Sigma QUick Infrared Camera User Manual download pdf (Page 10)

WHIRC Data Reduction Manual Version 2.0, 2014 June 26 10

1. Sky flats are best generated from a series of observations which give an appreciable

(several thousand ADU) sky level. This pretty much restricts nighttime observations to

the H and Ks filters. Twilight flats can be taken in the J and (perhaps) narrowband filters,

but the rapid onset of twilight in the infrared leaves limited time for this procedure. H

band sky flats may show fringing from the OH lines, which will complicate the process

(section 2.4.4).

2. Take a series (~ 10) of dark frames (using the OPAQUE filter) at the same integration

time as used for the observations being used to generate the sky flat; this will subtract out

any bias and/or dark current. Average the dark frames, perhaps with average sigma

clipping to prune out maverick pixel values.

3. Generate a sky image as described in section 3.1. Ideally, one should use a large

number of images (10 or more), at more or less the same airmass, with several thousand

ADU of background.

4. Subtract the averaged dark frame from the sky frame. Normalize and correct for

artifacts as with the dome flats.

2.4 Artifact Removal

WHIRC images contain several types of artifacts, some of which are intrinsic to the

detector, others of which result from multiple reflections within the optics. Some

correction techniques are discussed in this section, although experienced users may wish

to experiment with other procedures. Because infrared arrays utilize a unit-cell

architecture, the pixels are effectively independent of each other, unlike those in a CCD.

2.4.1 Bad Pixels

Like all arrays, the detector in WHIRC is not perfect and has several types of cosmetic

imperfections. Some of these can be corrected to a degree during the data reduction.

2.4.1.1 High Dark Current Pixels

These are scattered throughout the array, with certain areas, such as the upper right,

having a higher concentration. These show up as bright pixels, particularly against a low

background such as a dark or narrowband sky. These pixels may also be characterized as

“maverick” pixels, since the signal is not necessarily proportional to the frame time, as a

true dark current would be. The procedure of sky subtraction does seem to remove most

of the elevated signal level, although the noise in these pixels may be higher than in

others. Otherwise, there is no remedy for these artifacts.

2.4.1.2 Dimples

A well-illuminated image such as a flat (Fig. 2.4) shows numerous circular artifacts

which appear to be dead pixels. However, closer inspection reveals that these are not

dead, but areas of decreased response which appear to flatfield out reasonably well (Figs.

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Sigma QUick Infrared Camera User Manual Page 10