Detectors for IR astronomy

Where does infrared begin? Wavelength sensi?vity of the human eye vs. wavelength Note: the eye has some (limited) sensi?vity to IR light at ~1000nm (=0.5x energy of photons the eye is maximally sensi?ve to)! Total throughput (atmosphere + telescope + instrument op?cs + filter transmission + detector QE) for DECam (used for the Dark Energy Survey at the 4- m Blanco telescope)

Where does infrared begin? Total throughput (atmosphere + telescope + instrument op?cs + filter transmission + detector QE) for DECam (used for the Dark Energy Survey at the 4- m Blanco telescope) Filter response curve and detector Quantum Efficiency (QE) curve (purple) for the ESO VISTA infrared survey telescope. Note that the wavelength coverage of modern op?cal and NIR detectors overlap in the z and Y band.

Sub- division of the IR region Near- infrared (NIR; 0.7-5µm) Mid- infrared (MIR; 5-30µm) Far- infrared (FIR; 30-1000µm) The longest wavelengths in this region is also frequently referred to as sub- mm wavelength region Cooling necessary, using liquid nitrogen: 77K (NIR), liquid hydrogen: 4K (MIR) The exact limits are somewhat arguable The NIR region up to 2.4µm is best accessed using IR detectors on normal op?cal telescopes Beyond 2.4µm the Earth s atmosphere becomes a major limita?on à Large advantage from use of airborne (SOFIA), balloon, or space observatories.

Why bother with IR?

Examples of science cases planetary forma?on biomarkers high- redshih universe regions of high ex?nc?on (e.g. Milky Way centre)

T~275K T~5800K The spectrum of the zodiacal light is to good approxima?on a superposi?on of two blackbody curves from reflected light (solar spectrum) and thermal emission from interplanetary dust heated by the sun.

Spectrum of Vega (A0 main sequence star) 1983: The Infra- Red Astronomical Satellite (IRAS) observed infrared excess in the spectrum of Vega. Circumstellar disk of dust

brightness Brightness of planets rela?ve to the sun

Biomarkers in the Earth s transmission spectrum

Most distant galaxy (z=8.68) confirmed by measuring a spectral line (Lyman- alpha, 1216 Å) using the MOSFIRE IR instrument on Keck Zitrin et al. 2015 Radia?on emioed at 1216Å (UV) redshihed to l observed = l emioed (1+z) = 11770Å (NIR) No observed flux in the op?cal region.

Peering through Galactic dust Bok globule Barnard 68

8m VLT + FORS1 (optical) 3.5m NTT + SOFI (NIR)

Sensi?vity evolu?on Wavelength coverage of 39m E- ELT: Most instruments will cover NIR region

IR sensor material If photons have insufficient energy to lih a value- band electron to the conduc?on band, i.e. hν > U C - U V, the material becomes transparent For Si, this occurs at the cutoff wavelength λ cutoff = 1090nm, corresponding to hν = 1.1eV Photoconductors are widely used to detect NIR and MIR radia?on Bolometers are normally used in the FIR region

Photoconductors Photoconduc?ve cells change in conduc?vity with the intensity of the illumina?on (electrons lihed from valence to the conduc?on band) Monitored by a small bias current The light- sensi?ve material is bonded to a silicon substrate that contains the readout electronics No crosstalk (blooming) between pixels

Readout of photoconductors Non- destruc?ve readout Common readout modes: Reset- read- read or sampling up the ramp (useful against satura?on and cosmic rays hits) For mul?ple readouts, the results can be averaged to reduce the noise One bad pixel does not affect the others (cf. charge transfer (in- )efficiency issues for CCDs, bad columns etc.) Typical integra?on?mes can be 0.001s to 10s, depending on the sky background

Dealing with the background Faint targets are typically invisible in raw exposure data To know what is going on, one has to subtract an image of a blank background This can be done by nodding (moving the telescope) or chopping (moving a mirror), alterna?ng between the target and blank sky Flat fielding is s?ll needed to calibrate the sensi?vity differences between pixels

Bolometers Neither photoconductors or photodiodes Changes its electrical resis?vity as it gets heated by illumina?ng radia?on A typical detector consists of a small chip of the doped material supported by very thin wires which act as electrical conductors for the measurement of its resistance The doping level is chosen to provide an op?mum sensi?vity of resistance to temperature at the opera?ng temperature of the detector (typically 1-2K) The coefficient of change of resistance depends on the opera?ng temperature (which is also affected by the incident flux) à careful monitoring of changes needed for calibra?on Typical : Ga- doped Ge bolometers Examples of bolometer arrays: SCUBA and SCUBA2 (80x80) at the 15- m James Clark Maxwell Telescope (JCMT) SPIRE on Herschel Space Observatory (30x30) (250, 350, 500 µm) ALMA

~273K blackbody (ground- based telescope) ~275K blackbody (cryogenically cooled space- based telescope) ~5800K blackbody

Kuiper Airborne Observatory (KAO) 0.91m telescope 1974-1995 Discovered rings of Uranus (1977), atmosphere of Pluto (1988)

Stratospheric Observatory for Infrared Astronomy (SOFIA): 2.7m FLITECAM 1-5 µm FORCAST 5-40 µm HAWK 42-210 µm

NOTCam

Sensi?vity evolu?on Wavelength coverage of 39m E- ELT: Most instruments will cover NIR region

Sensi?vity evolu?on Field of view