Mid-infra-red Aperture Photometry

This is similar to near infra-red aperture photometry. The main difference is the source measured. Rather than measuring each positive and negative image, NOD_CHOP_APHOT integrates the average flux of the one or two pairs of positive and negative images in the chopped and nodded mosaic. The number of pairs depends on the chop-and-nod orientations and the nod throw. The four (or two) images are first registered using their centroids; extracted with a symmetric neighbouring background, but without any duplication of pixels from the original mosaic; and finally combined, averaging to preserve the flux per unit time. The exposure-time header, therefore, remains that of the mosaic.

The aperture size is 5 arcseconds. The background is determined from an annulus with an inner diameter of 7.5 arcseconds and an outer diameter of 15 arcseconds enclosing the source. The Michelle chip characteristics and the UKIRT chop throw limit the area of this combined-image mosaic--normally a 15-arcsecond square--leaving comparatively little background. Recommended apertures and background regions may change in the light of experience with Michelle. Already in below-average seeing, it's evident that a 6-arcsecond circle will leave some signal in the background.

The object name is compared with a 28-member catalogue of 10- and 20-micron standards, a handful of which are known optical semi-regular variable stars. In addition to deriving magnitudes, the recipe calculates an approximate flux in Janskys. It is approximate because being an absolute measurement, it does require a magnitude zero point to be applied to the relative instrumental magnitude. At the time of writing this zero-point is only roughly known, so the default fluxes should be taken with a pinch of salt. The flux is not yet written to the results file. If you have determined the zero point from standards, you can rerun the pipeline for your target observations with that zero point assigned to argument ZP of primitive _NOD_CHOP_APHOT_MAG_ invoked within recipe NOD_CHOP_APHOT. See the section on customising recipes for instructions to make and use a private version of a recipe.

There is an approximate extinction correction applied using a coeeficient of 0.18, because the coefficients for all but one of the various N and Q filters have yet to be determined.
[_NOD_CHOP_APHOT_MAG_, _FIXED_APERTURE_MIDIR_PHOTOMETRY_,
_MAKE_PHOTOMETRY_TABLE_, _GET_FILTER_PARAMETERS_,
_UKIRT_MIDIR_STANDARD_MAGNITUDE_, MICHELLE/_STANDARD_MAGNITUDE_, _FIND_SOURCE_CENTROID_, _CLIPPED_STATS_]