Electronic Imaging Devices

ELECTRONIC IMAGING DEVICES Roberto Bartali

The only way we can understand the Universe is capturing and measuring the light that emit stars, galaxies, gas and dust. Physical and chemical phenomena occurring at different temperature, pressure and density, emit photons at some specific wavelength, covering the full electromagnetic spectrum. Measuring the light emitted by those phenomena, require the development of detectors based on materials sensitive to each wavelength, instruments capable to focus that light and specific computer processing routines.

PHOTOMULTIPLIER TUBE

CHARGE COUPLED DEVICE (CCD) Semiconductor sensor based on the photoelectric effect, it absorb photons and release electrons which are stored in each pixel well. The number of photo-electrons generated depend on the exposure time, sensitivity, wavelength and quantum efficiency of the sensor. Resolution depend on the pixel size and the telescope focal length. Well content is transferred up-down in parallel form and left-right in serial form, toward the output amplifier and the output pin.

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Red=electrodes; Cyan=isolating oxide layer; Blue=charge storage well; Green=sensitive area; Black arrows=photons;

Pixel structure FRONT ILLUMINATED

BACK ILLUMINATED

Vacuum tube with one electrode (cathode) sensitive to light that release electrons because of the photoelectric effect. Focusing electrodes, direct photo-electrons toward a series of energized electrodes (dynodes). When accelerated electrons strike dynode surface, more electrons are released and directed toward the next dynode. The net effect is a multiplication of electrons at each stage. The electron avalanche, eventually reach the last dynode (anode) where is available to external measuring and processing circuit.

In this work, we will describe main characteristics of Charge Coupled Device and Photomultiplier Tube technologies developed to capture visible, ultraviolet and infrared light. Their performance, strongly depend on the working temperature, so a section is devoted to cooling systems technologies. Obtaining an image, as close as possible to the celestial object of interest, is not as simple as point the telescope to the object, open the camera shutter, wait for exposure time ends and send data to a computer display, but require a series of calibration images and processing software. A section is devoted to the detailed description of calibration frames and how to avoid problems during imaging capture and CCD operation.

MICROCHANNEL PLATE SiPMT

Semiconductor version of the vacuum tube photomultiplier. Incoming photons strike sensitive area and release photo-electrons (photoelectric effect). Electric fields direct them toward a thin channel carved into an isolating substrate, where they are multiplied. A shifting process move the content of each channel to the output pin. Structure of a SiPMT

Vacuum PMT

spectral response versus quantum efficiency of front and back illuminated CCD Spectral response versus quantum efficiency of vacuum tube PMT and SiPMT

CCD mosaic Structure of a PMT

COOLING SYSTEMS Image sensors, for astronomical observations, must be cooled to avoid thermally produced dark currents; especially when exposure times are long or during the detection of infrared light. THERMOELECTRIC COOLERS PELTIER MODULES

Peltier module Semiconductor device based on the Peltier effect. A variable electric current passing through a junction of different metals, produce a variable temperature. One plate became hot and the other cool. Sensor is placed in contact to the cool plate. Maximum temperature difference 80ºC.

LIQUID NITROGEN COOLERS

Liquid Nitrogen system assembly Liquid Nitrogen gas maintain the sensor at -110ºC to avoid formation of thermal dark current. Useful for IR observations.

vacuum

Schematic of a liquid Nitrogen cooler

Schematic of a Peltier module assembly

Size comparison Vacuum tube PMT versus SiPMT

CONVERTING PHOTONS TO A SCIENCE IMAGE

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RAW IMAGE, of galaxy M51, contains photons captured by the telescope and noise generated by the imaging system.

Corrected raw image Dark frame shows thermally generated electrons. Exposure length equal to raw exposure.

Flat frame shows difference in pixel sensitivity, dust on optic surfaces, fringes and vignetting.

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Bias frame shows internal noise, due to electronics, and defective pixels. Exposure time = 0.. Corrected flat frame

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CALIBRTION FRAMES

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SCIENCE IMAGE, of galaxy M51, corrected for internal and external errors and noise, ready for further image processing. (Raw – dark – bias) / (flat – dark – bias)

SUMMARY OF LIGHT DETECTORS FOR ASTRONOMICAL IMAGING IN THE UV, VISIBLE AND IR WAVELENGHTS ELECTROMAGNETIC SPECTRUM

Galaxy M82 in UV light

Galaxy M82 in visible light

SPECTRAL RESPONSE QUANTUM EFFICIENCY

PIXEL SIZE

PIXEL SIZE

PICTURE

PIXEL MATRIX

10 TO 100 MICRON

PIXEL MATRIX

DETECTOR

N/A

20 micron

10 to 24 micron

1K X1K

CONCLUSIONS Our knowledge of the Universe is strictly related to photon detectors for every band of the electromagnetic spectrum. When Galileo point his telescope to the heavens, until the application of photography to Astronomy, the Universe, suddenly increase its dimension, observations were subjective, depending on the eye of the observer, no matter of the telescope dimension. Astronomy, is today, inconceivable without electronics, we are able to take pictures of the Universe in different wavelengths thank to the precise understanding and application of the photoelectric effect on semiconductors materials. Very large scale integration technologies help to the development of larger sensors with many millions of pixels, incrementing the resolution and the field of view of astronomical images, taking the full advantage of the optics of modern large telescopes. Even when actual state of the art technology give us a near perfect sensor, many features will be improved in the future: flatness of spectral response, greater quantum efficiency, fast read out speed, low noise, large pixel count and selective read out, are characteristics that surely we will see in the next generation of photon detectors.

2K x 2K

4 to 24 micron

Si PIN CCD

10K x 10K

10 to 24 micron

BACK ILLUMINATE D CCD

2K x 2K

4 to 24 micron

HgCdTe CCD

8K x 8K

10 to 24 micron

FRONT ILLUMINATE D CCD

2K x 2K

variable

InSb CCD

HUMAN EYE

10K x 10K (rods) 6.3K x 6.3K (cones for each colour)

10 to 24 micron

SPECTRAL RESPONSE QUANTUM EFFICIENCY

PIXEL SIZE

PICTURE

2K x 2K

PICTURE

PIXEL MATRIX

DETECTOR

Si:As IBC CCD

DETECTOR

SiC CCD

MICRO CHANNEL DETECTOR

SPECTRAL RESPONSE QUANTUM EFFICIENCY

Galaxy M82 in IR light