IRAC description

The Infra-Red Array Camera [IRAC; Fazio et al., 2004] onboard Spitzer provides imaging at 3.6, 4.5, 5.8 and 8.0µm. Following Fazio et al. [2004], I will refer to these

as channels 1, 2, 3 and 4, respectively. Light from the telescope is focussed onto one of four detectors using a series of pickoff mirrors, lenses, dichroic beam splitters and filters (see Fig 2.2). Each detector measures light in a dedicated wavelength band

Figure 2.2: The optical layout of the IRAC instrument, from a side view (top image) and a top-down view (bottom image). These show the optical path of beams from the pickoff mirrors onto the IRAC InSb (channels 1 and 2) and Si:As (channels 3 and 4) detectors. The side view highlights that channels 1 and 3 image one field-of-view, while channels 2 and 4 image another field-of-view. Image taken from Fazio et al. [2004]

(see Fig. 2.3 for the response curves) and images one of two 5.2′

×5.2′

fields of view. One field is imaged by channels 1 and 3, while the other is imaged by channels 2 and 4. Data can be taken simultaneously by all four detectors, but since the two fields of view do not overlap, simultaneous observations of a particular target can only be made by the detector pairs that image the same field.

Each detector contains 256_×256 pixels, giving a pixel scale of 1.2′′ in all four bands. Each can be operated in a ‘sub-array’ mode where only a sub-region of 32_×32 pixels towards the edge of the full array are used. This mode is useful for taking shorter exposures (which are used to avoid saturation in bright stars) while not requiring large amounts of memory on the onboard computer.

The IRAC detectors are infra-red arrays. These share features with the charge-coupled devices (CCDs) used in optical astronomy, principally in that they are semi-conductor arrays comprised of p-n junctions that act as photodiodes [Dres-

Figure 2.3:Spitzer IRAC transmission curves for channels 1–4. The bands are cen- tred on 3.6, 4.5, 5.8 and 8.0µm, respectively and have full-width at half maximum

(FWHM) values of 0.7, 1.0, 1.4 and 2.8µm. Image from Fazio et al. [2004].

sel, 2012]. Incoming infra-red photons create free electron-hole pairs near the p-n junction, where electrons are raised from the valence band to the conduction band and thus are free to move. The electric field set up by the p-n junction separates the free electron-hole pair, with the electrons being stored in the n-type semi-conductor. The amount of charge accumulated is therefore proportional to the number of pho- tons illuminating the pixel.

In infra-red detectors the voltage change across the p-n junction associated with the charge accumulation is read out individually for each pixel by a dedicated readout amplifier. Pixel readouts are then sequentially connected to an output (mul- tiplexing). For the IRAC detectors, a four-channel readout is used, with four columns being read out row-by-row, simultaneously.

The reading of pixel voltages is non-destructive i.e. charge is not removed from the pixel in order to be read out (as it is in CCDs). This affects how pixel values for a single exposure are measured. Fowler sampling is used, where successive voltage reads are made at the beginning of an exposure (pedestal levels) and the same number are made towards the end (signal levels; see Figure 2.4 for a schematic diagram of this technique). A single measurement for the pixel for a given exposure is determined as the average of the pedestal voltages subtracted from the average of the signal voltages [Fazio et al., 2004]. By averaging in this way, readout noise, which can be significant for infra-red arrays, can be reduced by a factor of√N (where N

Figure 2.4: A schematic of how pixel count measurements are made for infra-red arrays like the IRAC detectors, using Fowler sampling. Successive voltage reads are made at the beginning (pedestal levels;pi) and end (signal levels;si) of an exposure. The final value for the exposure is given by the average of thesi−pi values. Effective exposure times are therefore the time difference between the correspondingpi and si measurements, while the total frame time spans all of these measurements. is the number of pedestal or signal reads).

A key choice for the IRAC detectors is the material used. The energy required to raise electrons from the valence band to the conduction band (the band gap energy) in the semi-conductor must be small enough so that infra-red photons can cause this change. Silicon, which is used in optical CCDs, has a band gap energy of

∼1.1 eV which corresponds to aλmax∼1.1µm. Infra-red photons would therefore

not be detected in a silicon detector. For the 3.6 and 4.5µm IRAC detectors, InSb is

used - a silicon-like material that has a band gap energy of_∼0.2 eV corresponding to aλmax∼6.2µm. As shown in Figure 2.3, the bandpasses for these detectors are

well matched to this cut-off. For the 5.8 and 8.0µm detectors arsenic doped silicon

(Si:As) is used. The band gap energy here is_∼ 0.05 eV which has λmax ∼25µm -

again a suitable material for these bandpasses.

The respective sizes of these bandgap energies explains why the 3.6 and 4.5µm detectors can operate during the warm mission, while the 5.8 and 8.0µm

detectors cannot. The longer wavelength detectors, with a smaller bandgap, are more susceptible to electrons being thermally excited into the conduction band, and at the temperatures of the warm mission (29 K) the resulting thermal noise at these wavelengths is too great for these detectors to be effective.