Astronomy Principles and Practice Fourth Edition.pdf

306 Astronomical optical measurements
designed which automatically scan the plate, hunting for each of the star images, even distinguishing
between a star and a distant galaxy. Highly automated instruments of this kind have been developed at
the Royal Observatory, Edinburgh. Its Super COSMOS scanning system can operate on 20 in × 20 in
glass plates and ‘celluloid’ films up to 14 in × 14 in. The information from the plate or film is digitized
with a spatial resolution of 10 µm with 10 15 bits (37 768 grey levels) for the intensity values. Special
software processes the resulting data to produce a catalogue of all the objects detected on each plate,
with 32 parameters associated with each object.
Prior to computerized systems, other special types of microscope for identifying moving objects or
variable stars were sometimes used. One instrument, known as a blink microscope, has two carriages
which support two plates taken of the same region of the sky at different times. An arrangement
is provided so that the identical star field on each plate can be viewed in quick succession. Any
object which has moved against the star background between the two exposures will appear to jump
between two positions across the fixed star background. Any variable star will appear to pulsate as the
microscope is operated.
Some types of positional measurement require only one coordinate to be measured and, in this
case, the microscope need not have both X and Y movements. For example, for the measurements of
radial velocity, comparison of the stellar spectral lines and laboratory spectral lines of a photographic
plate can be obtained by a single coordinate-measuring microscope.
The accuracy to which any positional measurements can be made depends on several parameters,
one of which is obviously the accuracy of the microscope, usually controlled by the quality of the
micrometer screw. The uncertainty of any position may be typically ±1 × 10 −3 mm and such a figure
may then be used to determine the uncertainty of the parameter being measured.
Because of the smaller size of the CCD chip relative to a photographic plate, large surveys
of stellar positional measurements have not been applied to ground-based surveys. However, very
accurate differential positional measurements are readily obtained from CCD frames under direct
computer control without the subjectivity of any eye-ball assessment of the XY positions of the image
seen either through a microscope or on a VDU screen. For example, in the case of stellar images,
the centroid of the distribution of the photons as collected by the local pixels is readily determined in
terms of the structure of the light-sensitive chip, the rows and columns providing the reference grid.
The positions are readily determinable as fractions of the basic pixel size, making for very accurate
measurements of angular separations according to the focal length of the telescope.
19.3 Broadband spectral photometry
The response of any detector system to energy falling on to it obviously depends on the strength
of the radiation received. It also depends on the frequency distribution of the energy and on the
spectral sensitivity of the detector. In many instruments, rather than using the full spectral range of
the detector, optical elements, such as colour filters, may be applied to select specific spectral zones
for measurement. The variation with wavelength of the relative response of the system may be written
as S(λ) and may be displayed graphically (see figure 19.1). The width of the embraced spectrum, λ,
may be defined as the spectral interval between points on the S(λ) curve where the response is reduced
to a half of the maximum value and this is termed the full-width half maximum or FWHM.
Each passband can be ascribed an equivalent wavelength, λ 0 ,definedby
∫ ∞
0
λS(λ) dλ
λ 0 = ∫ ∞
0
S(λ) dλ
this being the mean wavelength, weighted according to the response curve. To a first order, brightness
measurements made using passbands which are narrow in comparison to the broader energy spectrum
of a source can be considered as the monochromatic brightness value at the equivalent wavelength.