Astronomy Principles and Practice Fourth Edition.pdf

Unsharp masking 293
18.9 Unsharp masking
Much fine detail is often lost in an astronomical photograph of an extended field because it varies little
with respect to the background brightness. Unsharp masking is an image-processing technique that
enhances the fine detail. The original picture is processed to obtain a negative mask, such a mask being
of low density and slightly out of focus. This mask is then superimposed on the original, resulting
in a decrease in the brightness range of the original but without decreasing the fine detail. The tiny
brightness variations over the original can then be seen more clearly. The Australian astronomer,
David Malin, has produced many beautiful pictures of celestial objects such as the Orion Nebula
using unsharp masking, revealing by this method fine detail that otherwise would effectively be lost.
A selection of such images is available from the Anglo-Australian Observatory (AAO) W18.1 . This
processing technique can also be applied to images recorded in electronic form with the operation
undertaken by software rather than by using a physical mask.
18.10 Photoelectric devices
18.10.1 Introduction
When optical radiation falls on certain alkali materials, electrons are liberated and ejected from the
material’s surface. By encapsulating the photosensitive material in a vacuum and providing a plate
with a positive potential, the ejected electrons, or photoelectrons as they are called, can be collected
and their flow can be measured as an electric current. This type of detector provides the means of
performing extremely accurate photometry. One of the chief reasons for this is that the size of the
current is exactly proportional to the amount of energy which falls on the sensitive surface and this
holds for the whole of the range of energies which are likely to be presented to the device. The device
is said to have a linear response.
18.10.2 Spectral sensitivity
A range of photocathodes is available covering the spectrum from the ultraviolet to about 10 000 Å.
The cut-off in the ultraviolet is controlled by the glass envelope of the cell and, by using a quartz
window above the cathode, the ultraviolet sensitivity can be extended. The various spectral responses
are designated by a number (e.g. S–1) and three such sensitivity curves are depicted in figure 18.6.
18.10.3 Quantum efficiency
The quantum efficiency of the response is typically of the order of 0·1 or 10%, meaning that, on
average, one photoelectron is emitted for every ten photons which are incident on the sensitive surface.
Using this information, it is very easy to predict the current to be measured according to the energy
arrival rate. For example, if the photoelectric cell is receiving 200 photons s −1 (i.e. the photon rate just
detectable by eye), the number of photoelectrons emitted is 20 s −1 . Now the charge on an electron is
equal to 1·6 × 10 −19 coulomb (C) and, therefore, the total charge liberated is
20 × 1·6 × 10 −19 Cs −1 = 3·2 × 10 −18 A.
This, of course, is an extremely small current and it would require large amplification before it could
be registered by some recording device.