By : A E
Roy and D Clarce
From The
Book “ Astronomy Principles and Practice”
3.1 The subjectivity of simple measurements
One
of the drawbacks of making astronomical observations by eye, with or without
the advantage of supplementary equipment, is that they are very subjective.
When results taken by several observers are compared, inconsistencies become
apparent immediately. For example, if several observers time a lunar
occultation (i.e. the disappearance of a star behind the lunar disc) at a given
site by using stopwatches which are then compared with the observatory master
clock, the timed event will have a small range of values. If several
occultation timings are taken by the same group of observers, an analysis of the
spread of values of each timing will show that certain observers are
consistently later than others in operating the stop-watch. Each observer can
be considered to have a personal equation which must be applied to any
observation before comparing it with measurements taken by other observers.
The
problem is complicated further as the personal equation of any observer can be
timedependent. This might be a short-term variation depending on the well-being
of the observer or it may be a long-term drift which only becomes apparent over
a period of years as the observer ages. The first recorded example of such
effects appears to have been noted by the fifth Astronomer Royal, Maskelyne,
when he wrote in 1796 [Greenwich Observations 3]:
My
assistant, Mr David Kinnebrook, who had observed transits of stars and planets
very well in agreement with me all the year 1794 and for a great part of
[1795], began from the beginning of August last to set them down half a second
later than he should do according to my observations; and in January [1796] he
increased his error to eight-tenths of a second. As he had unfortunately
continued a considerable time in this error before I noticed it, and did not
seem to me likely ever to get over it and return to the right method of
observing, therefore, although with reluctance, as he was a diligent and useful
assistant to me in other respects, I parted with him.
It
is mainly due to this episode that the concept of the personal equation was
explored some years later. In Maskelyne’s account of the reasons for
Kinnebrook’s dismissal, he uses the term ‘right method of observing’, meaning
by this that there were discrepancies between Kinnebrook’s results and his own and
that Kinnebrook’s method of observation had deteriorated. It could well have
been, of course, that the drift in Maskelyne’s own personal equation had
occurred contributing to the discrepancies, or even accounting for them
completely.
An
example of short-term variations in the personal equation occurs in the determination
of colour differences made directly by eye. It is well known that the
sensitivity of colour depends appreciably on the individual observer; some
people have poor ability to differentiate colours and may even be ‘colour-blind’.
The colour sensitivity of each observer also depends on his or her condition.
Under normal conditions the average eye is most sensitive to the green region
of the spectrum. However, if the observer is removed to a darkened room, the
eyes become accustomed to the dark and maximum sensitivity shifts towards the
blue. After a period of about half an hour, the effect is very noticeable. If
an observer is made to do violent exercise, the slight rise in the bodily
temperature causes the sensitivity peak to move away from the normal position
towards the red end of the spectrum. Thus, any determination of colour, being
dependent on how the observer’s eye responds to colour, depends to a great
extent on the condition of the observer and the particular circumstances of the
observation.
3.2 Instrumentation in astronomy
As
in the case of all of the sciences, instrumentation has been developed in
astronomy so that the data provided by the observations are no longer
subjective. Again, as in other sciences, the application of instrumentation immediately
revealed that the scope for measurement is also extended. For example, when
Galileo employed the telescope for astronomical observation, a new range of
planetary phenomena was discovered and the number of observable stars was
greatly increased. Since Galileo’s time, the whole range of observable
phenomena has continued to grow with the application of each new type of
observing equipment.
The
instrumentation which was first applied to astronomy was designed so that the
actual measurement of record was made by eye. When photographic material became
available, the range of possible observation was immediately increased. This
has now been further extended by the introduction of solid state devices in the
form of CCDs (charge coupled devices). Whereas the eye is capable of being able
to concentrate on only a few stars at a time in a star field, the photographic plate
or CCD chip is able to record the light from every star in the field
simultaneously. For a star to be seen by eye, the brightness must be above a
certain threshold: the eye is not able to accumulate the energy it receives
over a period of time to form an impression. The photographic plate and CCD, however,
are able to do this and, if a time exposure is made, the resultant images
depend on the total energy which falls on to the detector. Thus, besides being
able to record many images simultaneously, these devices allow faint stars to
be recorded which would not normally be seen by eye (see figure 3.1).
The
variation of the sensitivity with wavelength of these detectors is also
different to the eye. For example, photographic plates of different types are
available with a range of spectral sensitivities. Some plates have their peak
of sensitivity in the blue while others have their peak in the red.
Bluesensitive plates will obviously give strong images for blue stars and not
for the red, while red-sensitive plates give weak images for blue stars and
strong images for red. By using two plates of different spectral sensitivity to
photograph a star field, the fact that stars are coloured is easily
demonstrated. Because of the physical process involved in the detection of
radiation by a silicon-based solid state detector, the natural peak sensitivity
tends to be in the red end of the spectrum but, again, the colour response of
an applied detector can be modified at its manufacture.
Some
special photographic materials are sensitive to colours which cannot be seen by
the normal eye. The colour range of astronomical observations can be extended
into the ultraviolet or the infrared by the choice of a particular photographic
emulsion.
Thus,
by recording the astronomical observation on a detector other than the eye, it
is possible to extend the scope of the observation by looking at many objects
simultaneously, by looking at a range of objects which are too faint for the
eye to see and by looking at a much broader range of colour.
The
range of available detectors has increased greatly since the photographic
process was first applied to astronomy. Detectors based on the photoelectric
effect have a common application. Detectors specially designed for infrared
work can also be attached to optical telescopes. After the discovery that
energy in the form of radio waves was arriving from outer space, special
telescopes were designed with sensitive radio detectors at their foci and the
era of radio astronomy was born. It is also apparent that our own atmosphere
absorbs a large part of the energy arriving from outer space but, with the
advent of high flying balloons and artificial satellites, these radiations are
now available for measurement. New branches of γ -ray, x-ray and infrared
astronomy are currently increasing the information that we have concerning the
extra-terrestrial bodies.
Although
the large range of detectors removes to a great extent the subjectivity of any measurement,
special care is needed to avoid the introduction of systematic errors. Each
detector acts as a transducer, in that energy with given qualities falls on to
the detector and is converted to another form; this new form is then measured.
For example, when radiation falls on the sensitive area of a photocathode, the
energy is converted in the release of electrons which can be measured as a flow
of electric current. The strength of the incident energy can be read as the
needle deflection on a meter or converted to a digital form for direct
processing by a computer.
The
process of converting the incident radiation to a form of energy which is more
acceptable for measurement is never one hundred per cent efficient and it is
essential that the observer knows exactly how the recording system responds to
a given quality and quantity of radiation. In other words, the whole of the
equipment which is used to make an observation must be calibrated. The calibration
can be calculated either by considering and combining the effects of each of
the component parts of the equipment or it can be determined by making
observations of assumed known, well-behaved objects. Because of the
impossibility of having perfect calibration, systematic errors (hopefully very
small) are likely to be introduced in astronomical measurements. It is one of
the observer’s jobs to ensure that systematic errors are kept below specified
limits, hopefully well below the random errors and noise associated with the
particular experimental method.
Although
every piece of observing equipment improves the process of measurement in some
way, the very fact that the equipment and the radiation have interacted means
that some of the information contained in the parameters describing the
incident radiation does not show up in the final record and is lost. All the
qualities present in the incident energy are not presented exactly in the
record. Each piece of equipment may be thought of as having an instrumental
profile. The instrumental profile of any equipment corresponds to the form of
its output when it is presented with information which is considered to be
perfect.
For
example, when a telescope is directed to a point source (perfect information), the
shape of the image which is produced (instrumental profile) does not correspond
exactly with the source. The collected energy is not gathered to a point in the
focal plane of the telescope but is spread out over a small area. The
functional behaviour of the ‘blurring’ is normally referred to as the point
spread function or PSF. For the best possible case, the PSF of the image of a
point source is that of a diffraction pattern but inevitably there will be some
small addition of aberrations caused by the defects of the optical system or
blurring by atmospheric effects. If the recorded image is no larger than that
of the instrumental profile, measurement of it gives only an upper limit to the
size of the object. Detail within an extended object cannot be recorded with
better resolution than the instrumental profile.
For
any instrument, there is a limit to the ‘sharpness’ of the recorded information
which can be gleaned from the incoming radiation. This limit set by the
instrument, is frequently termed the resolving power of the instrument. In all
cases there is an absolute limit to the resolving power of any given equipment
and this can be predicted from theoretical considerations. Certain information
may be present in the incoming radiation but unless an instrument is used with
sufficient resolving power, this information will not be recorded and will be
lost. When any given piece of equipment is used, it is usually the observer’s
aim to keep the instrument in perfect adjustment so that its resolving power is
as close as possible to the theoretical value.
As
briefly mentioned earlier, as with all sciences involving quantitative
observations, the measured signal carries noise with the consequence that the
recording values are assigned uncertainties or errors. One of the ways of
describing the quality of measurements is to estimate or to observe the noise
on the signal and compare it with the strength of the signal. This comparison
effectively determines the signal-to-noise ratio of the measurement. Values of
this ratio may be close to unity when a signal is just about detectable but may
be as high as 1000:1 when precision photometry is being undertaken.
3.3 The role of the observer
Observational
astronomy holds a special place in science in that, except for a very few
instances, all the knowledge and information has been collected simply by
measuring the radiation which arrives from space. It is not like the other
laboratory sciences where the experimentalist is able to vary and control the
environment or the conditions of the material under investigation. The
‘experiment’ is going on out in space and the astronomer collects the
information by pointing the telescope in a particular direction and then
analysing the radiation which is collected.
In
interpreting the accumulated data, the reasonable assumption is made that the
same physical laws discovered in the laboratory can be applied to matter
wherever it is assembled in space. Many of the astronomical measurements, in
fact, provide us with means of observing material under a range of conditions
which are unattainable in the laboratory. In order to understand these
conditions, it is sometimes necessary to provide an extension to the laboratory
laws or even consider invoking new laws to describe the observed phenomena.
Laboratory
analysis is practised on meteorite samples which are picked up from the surface
of the Earth and on micrometeoritic material which is scooped up by rocket
probes in the upper atmosphere. Some thirty years ago the Apollo and Lunakhod
missions brought back our first samples of lunar material for laboratory study.
Interplanetary space probes have sent and still are sending back new data from
the experiments which they carry. They are able to transmit information about
the planets that could not have been gained in any other way. Astronomers have
also gleaned information about the planets by using radar beams. However, all
these active experiments and observations are limited to the inner parts of the
Solar System, to distances from the Earth which are extremely small in relation
to distances between the stars.
When
it comes to stellar work, the experiments, whether on board space vehicles or
Earth satellites, or at the bottom of the Earth’s atmosphere, are more passive.
They involve the measurement and analysis of radiation which happens to come
from a particular direction at a particular time. It is very true to say that
practically the whole of the information and knowledge which has been built up
of the outside Universe has been obtained in this way, by the patient analysis
of the energy which arrives constantly from space.
As
yet, the greater part of this knowledge has been built up by the observer using
ground-based telescopes though in recent years a wide variety of artificial
satellite-based telescopes such as the Hubble Space Telescope and Hipparcos
have added greatly to our knowledge. The incoming radiation is measured in
terms of its direction of arrival, its intensity, its polarization and their
changes with time by appending analysing equipment to the radiation collector
and recording the information by using suitable devices. The eye no longer
plays a primary role here. If the radiation has passed through the Earth’s
atmosphere, the measurements are likely to have reduced quality, in that they
are subject to distortions and may be more uncertain or exhibit an increase of
noise. In most cases, however, these effects can be allowed for, or compensated
for, at least to some degree.
The
task of the observer might be summarized as being one where the aim is to
collect data with maximum efficiency, over the widest spectral range, so that
the greatest amount of information is collected accurately in the shortest
possible time, all performed with the highest possible signal-tonoise ratio.
Before the data can be assessed, allowances must be made for the effects of the
radiation’s passage through the Earth’s atmosphere and corrections must be
applied because of the particular position of the observer’s site and the
individual properties of the observing equipment.
It
may be noted here also that with the advent of computers, more and more
observational work is automated, taking the astronomer away from the ‘hands-on’
control of the telescope and the interface of the data collection. This
certainly takes away some of the physical demands made of the observer who
formally operated in the open air environment of the telescope dome sometimes
in sub-zero temperatures. Accruing data can also be assessed in real time so
providing instant estimates as to its quality and allowing informed decisions
to be made as to how the measurements should proceed. In several regards, the
application of computers to the overall observational schemes have made the data
more objective—but some subtleties associated with operational subjectivity do
remain, as every computer technologist knows.
We
cannot end this chapter without mentioning the role of the theoretical
astronomers. Part of their tasks is to take the data gathered by the observers
and use them to enlarge and clarify our picture of the Universe. Their
deductions may lead to new observational programmes which will then support their
theories or cast doubt upon their validity.
Several
comments may be made here.
It
goes without saying that an astronomer may be both theoretician and observer,
though many workers tend to specialize in one field or the other. Again, it has
been estimated that for each hour of data collecting, many hours are spent
reducing the observations, gleaning the last iota of information from them and pondering
their relevance in our efforts to understand the Universe. The development of astronomical
theories often involves long and complicated mathematics, in areas such as
celestial mechanics (the theory of orbits), stellar atmospheres and interiors
and cosmology. Happily in recent years, the use of the ubiquitous computer has
aided tremendously the theoretician working in these fields.
Selanjutnya>>
Postingan
Tidak ada komentar:
Posting Komentar