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Observational Astrophysics I

Detectors and Calibrations

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CCD Monolithic CMOS Hybrid CMOS

Silicon - Visible through near IR

HgCdTe Visible through IR

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CMOS detectors

n  Light-sensitive part is made out of silicon.

n  Each pixel is equipped with own amplifier and (occasionally) ADC.

n  CMOS multiplexor allows non-destructive readout, partial readout etc.

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Complementary Metal Oxide Semiconductor

6 steps of optical / IR photon detection

Sen

sitv

ity

6. Digitization

1. Light into detector Anti-reflection coating

Substrate removal

2. Charge Generation Detector Materials

Si, HgCdTe, InSb, Si:As

Quantum Efficiency

3. Charge Collection Electric Fields in detector collect electrical charge

Point Spread Function

4. Charge-to- Voltage Conversion

5. Signal Transfer

CMOS 4. Charge Transfer

5. Charge-to- Voltage Conversion

CCD

Converting photoelectrons to numbers

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Courtesy Jim Beletic, Teledyne

•  CCDs are more homogeneous but have slow readout

•  CMOS are much faster but ADC conversion is inferior

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Non-destructive readout

n  We can measure accumulated charges in each pixel without dumping the charges

n  This can be done several times before the dark current of detector catches up with the shot noise of the signal

n  Instead of using each individual frame we measure how charges grow (linear regression)

n  Typically we can make 16-64 readout before the array must be reset Dark current

Readout & Shot noise

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NIR detectors

n  NIR detectors are hybrid CMOS devices.

n  Special non-silicon layer is used to generate photoelectrons: HgCdTe (Hawaii) and InSb (Indium Antimonide, “insbe”, Aladdin) are sensitive between 0.9 and 25 microns.

n  Silicon electronics is well developed, therefore amplifiers and multiplexor use silicon.

n  Working temperatures: 1-60 K, connection problem.

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Hybrid Imager Architecture

Mature interconnect technique: •  Over 16,000,000 indium

bumps per Sensor Chip Assembly (SCA) demonstrated

•  >99.9% interconnect yield

H4RG-10 4096x4096 pixels

10 micron pixel pitch HyViSI silicon PIN

Teledyne Imaging Sensors

Photo courtesy of Raytheon Vision Systems

Human hair

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Thermal infrared detectors Raw frame Calibrations applied

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Thermal IR

n  HgCdTe (“mercad”) arrays depending on the exact structure are sensitive in 1-25µm range.

n  Detector needs to be cooled down to 5 K

n  Main problem is thermal emission:

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Fighting thermal background

n  Cooling the whole instrument

n  Taking short exposures n  Chopping

and nodding the telescope

n  Non-destructive readout 19-03-19

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PCD

n  Photomultiplier

n  Multi-anode microchannel array (MAMA)

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PCD properties n  Noise sources: shot noise and dark current n  No readout noise (since there is no ADC) n  Cosmic rays are minor concern – detector of

choice for many space missions n  Limited dynamic range (why?) n  Linearity problem n  Can easily be tuned to any spectral range, no

need for thinning or other risky operations n  Maximum QE is about 50% (why?) n  MAMA allows reading 2D frames

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Comparison CCDs C  Large dynamic range C  Large QE C  Extremely linear C  Large sizes (4k×4k) D  Sensitivity drops sharply in

the blue and the red D  Readout noise D  Cosmic rays D  Cooling

PCD C  Digital output in real time C  No readout noise C  Insensitive to cosmic rays C  No need for deep cooling C  Much easier to make and

therefore much cheaper D  Small dynamic range D  Small QE D  High voltages

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Calibrations

n  Goals: convert data from detector coordinates to physical coordinates and remove detector signatures as much as possible

n  Bias: 0 second exposure(s) with shutter closed Estimate of electronic signal offset for log amplifier

n  Darks: variable length exposures with shutter closed Estimate of dark current rate

n  Flat fields: short exposures with homogenous illumination and open shutter Estimate of relative pixel sensitivity

n  Calibrated source exposures Estimate of QE

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Calibrations should not add noise!

n  Take a sequence of bias frames (or dark frames) n  Combine them rejecting cosmic rays, replacing

cosmetic defects and increasing S/N ratio (master bias) n  Take a sequence of flat fields n  Combine them (master flat) and normalized the flat n  Subtract master bias from master flat and science

frames n  Divide science frames by master flat

Therefore we:

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CCD example: Bias

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Dark current vs. bias

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Flats in V and R bands

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Flat field can be taken through all the optical components

Fragment of a master flat field with HARPSpol

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Combining calibrations

I =s� b� d · tsf � b� d · tf

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Where: s – science exposure b – master bias frame d – dark current rate for 2D frame ts – exposure time for for science frame tf – exposure time for flat field frame If your exposures are short you might as well take several darks with exposures for science and flats and use them.