BASE and DEC_BASE |
| X_UPPER_BOUND | I | End column of array readout |
| Y_LOWER_BOUND | I | Start row of array readout |
| Y_REFERENCE_PIXEL | F | The pixel index of the reference position for RA_BASE and DEC_BASE |
| Y_UPPER_BOUND | I | End row of array readout |
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_ 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_ recipes. |
| MAKE_MOSAIC | B | Whether or not register the frames and make the mosaic. For EXTENDED_ 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 |
| Name | Type | Meaning |
| TARGET_NUMBER | I | When TARGET_OR_SKY is |
| 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). |