Techniques of Observational Astronomy
AST3722C

Detector Basics

The Electromagnetic Spectrum

Radiometry vs. Photometry

• Radiometry: measurement of radiant electromagnetic radiation; typical units are joules (energy) and watts (power)
• Photometry: measurements of response of human eye to incident radiation; Photometric units: typical lumen (energy) and candela (power)

Black Body Radiation

Energy given off by an object solely by virtue of its temperature is called Thermal or Black Body Radiation.

Types of Detectors

• Thermal detectors
  respond to temperature rise due to absorption of radiant energy
  - thermocouple/thermopile; bolometer/thermister; pyroelectric device; Golay cell
• Quantum Detectors
  respond to incident photons
  - photoemissive; photoconductive; photovoltaic; photochemical

Important Characteristics of Astronomical Detectors

Quantum Efficiency

QE = # of detections/# of incident photons

Detections may be:

• Crystals formed in a photographic emulsion (few %)
• Photoelectrons released (5-40% in photomultiplier tubes, up to 90% in PIN diodes)
• Charge-pairs created (> ~ 50% in CCDs)

Linearity of Response

Want response linear with exposure which is the product of light flux and exposure time; R = F·t
•Failure of the reciprocal relation R = F·t is “reciprocity loss and is a characteristic of photographic emullsions
•Photographic emulsions are not linear (note pre-flashing, log-log scale).
•CCDs and Photomultiplier Tubes (PMTs) are linear over a large range.

Dynamic Range

We want to have a large dynamic range. The ratio of the largest measured value to the smallest value should be as large as possible.

Spectral Bandwidth

Noise

• Photon statistics limited noise (photon arrival rate, by Poisson statistics, goes as qrt(n), n=number of photons)
• Sky noise caused by sky transparency changes and/or seeing/scintillation
• Johnson noise - thermal agitation of electrons in detector;
  (dark noise) reduced by cooling detector
• Readout noise
• Electronics noise

Detective Quantum Efficiency

Cooling

Liquid nitrogen often used in dewars to cool PMTs and CCDs. (above left image)
•Achieves 77 kelvins, lowest possible temperatures without extreme efforts (Liquid helium, hydrogen)
•Dangers (explosions if vent becomes clogged)
•Requires keeping topped off as it evaporates.
•May be too cold for PMTs (photomultiplier tubes)

Dry Ice (frozen CO2)
•Frequently used in past for PMTs
•sublimation temperature 195K
•"Cold Box" needs large opening for refilling
•typically needs refill every 4 -5 hours

Thermoelectric cooling (the Peltier effect; above right image)
•DC current passed through a semiconductor device makes one side hot and the other cold.
•Can achieve ~35-100C below reservoir
•Reservoir may be room air or re-circulated coolant (with or without refrigeration).

Ability to Integrate

The ability to collect photons for an extended period of time is one of the advantages of most detectors over the human eye.

Spatial Resolution

Spatial resolution determines the detail that can be "resolved" in an image. It should be well matched to the telescope and instrument.

Digital Output

In order for calculations to be carried out, the data received or recorded by an astronomical detector must be made available as numbers.

• In the case of photographic plates, devices called "photometers" or "microphotometers" were used to pass a beam of light through the plate and the transmission of the plate gave a measurement of the average photographic density in the region sampled. This transmission value could be displayed by a meter or recorded on a "strip chart" recorder. The development of digital electronics lead to values being displayed as numbers (and recorded by an "Armstrong" recorder - that is, written down by hand) and later printed on paper. The development of digital computers finally allowed the data to be made ready for computer analysis by being punched on paper tape or cards or recorded on magnetic tape.
• The output from photomultipliers was handled in the same fashion as the measured transmission of photographic plates (indeed, such transmission measures were usually made with PMTs).
• Image tubes yielded nuclear emulsions which were analyzed in a fashion similar to photographic emulsions (though with problems associated with the very high densities frequently achieved).
• CCDs are generally directly interfaced with a control computer and the data are converted from packets of electrical charge to a digital value in the CCD controller electronics.

Detectors of Interest

The Human Eye

• Overall, acts as a thin positive lens
• focal length 14 mm (near vision; focus down to 10 cm when young, 40 cm in middle age) to 17 mm (distant vision)
• iris aperture 2 mm - 8 mm
• adaptation (dynamic range) 100,000:1
  - iris varies
  - shift from cones (photopic vision: daylight) to rods (scotopic vision: darkness; QE=3%)

Photographic Plates

Photochemistry of Silver Salts (ref. http://www.cheresources.com/photochem.pdf)

To understand the fundamental chemistry of silver-based photography, we must look at the photochemistry of silver salts. A typical photographic film contains tiny crystals of very slightly soluble silver halide salts such as silver bromide (AgBr) commonly referred to as “grains.” The grains are suspended in a gelatin matrix and the resulting gelatin dispersion, incorrectly (from a physical chemistry standpoint), but traditionally referred to as an “emulsion,” is melted and applied as a thin coating on a polymer base or, as in older
applications, on a glass plate. The figure hows a schematic representation of the silver halide process. When light or radiation of appropriate wavelength strikes one of the silver halide crystals, a series of reactions begins that produces a small amount of free silver in the grain. Initially, a free bromine atom is produced when the bromide ion absorbs the photon of light:

The free silver produced in the exposed silver halide grains constitutes what is referred to as the “latent image,” which is later amplified by the development process. The grains containing the free silver in the form of Agº are readily reduced by chemicals referred to as “developers” forming relatively large amounts of free silver; that deposit of free silver produces a dark area in that section of the film. The developer under the same conditions does not significantly affect the unexposed grains.

Photomultipliers and Photodiodes

The key element in photoelectric cells is the photocathode. A metal plate which has a low work function can emit "photoelectrons" in response to incident photons. The energy of the photon must exceed the work function for a photon to be emitted. If the photocathode is placed in a vacuum with an electrode (the anode) to collect emitted electrons, a circuit can be completed allowing measurement of the rate of arrival of the incident photons. The anode must have a positive potential relative to the cathode.

Image Intensifiers

Image intensifiers were an attempt to gain the advantages of a photocathode device but retain the two-dimensional imaging of phtographic emulsions.

One of the first such devices to be used productively in astronomical research was the Lallemond Image Intensifier.

A variation on the Lallemond tube was the Kron Electronic Camera. This device moved the nuclear emulsion beyond an "airlock" coin valve so that plates could be changed without breaking the vacuum and destroying the photocathode.

CCDs

A Charge-coupled Device (CCD) is a two-dimensional quantum detector that provides a digital image output. An array of "pixels" (picture elements) is laid out in rows and columns to capture the photons as charge. These charge filled pixels are then manipulated to move the charge to the output and into a control computer.

The above illustration is of an 800x800 pixel CCD made by Texas Instruments (TI) for the Hubble Space Telescope WFPC. The inset shows the output amplifier.

This page was last edited October 24, 2002 1:29 PM