Dynamic Range. Can I look at bright and faint things at the same time?

Transcription

1 Detector Basics The purpose of any detector is to record the light collected by the telescope. All detectors transform the incident radiation into a some other form to create a permanent record, such as particles (photographic plates), molecules (eyes), or electrons (CCDs). There are eight important properties by which to gauge the utility of a detector: 1. Quantum Efficiency and Spectral Response 2. Temporal Response and Resolution 3. Dynamic Range 4. Linearity and Stability 5. Noise 6. Spatial Resolution and Field of View 7. Ease of Conversion to Digital Signal 8. Spectral Resolution

2 Dynamic Range Can I look at bright and faint things at the same time? Dynamic range = largest possible signal / smallest possible signal The dynamic range is relevant because it tells you for a single image the range of object brightnesses you can observe. A related quantity for digital detectors is the full well depth (capacity), which tells you how many detections your detector can record before saturation. Concrete example: CCD # of detections is an integer (so min=1) Assume full well depth = 2 16 (65,536) counts Dynamic range = 65,536/1 = In magnitudes, this is mag of dynamic range.

3 Linearity 2+2=3? Ideally, you want the response of your detector to be linearly proportional to the number of photons (1 photon 1 detection). Human eye and photographic plates highly nonlinear CCDs and most other modern detectors are nearly linear In some cases, must still apply a linearity correction. Stability Will it count the same tomorrow as today? How reliable is your photometry? Is the efficiency of your detector stable in time? Sensitivity of photographic plates degrades with time, especially in high humidity. Sensitivity of detectors on satellites can degrade with time due to hard radiation and cosmic rays.

4 Linearity Comparison for Photographic Plate and CCD CCD Photographic plate (Note that the scale is logarithmic) Figures courtesy John P Oliver,

5 Noise It is a sad fact of life that detectors introduce additional noise into observations Types of noise Poisson noise (aka shot noise): Goes as N 1/2, where N is the number of photons. For an ideal detector this is the only source of noise. Two components that contribute to the Poisson noise are (1) source photons, and (2) background sky photons. Read noise (RN): Some detectors, like CCDs, generate additional noise when the signal is read out Thermal noise (dark noise): thermal agitation of electrons in detector. Noise in electronics: self-explanatory Noise components add in quadrature, so Total noise = [N + RN 2 + dark noise 2 + electronic noise 2 ] 1/2 For research instruments, Poisson and read noise are usually the dominant sources of noise.

6 Angular Resolution and Field of View Angular resolution Recall that the image scale (in arcsec/mm) is determined by the design of the telescope Two pixels are required to resolve an object. Consequently, for a detector the angular resolution = image scale 2 pixel size Want the resolution to be well-matched to the telescope. If the detector resolution is worse than the seeing, you re sacrificing performance. If the detector resolution is much better than the seeing, you re sacrificing field of view. Field of view FOV = image scale N pix pixel size

7 Angular Resolution and Field of View In which cases is the detector well matched to the PSF? Nyquist sampling Want at least 2 pixels within FWHM for the PSF to be well sampled

8 Ease of Conversion to Digital Signal For quantitative analysis, you want to convert your data into a digital format. Eye: out of luck (for now) Photographic plates: densitometer CCDs and other modern detectors: direct digital output Spectral Resolution Spectral resolution is a moot point for current optical and infrared detectors, but at some wavelengths (particularly high energies like X-rays and Gamma rays) the detectors return energy information for the individual photons. In these cases it is relevant to quantify the precision of this energy information.

9 Photomultiplier tubes The ideal photoemissive detector? N 1 = fht N out = (dynode gain) m N 1 Photoemissive cathode anode V sig DC voltage Only e (typically ~ 20%) limits performance

10 Semiconductors conduction band conduction band conduction band E c E c E G E v E G valence band E v valence band valence band Insulator Semiconductor Metal

11 Semiconductors Semiconductor Eg (ev) c IV Si IV Ge III-V AlAs III-V AlP III-V AlSb III-V GaAs III-V GaP III-V GaSb III-V InAs III-V InP III-V InSb II-VI CdS II-VI CdSe II-VI CdTe II-VI ZnSe II-VI ZnTe IV-VI PbS IV-VI PbSe IV-VI PbTe Cutoff wavelength: c For E g in ev c = 1.24/E g

12 CCD pixel +V Metal electrode photon SiO Depletion region P- doped silicon

13 Charge transfer f1 f2 f3 t 0 t 1 t 2 Direction of charge transfer

14 The 4Kx4K CCD in the TOU spectrograph for the Dharma Planet Sutvey

15 Near IR Arrays: HgCdTe IR photodiode bonded to electronic multiplexer Indium Bump ZnS n-type implant Ti/Au/Ni Si 3 N 4 Hg (1-x) Cd x Te p-type Sapphire CdTe Indium bumps grown on each pixel of array and readout IR detector array grown on a separate substrate Two arrays are carefully aligned and pressed together indium acts as electrical connection between detector material and readout Epoxy fill to support detector material

16 Near IR Arrays Sensitivity from 0.5 to 5 m Photodiode detector elements each pixel must have Photodiode Reset switches Output amplifier Detector and readout arrays fabricated separately Bump bonded to multiplexer readouts Capability to non-destructively read pixels

17 Quantum Efficiency Hawaii 2 QE Wavelength ( m)

18 HgCdTe II Arrays Dector Material Hawaii 2 HgCdTe Operating temperature 78 Array Geometry Pixel Pitch 18.5 m Fill factor >90 % Number of pixels 2048x2048 Size 40 x 40 mm Architecture Quadrants X 1024 pixel Readouts 4 1 per quadrant Pixel 0.5 VDC e - o K Readout Noise <10 e - rms Readout Noise (Fowler Sampling) <3 e - rms Dark 77 o K 0.03 e - sec -1 Quantum Efficiency 65 % Wavelength Range m Frame Rate 0.2 Hz Bias -0.5 VDC Response Uniformity 2 % Crosstalk <10 %

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20 ALADDIN InSb Arrays ALADDIN Dector Material InSb Operating temperature 35 o K Array Geometry Pixel Pitch 27 m Fill factor 100 % Number of pixels 1024x1024 Size mm Architecture Quadrants X 512 pixel Readouts 32 8 per quadrant Pixel 0.4 VDC e - Readout Noise <25 e - rms Dark 35 o K 0.1 e - sec -1 Quantum Efficiency >80 % Wavelength Range m Frame Rate 20 Hz Bias 0.4 VDC Response Uniformity 8 % Crosstalk 5 %

21 Quantum Efficiency ALADDIN Quantum Efficiency Wavelength ( m)