21 Photometry-II

Naseer Iqbal

Learning Outcomes

After studying this module, you shall be able to study:

Different types of Detectors which are used in astronomy for photometry.
Basic idea about how Photographic emulsions and photomultiplier tubes are used as detectors in astronomy.
A detailed description about the development of CCDs as Detectors.
Construction and Mode of operation of CCDs as detectors.

1. Introduction

Astronomy is an exciting field to study. The marvelous pictures of celestial objects taken by using large ground/space telescopes are the most visible manifestation of modern research astronomy. Pictures are needed as a first step in classifying objects based on their appearance (morphology). However, the transition from observational astronomy to science requires the quantitative information about the celestial objects. The quantitative information is the measurements of the properties of the objects such as: How far away is that object? How much energy does it emit and How much hot it is?

The amount of energy, in the form of electromagnetic radiation that we receive from celestial object is called as flux. The science of measuring the flux we receive from celestial objects over broad wavelength bands of radiation is called Photometry. Measurement of flux, when combined with some estimate of the distance to an object, can give us information on the total energy output of the object (its luminosity), the object’s temperature, and the object’s size and other physical properties.

Photometry is the measurement of electromagnetic radiation received from the celestial objects. This response changes with wavelength, and to an extent, from person to person. In photometry, the word ‘luminous’ is used to indicate that measurements have been made using a detection system (called a photometer) that has a spectral response similar to that of a human eye. Telescopes simply collect photons. To make useful observations it requires some sort of gizmo to detect those photons and make a measurement or record of them. The natural detector of visible photons is the retina of the human eye. However, eye is not a very good photometric device in the sense of being able to make quantitative measurements of flux. Also, the eye is not an integrating device i.e, signal doesn’t build up with time. The eye does not provide a permanent record of what it sees. Thus eye is not used as the primary detecting device at professional research level telescopes. Thus there was a need for a technological advance in astronomical detectors. The functional components of a detector are:

1: Position sensing component.

2: Total energy measurement component.

3: Timing component.

4: Particle identification component.

2. Detectors:

In astronomy, we have three main detectors:

a) Photographic emulsions.

b) Photo-multiplier tube.

c) Charge coupled device (CCD).

2.1. Photographic Emulsion

The importance that photography could have in the field of astronomy was realized by LIOUS JACQUES. It would allow an accurate easy recording of brightness, position, spectra and physical aspects of celestial bodies. However, the early photographic plates weren’t sensitive enough to image faint objects.

During the decade of 1870’s there were several dramatic technological breakthroughs in the field of photography. In 1871 Richard Leach produced the first positive dry emulsion for physical development using gelatin and then in 1874, Johnson made the first negative emulsion for chemical development. Later Charles Bennet increased the speed of sensitivity of light of gelatin-silver bromide emulsion by ageing them at 32 degree C in a neutral medium. This was the first technological advance in astronomical detectors, since the universe is filled with very faint objects and astronomers wanted to be able to photograph them without waiting for days to get an image on photographic plate. Emulsions help in the study of spectra. The first good photographs of Jupiter and Saturn were made in 1879-1886 and of comets in 1881 using emulsions. However emulsions are not very good photometric devices for astronomy because of several drawbacks. Photographic emulsions can make a permanent record of astronomical objects imaged by telescopes. However, photographic emulsions are not very good photometric devices for astronomy because of several drawbacks. These record a small fraction (around 1%) of the photons that hit the emulsion. Because of the analog (rather than digital) nature of the image record on an emulsion, it is difficult to make quantitative measurements of star brightness’s. Photographic emulsions are also nonlinear- twice the input light does not produce twice the output on the film. This feature is called reciprocity failure.

2.2 Photo multiplier tube (PMT)

Photoelectric photometry is the electrical measurement of the light intensity of stars and other objects. Photomultiplier tube (PMT) is one such detector which finds extensive use in photometric observations of stars and other objects. The functioning of the photomultiplier tube is based on the well known principle, the photometric effect, which converts light into electricity. A photomultiplier tube consists of a light sensitive photocathode which emits photoelectrons at a rate modulated by the signal, and a multiplier section which amplifies the signal to provide a measurable current output. Photomultiplier tubes are sensitive to radiant energy from the ultra-violet (105nm) to the near infrared (1100nm). Their ability to detect single photons makes them particularly useful in low-light astronomical applications.

The PMT is the first modern detector. It is inherently digital. It detects individual photons and outputs a number which is directly and linearly related to the number of photons that were incident on the detector. A PMT consists of an evacuated glass tube one end of which is deposited a film of material such as indium antimonide called as a PHOTOCATHODE. This material has the property that when it is struck by a photon an electron is often liberated from the material. Each electron liberated from the cathode is directed away from the cathode by an electric field and is amplified into a pulse of electron by a series of metal plates (called dynodes) and an accelerating electric field in the tube. Electronics coupled to the PMT count these pulses. Thus, a single photon hitting the cathode results in an easily counted pulse of many electrons.

In the search to understand the universe, the astronomer seeks even more powerful observing instruments. Today progress in optics (thin mirrors), materials (light and rigid structures), and computer science (image quality improvement in real time) allows giant telescopes to flourish around the world.

Compared to their ancestors of just 20 years ago, new telescopes are often located without regard to onsite living amenities. Rather the controlling factors are the maximum number of clear nights and a calm, transparent atmosphere. The atmosphere is often found half a world away sitting comfortably in front of a computer screen from which he controls the telescope and collects the data via satellite link.

Throughout the history of the telescope the sensor at the focal point has been recognized as vital. Nobody can afford to waste the few rare and precious photons gathered by expensive telescopes because of an inadequate sensor. The practical sensor has been the human eye. However, its sensitivity, limited by physiological considerations and human subjectivity, placed serious limits on what could be discovered. The advent of photography in the last century was a monumental step forward.

Fig 5.1 : Construction of Photomultiplier Tube.

A fundamental advantage of photographic plate was its ability to record unseeable faint stars with long exposure times. Photographic technology has evolved over the years with ever more sensitive emulsions and treatment techniques.

Despite these improvements, the efficiency of a photographic plate remains relatively low. For every 100 photons that strike the plate, at best only 3 or 4 react with the silver sensors in the film‘s emulsion to create the image. This means that a one meter telescope using photographic techniques is no better than a 20cm telescope equipped with a 100 percent efficient sensor. We can therefore understand why the astronomers continue to seek a better sensor the 3 to 4 percent efficient photographic plate. Modern detectors are inherently digital. The first modern detector in this sense is the photomultiplier tube (or PMT). PMT’s offer various interesting advantages:

Extremely fast response time: reaction time can be as short as a fraction of nanosecond.
PMT allows the measurements of very weak signals with signal to noise ratio limited by electron statistics.
The nature of output of a PMT enables application of pulse counting technique.

The drawback of a PMT is that it is essentially a single channel device, meaning there is no positional information in the signal. The output signal does not depend on where on the cathode the photon will hit, so we get only a measure of all the light following on the photocathode. However the PMT, unlike a photographic plate, has a digital output, meaning we can easily make quantitative measurements. The fraction of the photons which hit the cathode that are actually detected by the PMT is set by the fraction of the photons that hit the cathode that liberate an electron. This fraction is typically 20 percent or so, so the efficiency of observing is much higher for a PMT than for the best photographic emulsion. The detector of choice for optical astronomy is now the CCD (charge coupled device) due to various advantages.

2.3. Charge Coupled Device: A charge coupled device (CCD) is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example converting it into a digital data. Most astronomical detectors in use today at professional observations as well as with many amateur telescopes are charge coupled devices. The charge coupled device image detector or CCD was developed in 1970 by Boyle and Smith at Bell laboratories. The original CCD was designed to store and transfer analog information in the form of packets of electrical charges within a semiconductor structure. The charges have memorized in storing sites usually composed of MOS capacitors (metal oxide semiconductor). There can be several hundred to several thousand MOS capacitors on a single silicon chip. The sites are coupled together by transfer circuitry which makes it possible to move the charges in an orderly manner to appoint where they can be measured.

CCDs have various uses: memories, delay lines, corelators, and optical detectors. This last category is of most interest here. In a photo sensitive detector with CCD registers, the electric charges are created through the photoelectric effect. The storage sites are also photo elements (they are sensitive to light). They are organised either in lines (linear array CCD) or in a matrix (area array CCD).

After a period of being exposed to light, called the integration time, the photo charges are transferred, one behind the other, to an output stage. The electrical signal delivered at the output stage is proportional to the incident illumination falling on the read photo site. We therefore observe at the output the variation of an electric signal, synchronized with a read out rhythm of the CCD, and corresponding to the number of charges contained in the packet. The injection of charges, their storage, their transfer and their readout are the basic functions of CCD.

Principle of operation:

Basically a CCD is a light sensitive silicon chip which is electrically divided into a large number of independent pieces called pixels (for picture elements). Present day CCDs have (512)(512) to at least (4096)(4096) individual pixels for astronomical use, we use the CCD as a device to measure how much light falls on each pixel. When a photon is absorbed in silicon an electron hole pair is created. In a CCD electrodes create potential wells in the silicon that collect these electrons. The surface of CCD is covered with electrode biased so that the electrons preferentially are diffused towards the electrodes with the highest applied potential. The holes diffuse away into the bulk of the silicon and are effectively lost. The output is a digital image, consisting of a matrix of numbers, 1 per pixel, each number being related to the amount of light that was on that pixel. The image coming out in a digital form is readily manipulated, measured and analyzed by computer.

The MOS capacitor: A MOS capacitor is composed of a doped semiconducting substrate, covered by an insulating layer (silicon oxide, SiO2), on which a metallic electrode, also called a gate, is placed. The insulating oxide is a thin layer of silicon several tens of micrometers thick. The metallic electrode is a deposit of aluminium or polycrystalline silicon heavily doped to become a conductor. The layer of silicon oxide makes the MOS structure insulating and forms a capacitor.

Fig 5.2 : Cutaway of Moss cell

Let us study the mechanism that stores the electrical charges in a MOS capacitor. A silicon atom has its valence band occupied by 4 electrons. Each valence electron associates two by two with those of neighbouring atoms to form a crystalline lattice. In the case of crystalline Si, 8 electrons would complete the valence band. If trivalent impurities are introduced into the lattice, each atom of impurity introduces only 3 electrons, and there will be only 7 electrons in the outer layer. A hole is therefore created. The hole is caused by the absence of an electron and because of this deficiency we say that the semiconductor is P- type doped .On the other hand, N-type semiconductors are doped with pentavalent atoms which have 5 electrons in the valence band.

Fig 5.3 : Surface voltage of a MOS cell.

Suppose that the substrate is of P-type doped Si as is most frequently the case with CCD’s. The holes are then called majority carriers, there are several free electrons, resulting from the action of thermal energy which breaks the covalent bonds and brings the electrons from the valence band towards a band where they can move freely: the conduction band. These electrons are called minority carriers.

The gate of a MOS capacitor is biased positively with about 10volts. At the instant that the bias is applied, the majority carriers (the holes) present in the neighborhood of the SiO2-Si interface are pushed back into the interior of substrate. A zone that is almost empty of majority carriers is then created in the area of the interface. A zone where charges are rare is called a DEPLETION ZONE or a SPACE-CHARGE ZONE.

A space charge zone is not in a state of equilibrium over time, hole pairs are either generated from within the depletion zone or diffused at the frontier of this zone. These electron hole pairs are separated by the electric field and the electrons accumulate in the neighborhood of the SiO2-Si interface. This concentration of minority carriers of a type opposite to those of the substrate creates an INVERSION LAYER. The presence of minority carriers reduces the voltage of the surface from V1 to V2 (decrease in the thickness of the depletion zone).

Fig 5.4 : CCD voltage well.

After a certain period of time called “thermal relaxation time”, there are as many electrons on the surface as there are holes in the substrate. This phenomenon is known as dark current. The state of equilibrium that results from this situation is reached between one and several dozen seconds after biasing the electrode. Relaxation time depends on the type of Si, on the state of the surface between Si and the oxide and on the temperature.

The CCD is used when the MOS capacitor is in disequilibrium that is when the minority carriers produced by thermal effect are negligible. The useful electron carriers are those that are generated by photoelectric effect and not those caused by dark current. Useful electrons can only be accumulated for a period that is less than the relaxation time. These carriers are stored at the SiO2-Si interface, and the inversion layer thus produced carries the information. The operations of charge injection, transfer, and readout must take place in a very short time compared to the thermal relaxation time.

3. Basic Concepts to CCDs:

Quantum efficiency: A CCD detects individual photons, but even the best CCD does not detect every single photon that falls on it. The fraction of photons falling on a CCD that are actually detected by the CCD is called the QUANTUM EFFICIENCY (QE), usually expressed as a percentage. Another way of defining QE that involves the signal to noise ratio of the input and the detected signal. For CCD’s in which photon noise is the dominant noise source, the fraction of photons detected and the signal to noise definitions of QE are equivalent.

QE is a function of wavelength. For optical detection, there are two basic styles of chip: thick chips or “front side illuminated chips”, in which the light passes through some of the electronic layers of the CCD before hitting the silicon detecting level, and thin chips or ”backside illuminated” in which the silicon layer is mechanically or chemically thinned and the light enters the silicon directly. Thick chips have low QE in the blue, because the electronic layers absorb much of the blue light. Thin chips have better blue QE. Thin chips and thick chips have more similar red QE, but the thin chips usually have higher QE at all wavelengths than the thick chips.

The dark current: The main enemy of the CCD user is the dark current. Even when the device is placed in complete darkness, a signal is observable at the output of the detector. This signal is the result of charges created in the silicon because of thermal agitation of the crystalline lattice. Charges of thermal origin are created in the photo elements as well as in the thermal registers (which share the same technology as the CCD). Their number, like that of the photo charges, is obviously tied to the integration time. The production rate is such that at room temperature a standard CCD in darkness is saturated after only a few seconds of integration. On the other hand, the dark current becomes nearly negligible at a temperature of about -100 degree Celsius.

4. Photometry with CCDs:

The main stay of astronomy is photometry. Technically this means measuring the light output of an object such as a star or galaxy. In the past, photographic and photoelectric photometries were used. The former is essentially obsolete, and the later is not so accurate. The great advantage of CCD detectors is their quantum efficiency, which enables us to use not only big telescopes in major observatories, but also small amateur ones for unique data collecting. This opens up many new opportunities in the astronomical research e.g. a long term monitoring of variable stars, search for new variable stars. Search for novae and supernovae eruptions in other galaxies, search for Near Earth Asteroids and their photometry.

The image produced by a CCD camera is by a number of astronomical effects, which must be corrected before reasonable results can be obtained. The image which is not yet corrected, will be called a raw image. The raw image contains not only information about the object of interest, but also a contribution from the electrons introduced by read-noise and thermally generated electrons. The process of CCD image reduction is based on the use of three calibration images: the bias (or off-set) frame, dark frame and flat field frame.

Bias Frame: Despite the fact that CCD’s are inherently linear, there is an offset or signal even with no exposure. This is called the bias level. A bias frame, that is, an image of zero duration with the shutter of the CCD camera closed, can be taken, stored, and then subtracted from every exposed frame. The preferred practice is to use the average of several bias frames.

Dark Frame: The frame is exposed with the closed shutter, usually with the same exposure time as the raw image. The dark frame measures the thermal (also called the dark current) on the CCD chip during the exposure. The dark frame can also give information about bad or hot pixels on CCD chip as well as an estimate of the cosmic ray strikes at the observing site.

Flat Field:

The frame is used for correction of the pixel to pixel response variations. These pixel-to-pixel sensitivity variations also change with the wavelength of the incident light. Flat field frames could be obtained by illumination of the CCD by constant and uniform light (e.g. expose of the twilight sky, dome or projector lamp) to provide high S/N image. The benefits of the photometry with CCD detectors are as follows:

All objects are observed by the same detector and through the same filter.
The high quantum efficiency, which depends on the spectral range, may reach upto 80 percent.
The great advantage of CCD photometry is that more than one or two comparison stars can be used at once.
Sky background can be estimated directly near the measured stars and contribution from other sources could be eliminated.
Photometers are unable to do crowded field photometers, as unwanted star may drift in and out of the aperture. Also the comparison stars must be chosen ahead of time and cannot be easily changed during the night. With CCD’s, this is not the problem, as the entire field of view is saved. This allows data reduction using varying aperture that can be compared to ensure that only the flux from the target star is observed, and also the comparison star can be chosen at the time of reduction, with as few as many as the observer wants.

For More Details on topics discussed in Text Module, See

Introduction to Astronomical Photometry by Edwin Budding and Osman Demircan.
A Practical Guide to CCD Astronomy by Lena P.
Electronic Imaging in Astronomy: Detectors and Instrumentation by M c lean, I. S.