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

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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!