Introduction to Optical Detectors: Plates, PMTs and CCDs Matt A. Wood Florida Institute of...
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Transcript of Introduction to Optical Detectors: Plates, PMTs and CCDs Matt A. Wood Florida Institute of...
Overview● Photographic Plates● The Photoelectric Effect● Photomultiplier Tube Basics● CCD Basics: Structure and Operation● Quantum Efficiency● Binning ● System Gain● Noise Sources● Optimal Data and Calibration Images● Data Reduction Basics
Photographic Plates
● Used historically● Wide FOV, high resolution● But terrible Quantum Efficiency (QE ~ 1%)
– QE = (#photons detected) / (#photons incident) x 100
– QE = 100% means you count every photon that hits detector.
● Non linear behavior, so difficult to get good magnitudes using plates
● Photographic plates no longer used at major observatories
The Photoelectric Effect• Photon with energy greater than the work
function of the material can free an electron
h = W + KEmax
– No emission for frequencies below c = W/h
– Current proportional to light intensity above c
– Current proportional to frequency above c
– Einstein Nobel work, established photon nature of light
• In a Photomultiplier Tube (PMT), photon ejects electron, starts cascade (see diagram later)
• In a Charge Coupled Device (CCD) photon creates electron-hole pair. Electrons attracted to buried electrode
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Photomultiplier Tube Basics
● “Electron multiplier phototube” = “photomultiplier tube”
● Basic need: single electrons released via photoelectric effect can’t be measured, so stack a series of plates, and let electrons cascade– Photon releases electron at cathode– A dynode is placed close, and with a potential
difference of ~100V. When electron strikes the dynode, 2-3 electrons are released.
– Stack several dynodes, and finally detect pulse of ~106 electrons at the Anode
PMT Applications
• Historically, preferred to photographic plates for measuring magnitudes
• High time-resolution astronomy (still beats CCDs in this area)
• Describe 3-star photometer …
Sources of Noise
● Dark Current / Thermal noise● Random pulse sizes● Cosmic Rays● Magnetic Fields (use -metal)
● Also, aging from vacuum leakage, bright illumination
● Keep in light-tight enclosure
Photoelectric Effect in Semiconductors
• Atomic Energy levels perturbed if nearby atoms
• Split to 2 levels if 2 atoms
• N levels if N atoms -> BAND structure
Photoelectric Effect in Semiconductors
• N levels if N atoms -> BAND structure
• In metal, outermost electrons are in the valence band – free to conduct charge
• If valence level filled – insulator• Require vacant sublevels, “dope”
with impurities to make semiconductor– n-type: current carried by e-
– p-type: current carried by “holes”
Photoelectric Effect in Semiconductors
• In semiconductor, there is only a small gap between valence and conduction band
• Thermal motions, photon absorption can excite e- to conduction band.
• Once in conduction band, the e- can move through the semiconductor, for example towards an electrode with +charge
CCD Basics
● Physical Structure● Transferring Charges
● Binning
(Figures from Apogee ccd.com website)
CCD Basics: Single Pixel
• Basic Structure of a single pixel
• Electrode insulated from semiconductor via thin oxide layer.
• +Voltage attracts e-, repels holes
• In effect a radiation-driven capacitor
Figures from Astrophysical Techniques by Kitchin
Basic Structure: Array
• Array of pixels with insulators between (high p-type doping)
• Each develops charge proportional to illumination intensity
• Now just need to read it out
Multiple Electrodes & Charge Transfer
• If charge is under B, and all A-D are at +10V, then charge will diffuse to be equal under all 4
• If A and C kept at +2V, though, then charge remains under B
• This allows us to transfer charge
• Charge transfer efficiency 99.999%
2-Phase CCDs Are Most Common
Front Illuminated vs. Back Illuminated
Noise:Dark currentSensitivity Variations (pixel-to-pixel)Cosmic Rays
Bias counts from electronspulled to electrodes even ina zero-second exposure
Requires only a single clock, but requires buried electrodes
Quantum Efficiency● QE = (#photons detected) / (# photons Incident)
The closer to 100% the better!
Detector absolutely needs to be linear for you to do photometry
Note: Different sensitivitiesat different wavelengths, so must calibrate through eachfilter. Also, must integrate longer for same S/N in regionswith lower QE.
Sources of Noise
• Readout Noise: Imperfect repeatability when charge read through A/D converter, and other unwanted counts from electronics
• Dark current: Thermal motions of atoms bump electrons into valence band – lower temp for lower dark current
More Noise
● Shot noise: if Poisson statistics (which photon arrival times obey):
Signal proportional to counts S C
Noise proportional to sqrt(counts) N C
So S/N ~ C
So to get S/N of 100:1 -> need 10,000 photons
(I usually aim for 10k ADUs for sky flats, target star, etc., if possible)
Practical Aspects● Bias Frames: Take early evening, and/or at end of
night. I take 30 bias frames and median filter to produce a Master Bias.
● Dark Frames: CCD temp must be same as data frames. Exposure must be at least as long as longest data frame – longer is ok. (e.g., 20 5-min exposures). Bias subtract and Median Filter to remove cosmic rays -> Master Dark
● Sky Flats: Take as many as possible, but at least 3/filter. Shoot for 10k-20k ADU per pixel (definitely <30k/pixel). If using filters, go UBVRI – do less sensitive wavelengths first when sky is brighter. Either Tel drive off, or dither b/n frames. Bias subtract, Dark Correct, then Median filter weighted by counts for Master Flats. Note these are normalized to unity.
Data Reduction Steps● Download raw frames● Make master Bias frame (IRAF:
zerocorrection)● Apply Bias correction to dark frames and
make master Dark (IRAF: darkcorrection)● Apply Bias and Dark corrections to sky flats
and make master Flat(s) (IRAF: flatcorrection)● Apply Bias/Dark/Flat corrections to data
frames (IRAF: ccdproc)● That's it – now you're ready to extract
photometry or do other image analysis!