Image Items

In this Appendix we will describe the items that can be present
in an image dataset. *Miriad* utilities such as `prthd`, `imlist`,
`itemize`, `puthd`,
`copyhd` and `delhd` may
be used to browse or modify these `header variables'.

Note that the units used in *Miriad* are not always FITS-like,
*e.g.* frequencies are stored in GHz in *Miriad*, whereas in FITS
one finds Hz. For example, *Miriad* keeps
velocity in km/s, sky coordinates in radians and frequencies
in GHz.

Item | Type | Units | Description/Comments |

bmaj,bmin | real | radians | Beam major and minor axis FWHM. |

bpa | real | degrees | Beam position angle, measured |

east from north. | |||

bunit | character | The units of the pixels. | |

btype | character | The type of the image. Possible values: | |

`intensity` - Normal map. |
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`beam` - Point spread function. |
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`depolarisation_ratio` |
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`fractional_polarisation` |
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`optical_depth` |
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`polarised_intensity` |
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`position_angle` |
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`rotation_measure` |
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`spectral_index` |
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cdelt1,cdelt2,... | double | See ctype | The increment between pixels. |

cellscal | character | Latitude/longitude cell scaling | |

convention with frequency/velocity. | |||

Possible values are: | |||

`CONSTANT` |
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`1/F` |
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crpix1,crpix2,... | real | pixels | The pixel coordinate of the reference pixel. |

crval1,crval2,... | double | The coordinate value at the reference pixel. | |

radians | if ctype represents a celestial coordinate. | ||

km/s | if ctype VELO or FELO |
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GHz | if ctype FREQ |
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ctype1,ctype2,... | character | The type of the nth axis (as FITS CTYPE). | |

datamin,datamax | real | See bunit | The minimum and maximum pixel values. |

epoch | real | years | The epoch of the coordinate system. |

history | text | A text item containing the history of | |

processing performed to the data set. | |||

image | real | See bunit | The pixel data. |

llrot | double | radians | Eastward rotation relative to north of the |

sky grid relative to the pixel grid. | |||

lstart,lstep,lwidth | real | ||

ltype | character | The linetype used in making the map. | |

mask | integer | # | Bitmap used to determine which pixels in |

the image have been blanked. | |||

mostable | binary | Mosaic information table. | |

naxis | integer | # | Number of dimensions. |

naxis1,naxis2,... | integer | # | Number of pixels along each dimension. |

niters | integer | # | The total number of deconvolution iterations |

performed on the image. | |||

object | character | source name | |

observer | character | observers name | |

obsra,obsdec | double | radians | The apparent RA and DEC of the observation |

centre. | |||

obstime | double | Julian date | The mean observing time. |

pbfwhm | real | arcsec | Gaussian primary beam size (deprecated). |

pbtype | character | Primary beam type. Standard values include: | |

HATCREEK, VLA, ATCA, WSRT |
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restfreq | double | GHz | The rest frequency of the observed data. |

telescop | character | Telescope name. Standard values include: | |

HATCREEK, VLA, ATCA, WSRT |
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vobs | real | km/s | Velocity of the observatory |

with respect to the rest frame, | |||

during the observation. |

2016-06-21