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Kamera CCD Astronomi (1) AS3100 Lab. Astronomi Dasar I Prodi Astronomi 2007/2008 B. Dermawan.

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Presentation on theme: "Kamera CCD Astronomi (1) AS3100 Lab. Astronomi Dasar I Prodi Astronomi 2007/2008 B. Dermawan."— Presentation transcript:

1 Kamera CCD Astronomi (1) AS3100 Lab. Astronomi Dasar I Prodi Astronomi 2007/2008 B. Dermawan

2 Prinsip CCD Astronomi (1) Bradt Struktur & Exposure Cowan

3 Prinsip CCD Astronomi (2) Bradt Cowan Readout

4 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

5 Charge Transfer Efficiency ( CTE ) Poor CTE  blurring Only slight transfer inefficiency is tolerable; of > 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

6 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 = distinct values Majewski

7 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

8 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

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

10 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

11 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

12 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

13 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

14 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 x 1020No binning 2 x 2765 x 510Final image size 3 x 3510 x 340Final image size 2 x 1Not availableSpectroscopy

15 Binning (Cont’d) Mechanism Majewski

16 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 x s 2 x x s 3 x x s Majewski

17 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

18 Undersampling Well sampled image Undersampled image Howell

19 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

20 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)

21 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

22 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 I NOAO 4-m/Mosaic MMT/Megacam Sloan WIYN/ODI Pan-STARRS4 x LSST/DMT (~2012) x  /  2

23 Perkembangan CCD Astronomi Pan-STARRS SDSS Majewski


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