7 Infrared Arrays

This section summarises the differences between reducing data acquired with arrays sensitive to infrared (IR) wavelengths and those acquired with conventional CCDs. The purpose here is not to describe the principles of the construction and operation of infrared arrays; see McLean’s Electronic Imaging in Astronomy[20], especially Chapters 8 and 9 (pp195-263) for a thorough introduction to these topics. Infrared cameras were introduced into astronomy in the mid to late 1980s. The instruments are more technically demanding to construct than optical CCD cameras, largely though not entirely, because of the additional cryogenics required. The arrays are ‘similar but different’ to CCDs. They are usually smaller than CCDs and the quantum efficiency may be less. Like CCDs, infrared array instruments are entirely electronic, and the images are copied directly into computer memory without any intermediate analogue stage.

The data reduction procedures for infrared arrays are very similar to those for CCDs. However, some of the differences are outlined below. The following notes are largely based on practices at UKIRT (United Kingdom Infrared Telescope) on Mauna Kea, Hawaii and the procedures at other observatories may differ somewhat.

Observing modes

In practice infrared arrays saturate very quickly. Consequently they are read-out very frequently in order to produce a stack of frames (or co-adds) which are subsequently added. You will not necessarily see the individual frames: they may be co-added before you receive them, either by bespoke circuitry in the instrument or by the instrument control software.

Also the instrument and telescope may be ‘chopping’ and ‘nodding’ during the observation: rapidly switching between observing the target object and neighbouring sky in order to allow the otherwise dominant contribution from the sky background to be estimated and and subtracted. This effect is usually achieved by oscillating some component of the optical system, often the telescope secondary mirror. See Electronic Imaging in Astronomy[20], pp201-203 for further details. Chopping and nodding were the usual modes of operation with earlier single-element photometers, but are less common with modern array detectors.

Bad pixels

Though infrared arrays contain individual bad pixels the way that they are read-out means that they are unlikely to contain bad rows or bad columns.

Bias

Infrared array data may appear to have no bias strips or bias frames. This absence may be due to the bias having already been automatically subtracted or it may be because the bias correction is subsumed into the dark current correction (see below).

Dark current

Unlike optical CCDs, the dark current correction is important for infrared arrays. Dark frames should be taken frequently throughout the observing session.

Flat fields

At infrared wavelengths the night sky is sufficiently bright that it can be used to construct flat field frames and this is the usual procedure, rather than acquiring twilight or dome flat fields. A particular disadvantage of dome flats is that they may contain a blurred image of the telescope reflected off the dome because the telescope glows at these wavelengths.

In order to generate the flat field a series of jittered frames will be acquired. Here jittering means slightly shifting the position of the telescope between exposures, typically by about ten seconds of arc¹. At least three frames are required. The region of sky observed may be either offset from the target object and contain only field stars (a sky flat) or be centred on the object and have background sky and field stars around the periphery of the frame (a self flat). The dark current is subtracted from the individual frames, which are then combined by computing the median or clipped median for each corresponding pixel. The median for some pixels may still be biased by the presence of field stars (and the target object in a self flat) so it may be necessary to locate any objects in the frames and mask them out prior to combination.

The frequency with which flat fields need to be taken depends (amongst other things) on the type of coating on the array detector. Arrays with an indium antimonide (InSb) coating, such as IRCAM on UKIRT, require frequent flat fields at intervals of about every half an hour. Arrays with a mercury-cadmium-telluride (HgCdTe) coating, such as UFTI on UKIRT, are more stable and taking only a few flat fields throughout the night may be adequate.

7.1 Reduction procedure

Infrared images of target objects are dominated by the large, additive sky background. Various reduction schemes are possible. One common approach is as follows.

(1)

subtract the dark current from all the frames (both flat fields and target objects),

(2)

create a master flat field,

(3)

apply the flat field to the target frames,

(4)

resample the target images onto the same pixel grid and assemble them into a mosaic,

(5)

optionally any remaining residual non-zero sky can be subtracted later, if required.

Obviously observations made with different filters and on different nights are reduced separately. An alternative, though less usual, approach is to subtract the sky background before making the flat field and dark correction:

The flat field will usually be normalised. The sky frame should have been acquired at a similar time to the target frame, have the same co-adds and be an average of several frames taken before and after the target observation. Median filtering and pixel masking can be used to remove cosmic-ray hits and bad pixels, respectively.

The dark frame includes the bias offset. It should have the same exposure time and co-adds as the flat field and both should be averaged from numerous individual frames. Although in principle it is possible to combine exposures of different duration by scaling, there may be subtle effects which do not scale, so it is better to ensure that the exposures are of the same duration.

For some further details see Section 6.3, IR data reduction, of SUN/139[10].

Part II
The Recipes