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Object masking

In recipes which make a flat using the frames taken of the targets, the so-called self flat, any sources present can bias the flat field, and result in blotchy mosaics. The full versions of such recipes, as opposed to the _BASIC versions, and SKY_FLAT_MASKED greatly reduce these artifacts using the following algorithm. After the application of the approximate self-flat field, an EXTRACTOR inventory is made of objects having at least 12 connected pixels above 1.0 $\sigma$ above sky. (The thresholds can be changed in $ORAC_DATA_CAL/extractor_mask.sex through the DETECT_MINAREA and DETECT_THRESH parameters.) The locations, shapes, orientations and sizes are used to make a mask. The mask is applied to the dark-subtracted frames and a new flat created. As the outer parts of bright objects often leave residual unmasked blobs, a circular central occulting mask is used. The diameter is normally 7 arcseconds, but it can be adjusted through the OCCULT argument of primitive _MAKE_OBJECTS_MASK_. In the QUADRANT_JITTER recipe the central mask's diameter equals the length of the shorter side of a quadrant. The disadvantage is that the noise is higher within the occulted circle, and its variance is non-uniform across the central ninth of the mosaic. You can modify the the central mask size and shape; see primitives _FLAT_FIELD_QUADRANT_JITTER_ and _MAKE_OBJECTS_MASK_.
[_MAKE_OBJECTS_MASK_, _MASK_OBJECTS_, _DEFINE_QUADRANT_MASKS_,
_MASK_QUADRANT_]

After masking biases can be introduced as the objects or masks move to different locations on the detector each with a different response in the flat field. This is most pronounced for QUADRANT_JITTER where a quadrant of the detector is masked, and IRCAM2 which had a strongly sloping response. in which the mean flat is considerably different from the remaining quadrants. Merely taking a median at each pixel will preferentially select values from certain frames. Thus there has to be an allowance for these systematic differences before the data are combined to give representative relative intensities. The first frame becomes a reference frame against which the recipes scales the modal values of the other frames.
[_NORMALISE_TO_MODE_, _NORMALISE_TO_MODE_EXTENDED_,
_CLIPPED_STATS_]

The improved flat typically shows a uniformity at $\sim$0.02% of the sky. It is this flat which produces the flat-field frames for mosaicking. Systematic errors in the sky--a major uncertainty in infra-red point-source photometry--are also reduced significantly by this algorithm.

EXTRACTOR on occasions underestimate the sizes of objects, so there is an enlargement factor from 1.0-1.5, defaulting to 1.0, applied in primitive _MAKE_OBJECTS_MASK_. If at high contrast you find residual dark rings in your flat-fielded images, try adjusting the ENLARGE argument either in the recipe or the primitive default. See Customising Recipes and in particular editing recipe primitives on how to tailor primitives.


next up previous 309
Next: Field-distortion Correction
Up: Flat-fielding
Previous: Flat creation

ORAC-DR -- imaging data reduction
Starlink User Note 232
Malcolm J. Currie
Brad Cavanagh
Joint Astronomy Centre, Hilo, Hawaii
2004 June
E-mail:starlink@jiscmail.ac.uk

Copyright © 2004 Particle Physics and Astronomy Research Council