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, 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.
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, 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.
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.
Infrared images of target objects are dominated by the large, additive sky background. Various reduction schemes are possible. One common approach is as follows.
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.
1The jitter offset may be larger, depending on the array size and target characteristics. For example, the UFTI instrument on UKIRT can have an offset of up to forty-five seconds of arc.