Sigma QUick Infrared Camera User Manual download pdf (Page 37)

WHIRC Data Reduction Manual Version 2.0, 2014 June 26 37

4.0 Appendices

4.1 Appendix A – Observational Test of Pupil Ghost Removal

On the (apparently) photometric night of 9 July 2009 UT we tested the pupil ghost

removal technique described in section 2.4.2 by sampling a region approximately 80

arcsec square centered on the array. An arbitrary star (Ks ~ 11) on the outskirts of M13

was measured using four 5 × 5 dither sequences of amplitude 10 arcsec, each of which

was offset by 18 arcsec in both RA and DEC from the center of the array in the four

possible directions. This gave high spatial sampling of the area around the center of the

pupil ghost (approximately [1032:1002] on the array). The experiment was done in the

Ks filter where the pupil ghost is most pronounced. The focus was checked between each

of the four sequences to ensure that the image quality did not drift significantly during the

experiment; seeing was modest (0.6 – 0.7 arcsec).

Data reduction was done in the standard way. The 100 frames were median combined to

generate a sky frame, which was subtracted from each of the input frames to yield the

sky-subtracted images.

Good quality dome flats in the J and Ks filters were obtained using the standard “lights

on, lights off” technique, subtracted and then corrected for bad pixels using the fixpix task

and the WHIRC bad pixel mask. The flats were normalized using regions

[400:600,400:1600] and [1400:1600,400:1600], which avoids the pupil ghost area, and

any other pixels of value < 0.02 were set to 1.0 to avoid arithmetic blowup in the

flattened images. These flats, which contain the pupil ghost, are referred to as the

uncorrected flats.

The template for removal of the pupil ghost was generated as follows:

• The normalized Ks and J band flats were ratioed to give a flat ratio image

• A new 2048 × 2048 image with pixel value = 1.0 was generated with mkimage

• The central 800 × 800 of the flat ratio image centered on the pupil ghost

[625:1424,601:1400] was copied into the new image

• The image was smoothed using the gauss task with sigma = 31

• Subtract 1.0 from the result to yield the pupil ghost template image

The uncorrected flats were run through the rmpupil task using the pupil ghost template

image as the ‘pupil’, and the pupil mask file pupmask.pl as the ‘pupilmask’ to restrict the

procedure to the central 600 pixels [725:1324,725:1324]. The parameter ‘outtype’ was

set to “sdiff”, since one is subtracting the ghost, and the column and line smoothing were

set to 20. The resulting flats were the “nopupil” flats.

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Sigma QUick Infrared Camera User Manual Page 37