Near Infra-red Aperture Photometry

The recipes with an _APHOT suffix (except NOD_CHOP_APHOT) perform aperture photometry on the mosaic and the contributing flat-fielded frames. The method assumes that the target, usually a standard star, is approximately centrally located after allowing for the jitter offsets. If you have data where the star lies outside the aperture, it is possible to apply an offset. See the RAOFF and DECOFF arguments of primitive _FIXED_APERTURE_PHOTOMETRY_ in _APHOT_MAG_ to adjust the aperture's position. The thermal chopped data have sources displaced from the centre, but the reductions allow for the symmetric offsets about the mean jitter position. For UIST thermal data, the search algorithm does not assume a central or centrally symmetric distribution of the positive and negative signals.

Residual bad pixels (usually in the individual flat-fielded frames are removed by median filtering. This does leave a bias in the wings of stars, but certainly the underestimate is far less than ignoring the bad pixels, and is typically far less than the other photometric errors.

The photometry is through a circular aperture located at the centroid of the source, with the sky measured from a concentric annulus outside the aperture. The default aperture size is 5 arcseconds (3 arcseconds for NACO). The annulus diameters are 6.5 to 10 arcseconds (all instruments but UFTI and NACO), 6.5 to 12.5 arcseconds for UFTI, and 3.9 to 6 arcseconds for NACO. The default estimator of the sky flux is the mode calculated from $3*{\rm median}-2*{\rm mean}$ and Chauvenet's rejection criterion. The photometry accounts for fractional pixels at the aperture edge but without allowance for the local gradient.

The magnitudes are given by the expression -2.5*log10(abs(counts) per second exposure time). Therefore negative sources can be measured too, as presented by the thermal photometry recipe NOD_SELF_FLAT_NO_MASK_APHOT. The photometry also yields an internal error determined from the sky variance.

A case- and space-insensitive comparison of the object name with the entries in a table provides a catalogue magnitude for a standard star in I, Z, J, H, or K for all instruments, and in L or M for IRCAM and ISAAC. Also a mean extinction is applied for the mean of the start and end airmasses. Thus the primitive calculates an approximate zero point. Note that ISAAC and NACO standard stars include additional objects not present in the UKIRT faint-standard list or Persson's HST list; for these the magnitude and derived zero point will not be determined automatically. For accurate photometry the actual extinction coefficients should be determined. As the output from the photometry is a small text list, you can use catphotomfit command of the CURSA package to achieve this. The units and meanings of the columns are documented within each small text list.

The seeing is estimated for each frame and the mosaic by fitting a two-dimensional Gaussian to the star, although in good seeing the UKIRT images are more centrally concentrated than a Gaussian. The full-width-half-maximum so derived is also tabulated in the small text list.
[_APHOT_MAG_, _APHOT_MAG_NCOLOUR_, _NOD_APHOT_MAG_ (plus NACO and UIST variants), _FIXED_APERTURE_PHOTOMETRY_, _MAKE_PHOTOMETRY_TABLE_,
_GET_FILTER_PARAMETERS_ (and several instrument variants),
_PERSSON_STANDARD_MAGNITUDE_, _UKIRT_STANDARD_MAGNITUDE_,
_STANDARD_MAGNITUDE_ (and instrument-specific variants), _CLIPPED_STATS_,
_FIND_SOURCE_CENTROID_, _GET_FRAME_CENTRE_]