1. Detektor & CCD Astronomi AS4100 Astrofisika Pengamatan Prodi Astronomi 2007/2008 B. Dermawan

2. Position-insensitive Detectors (1) Do not record the location of an impinging photon Photomultiplier Tube (optical radiation) Proportional Counter (x-rays) Principles: Convert the incoming photon to an electron via the photoelectric effect Multiplication of electron number within the detector  sufficient charge

3. Position-insensitive Detectors (2) Photomultiplier Majewski

4. Position-insensitive Detectors (3) Basic Proportional Counter Bradt

5. Position-sensitive Detectors (1) Record the position of photons incident upon the detector Position-sensitive proportional counters Bradt

6. Position-sensitive Detectors (2) Charge-coupled device (CCD) Solid state device that can record the integrated intensity of light falling on it as a function of position on its surface Divided into rows and column of pixels. Ex: 2048 x 2048 pixels each of size 15  m x 15  m giving an overall size of 30 mm x 30 mm Imaging/photometry: placed in the focal plane of a telescope and a portion of the sky is focused onto it Spectroscopy: the light is dispersed into its component frequencies (colors) and then focused onto the CCD X-ray: modified to take into account the greater penetrating power of x-rays

7. Parameter Detektor (1) 1. Quantum Efficiency (QE): The fraction of incoming photons converted into signal 2. Spectral Response: Wavelength coverage Majewski

8. Parameter Detektor (2) 3. Noise Uncertainty in output signal. Ideally, only statistical fluctuations in the input signal. But not usually. Majewski

9. Parameter Detektor (3) 4. Linearity Degree to which output signal is proportional to incoming photon numbers Majewski

10. Parameter Detektor (4) 5. Dynamic Range Maximum variation in signal over which detector output can represent photon flux without losing signal 6. Pixels Individual, independently detecting elements in multiple detector system; "Picture Elements" Majewski

11. Parameter Detektor (5) 7. Time Response Minimum time interval over which changes in photon rate are detectable. E.g. photomultiplier Majewski

12. CCD Camera Control Malasan

13. Prinsip CCD Astronomi (1) Bradt Struktur & Exposure Cowan

14. Prinsip CCD Astronomi (2) Bradt Cowan Readout

15. Prinsip CCD Astronomi (3) Conveyor belt & Multiplexer Line Address Readout Shifting all columns by one pixel into multiplexer (MUX) Readout the full MUX pixels in order by shifting charges along MUX to amplifier Repeat again when MUX completely empty after transfer of entire row of charge Interline Transfer Frame Transfer Majewski

16. Charge Transfer Efficiency ( CTE ) Poor CTE  blurring Only slight transfer inefficiency is tolerable of > 0.99999 Why does it take so long to read out a CCD image? m  # transfer phases T  the slower of the two time constants (  si &  th ) t  duration that the gates are in each voltage phase state Majewski Problems: Fringing fields; Traps

17. CCD Output Amp., Gain, ADU An amplifier at the end of the MUX converts the electron packet net charge into a digital signal Gain ( G ) [more accurately: inverse gain] is the number of electrons combined to make one picture "count"  e - / ADU ; (Analog-to-Digital Units) The dynamic range of the image output is limited by the Analog to Digital Converter (ADC) which is capable of converting to a certain number of distinct digital "bits" Ex: 16 bit = 2 16 = 65536 distinct values Majewski

18. QE vs Wavelength Majewski Frontside & backside illuminations

19. CCD Spectral Response (Sensitivity) Majewski

20. Quantum Efficiency (QE) Peak QEs for CCDs are 40-80% Vary with wavelength Important source of variation: the ability of photons of different wavelengths to penetrate the silicon Flux at some depth of the silicon:  is the coefficient of intrinsic absorption Majewski

21. Sources of Noise Shot-Noise (Photon Counting): The statistical noise from Nature itself (can’t be removed) Read-Noise (Readout-Noise): Detector electronics subject to uncertainty in reading out the number of electrons in each pixel. Independent of the signal ( S ), regardless of exposure time F  the average photon flux, t  the time interval of the measurement n pix  the number of pixels RN  the readout noise per pix Majewski

22. Sources of Noise (Cont’d) Dark Current: Can be “removed” by subtracting image obtained without exposing CCD. Depend on length of exposure/integration D  dark current ( e - /pix/s ) Majewski

23. Sources of Noise (Cont’d) Sky Background: Scattered light from other sources Cosmic rays R sky  e - /pix/s from the sky Majewski

24. Noise & Signal-to-Noise Ratio ( S/N, SNR ) Crucial for efficient detection: NOT strength of signal but signal-to-noise ratio The higher the S/N, the more reliable the measure Majewski

25. Special Cases of Signal-to-Noise Poisson statistics: If detector has very low read noise, sky background is low, dark current is low  RN >>  Poisson   total   RN  Poisson >>  RN   total   Poisson Read-noise dominated: If there are lots of photons but read noise is high For multiple images: SNR ~ Majewski

26. Special Cases of Signal-to-Noise (Cont’d) Flux from the sky (Poissonian) contributes to every pixel (scattered moonlight, unresolved starlight, reflected/scattered sunlight, auroral emission, light pollution) Sky-limited It can reach the sky-limited regime if the sky level yields a number of counts N ( ADU ) per sky pixel (given in ADUs ) when  Poisson >>  RN   total   Poisson Majewski

27. Note on Thermal Noise Astronomical exposures tend to be long (a few second to many minutes) and many thermally induced electrons appear in that time There is no way to distinguish these from the photo- electrons which we wish to measure Solution: to cool enough the CCD so that thermal noise isn’t a problem Professional CCD systems are in evacuated chambers and cooled to ~  100  C; amateur CCDs barely manage ~  30  C Majewski

28. Binning Combine signals from adjacent pixels before they get to the readout amplifier Combining sets of electron packets from more than one adjacent physical pixel to create one image pixel Rieke Majewski Binning# pixelsRemarks 1 x 11530 x 1020No binning 2 x 2765 x 510Final image size 3 x 3510 x 340Final image size 2 x 1Not availableSpectroscopy

29. Binning (Cont’d) Mechanism Majewski

30. Binning (Cont’d) Reduces the effects of readout noise 4 pixels, no binning 4 pixels, 2 x 2 binning A faster total chip readout Binning# ReadoutRead Time 1 x 11.56 x 10 6 52 s 2 x 23.90 x 10 5 13 s 3 x 31.73 x 10 5 6 s Majewski

31. When bin? Faint or low surface brightness objects where you are starved for photons When you are taking short integrations and the sky flux will contribute very little to the "blank sky" pixels When CCD readout speed is needed When loss of resolution is not important Majewski 3x3 pixels about 0.6" x 0.6" But seeing typically > 1.5" Always want to Nyquist sample ( r = FWHM / p > ~2) For a star with seeing width 1.5" need pixels smaller than 0.75" For r less than about 1.5, the data are considered undersampled

32. Undersampling Well sampled image Undersampled image Howell

33. Other Speed Configuration Largest CCDs now is 4096 x 4096 pix At 30 kHz, it would take 560 sec = 9 minutes to read! Majewski Quad-Amp Readout: 4x Faster Latest electronics approaching > 100 kHz Non-destructive readout. Send same charge packet to amplifier M times - readout noise reduced to  /  M

34. Time Delay Integration (TDI) or Drift Scanning Read the chip along columns at exactly the rate the CCD camera sweeps past a fixed scene to build a long strip image Turn off clock drive on telescope and have CCD / scope move with Earth past stellar scene; clock CCD at sidereal rate Majewski Sloan Digital Sky Survey (SDSS)

35. Orthogonal Transfer Arrays (OTA) Traditional CCDs are designed to move charge in one dimension (from row to row along columns to the MUX) However, a new CCD structure has been designed that allows motion of charge packets in TWO dimensions  OTA Majewski To move charge left-right, electrode 3 is held negatively biased (to act as a repelling channel stop) and electrodes 1,2,4 are operated like a normal 3- phase CCD. To move charge up-down, electrode 4 negative, and using electrodes 1,2,3 as a three-phase CCD On-chip tip-tilt compensation! Localized tip-tilt compensation! Being used for billion pixel (gigapixel), 40 cm X 40 cm cameras for the Pan- STARRS experiment Rieke

36. Large Imaging Camera A standard metric in common use these days is A , where A is the aperture area, and  is the area of the sky that can be imaged simultaneously Majewski Telescope/ImagerAperture (m)CCD field (deg 2 ) A  (m 2 deg 2 ) FMO 1-m/Gen I1.00.040.03 NOAO 4-m/Mosaic4.00.364.5 MMT/Megacam6.50.165.3 Sloan2.51.57.5 WIYN/ODI3.51.09.6 Pan-STARRS4 x 1.83.030.5 LSST/DMT (~2012)6.99.097.5 x  /  2

37. Perkembangan CCD Astronomi http://www.ucolick.org/~bolte/AY257/ay257_2.pdf Pan-STARRS SDSS Majewski

38. Notes on CCDs Bias Level Due to the readnoise problem, it is possible for a normal CCD pixel to have a (slightly) negative value Other effects can also yield negative output values for normal pixels To properly account for this problem, but get the substantial dynamic range benefit of 16 bit unsigned output, CCD electronics will add a pedestal level, called the bias level, to shift all pixel levels up into the positive range This "zero" level is typically a few hundred to ~1000 ADU Howell Majewski

39. Notes on CCDs Drifts in the Readout Amplifier and the Overscan During readout, reference voltage can drift, changes "bias" level -- results in varying "0-point“ Solution: Overscanning Majewski

40. Notes on CCDs Linearity Majewski For a given pixel, let: F be the incident flux (in photons per second) S be the recorded signal level (in ADU) Q be the quantum efficiency G be the gain t be the integration time (in seconds) Then, for a strictly linear system: In reality, Q is a function of the accumulated charge:

41. Notes on CCDs Blooming Majewski Depends very much on the electronic design of the CCD During readout, not all the charge can be shifted – some is left behind (streaks – blooming or blending-- forming behind saturated pixels This can be minimized somewhat by the inclusion of electronic “drains” in the CCD, called an Anti-Blooming Gate (ABG) However, also drains off wanted charge and so reduces the QE of the device

42. Notes on CCDs Dynamic Range The dynamic range of a CCD is limited by its maximum useful level (the full-well capacity ( FWC ) is an ultimate limit for a pixel) Adopting the definition use din acoustics, the dynamic range, D, of a CCD is given in decibels by: D (db) = 20 log 10 (maximum level / RN ) A CCD with 100,000 e - = FWC and RN = 10 e - has an 80 dB dynamic range Majewski

43. Notes on CCDs Cosmetic Defects Dead pixel - Pixel unresponsive to light due to defective gate, depletion zone, substrate, insulator, etc. Hot pixel - Pixel with much larger dark current than neighbors Bad column - A defective pixel where the defect affects CTE and all charge packets that pass through the defective pixel will be destroyed (e.g., fall into a trap) resulting in a bad column in the final image Majewski

44. Notes on CCDs Electroluminescence In some cases, diodes in the output amplifier can actually act as Light Emitting Diodes (LEDs) and can cause serious problems of excess light near the amplifier Majewski

45. Notes on CCDs Other "Defects" in CCD Images Not Related to Chip Itself Dust Majewski Interference Sky pollution “Cosmic pollution“ Fringing

46. Notes on CCDs Radiation Damage in Space CCDs The harsh radiation environment in space can temporarily or permanently degrade the performance (e.g., the CTE) of a CCD: Solar wind, solar flares and the general background of cosmic high energy particles in unprotected environment away from Earth The South Atlantic Anomaly (SAA) is the point on the Earth's surface where its inner van Allen belt comes closest http://www.astro.psu.edu/users/niel/astro485/lectures/ lecture09-overhead02.jpg srag-nt.jsc.nasa.gov/AboutSRAG/What/What.htm Majewski

47. Advantages of CCDs The increase in QE over film is like making a telescope into a much bigger one –effectively allowing a 1-m telescope to perform like a 4-m The accuracy of CCDs in both linearity and stability means the measurements made are of the highest quality, and a wider band of the spectrum is utilised The digital nature of CCDs allows new techniques to be devised, both in taking the data and extracting the most from it Majewski