AST 443 / PHY 517. Photon Detectors

Transcription

1 AST 443 / PHY 517 Photon Detectors

2 Photons Light is electro- magne>c radia>on Crossed electric and magne>c vectors Self- propaga>ng Travels at speed of light c c= x 10 8 m/s (vacuum) λν = c n=c/v light

3 Photons Light has characteris>cs of both waves and par>cles At long wavelengths (sub- mm, radio), the wave characteris>cs dominate. At wavelengths up to and including the mid- IR (10s of μms), light is best thought of a discrete photons a self- bounded packet of energy. E=hν=hc/λ h=6.62 x erg s (Planck s constant)

4 Units Energy 1 erg = 10-7 J 1 ev = 1.60 x erg 1 kev = 10 3 ev =1.60 x 10-9 erg Energy Density Jy = J / (m 2 s Hz) = erg / (cm 2 s Hz) Jy = 3.34 x 10 4 λ 2 F λ erg/(cm 2 s A) Jy = 6.34 x 10-4 λ f λ photons/(cm 2 s A)

5 Basic Detec>on Schemes Antenna: radia>on makes electrons oscillate Appropriate at radio wavelengths Generally analog devices Bolometer: detect absorbed energy Can be either analog or digital Photon- counter: detect individual photons Requires high >me resolu>on, or low photon rates Generally digital

6 Electromagne>c Radia>on

7 Electromagne>c Domains Gamma- ray: E>100 kev. hard to focus. X- ray: defined by detector technology. Originally 2-6 kev. Hard X- rays: >10 kev (<1 A) Sol X- rays: <0.25 kev (<44 A) Extreme UltraViolet: between sol X- rays and ~ 912 A. Needs window- less detectors UltraViolet: 912 A to atmospheric cutoff (3200 A) Op;cal: 3200 A about 1 micron (10,000 A). (Roy G Biv) IR: >8000 A to ~100 microns near- IR: <5μm mid- IR: ~5-50μm Sub- mm/mm: ~100 microns 1 cm Radio: λ > 1 cm ν<30 GHz

8 Photon Detectors Considera;ons in a detector Quantum Efficiency (QE): What frac>on of the incident photons are detected? Noise: Does the detector add photons or electrons to the signal? Time Resolu;on: How fast can the detector count? Energy Resolu;on: How well can you determine the total energy? Pixel size (spa;al resolu;on): How well can you localize the posi>on? Dynamic Range: what is the brightest and faintest signal that can be detected in the same exposure? Response/bandpass: set by product of QE, filter response, telescope throughput Size of field: How many pixels?

9 The Eye

10 The Eye as a Simple Lens

11 The Eye - a simple, versa>le detector with a complex data processor A digital detector. Does not integrate. Time resolu>on ~ 13 msec. Lens focuses on curved re>na Dynamic Range: ~ ~10 8 rods provide high sensi>vity at low light levels (QE~1). Rods insensi>ve to red light. Entrance pupil expandable to about 5 mm. ~6 million cones concentrated at field center provide color sensi>vity Peak sensi>vity ~ 5500 A (QE~0.01). Faint limit: typically V~6 (~300 photons in 0.1 seconds) Rayleigh limit:~ 20 arcsec (but actual resolu>on about 1.2' due to density of re>nal cells; 5-10' off- axis due to aberra>ons.) Opera;on: Photon energy deposi>on alters structure of Rhodopsin, which changes the permeability of the cell membrane and sends a signal to the brain.

12 Photography Invented about 1840 Large format stable detectors (glass substrate) Good storage medium Low QE, <1-2% Long integra>ons possible non- linear response (the characteris>c curve) Limited dynamic range Sensi>vity/spa>al resolu>on tradeoff

13 Photography: the Characteris>c Curve

14 The Photographic Process Ag halide grains suspended in gel Ag halide absorbs photon - > e -, hole pair e - moves from valence band to conduc>on band, and neutralizes Ag + ion Grains of ~100 Ag atoms form latent image. Developer converts Ag halides to Ag, but is catalyzed by neutral Ag (the latent image). Latent image is amplified ~ 10 9 >mes. Fixer removes remaining Ag halide

15 Photographic Plate Collec>ons Plates were the primary astronomical detector un>l the 1980s Harvard plate stacks Over 525,000 glass plates Taken Par>ally digi>zed Strasbourg plate stacks Many observatories have small collec>ons

16 Bolometers Resis>vity in a metal is a func>on of temperature Place conductor inside a heat bucket Measure change in resis>vity First use: 1880

17 Bolometers Examples: ACRIM: measure solar constant DIRBE Herschel SPIRE Advantages: Simple High QE

18 Photomul>pliers Uses the Photoelectric effect Photon ejects electron from photo- emissive surface Electron is amplified >mes Detec>on of current - > detec>on of single photon.

19 Photomul>plier Tubes Good >me resolu>on (micro- seconds) No spa>al resolu>on (make an array!) Generally no energy resolu>on No dynamic range Short wavelength cutoff set by window/filter Long wavelength limit set by work func>on NaCl: 150 nm CsSb: 700 nm GaAs: 1000 nm

20 Semi- conductor Detectors Photon can: eject electron from metal, or from valence band to conduc>on band

21 Photon- coun>ng Imaging Systems A photomul>plier using photo- emissive surfaces Read as a TV in reverse

22 Charge Coupled Devices Charge Coupled Devices, or CCDs, are the workhorse of op>cal astronomical instrumenta>on. For many purposes they are as close to an ideal detector as we can get. CCDs feature: High QE low instrumental noise (read noise) stable response good fla{enability large dynamic range (generally > the 16 bit digi>za>on) linear response small pixels (typically microns) large format (up to pixels) can be >led

23 CCDs Conceptually

24 CCD Clocking

25 CCD Limita>ons Small formats Slow readouts Faster readout - > more read noise Generally not photon- coun>ng in op>cal Imperfect Charge Transfer Efficiency (CTE) Deferred charge /traps/ blooming Digi>za>on limits Non- uniform responses

26 Spa>al Resolu>on CCD pixel size limited by manufacturing process Typically microns Plate scale = 1/f l Pixel scale = size/f l For HST, effec>ve f l ~ 57.6m; D=2.4m Arcsec/pix = 24 μm/57.6 x 10 6 μm x ~ 0.09 / pix HST resolu>on: R = 1.22 λ/d = 0.06 at 5500A

27 CCD Quantum Efficiency