Astronomy Principles and Practice Fourth Edition.pdf

290 Detectors for optical telescopes
Figure 18.5. A density scan through a recorded image with a sharp change in strength illustrates the Eberhard
effect.
of incident energy but on the rate at which it arrives. In effect, reciprocity failure means that there
are different characteristic curves available according to how the calibration exposures are imposed so
that, before any photometry can be attempted, the characteristic curve, as obtained via the calibration
exposures, must be corrected for reciprocity effects. On this account alone, photographic photometry
is open to systematic error. By a similar argument, the brightnesses of astronomical objects recorded
on two different plates cannot be compared directly if the exposure times are different.
It is also important to consider the effects of reciprocity failure when an emulsion is being chosen
for a particular type of photograph. For example, it might be thought that the fastest possible plate
should be chosen to record faint stars in the shortest possible time. Now the speed of a film is normally
determined according to how it behaves under the circumstances of everyday photography with very
short exposures. If the reciprocity failure is severe, as it usually is for everyday fast emulsions, the
effectiveness of the plate with long exposures can be so reduced as to be inferior to other plates with
small reciprocity failures but with lower everyday photographic speeds.
There are several other effects besides reciprocity failure which can lead to distorted
measurements unless they are allowed for. The following two result from the development process
itself, the first affecting photometry and the second, positional measurements.
If an object has detail in it which is recorded on the photographic plate, during the process of
development the chemical reactions will be under way at different rates, according to the position in
the image. In regions where the image is most dense, the reaction is most speedy; however, in other
regions, the development will proceed at a lower rate. Because of this, there will be localized variations
in the rate at which the developer is being used and, therefore, in its strength. Unless they are disturbed
adequately by agitation, these variations will control the development rate to some extent. The effect
of unequal development rates is usually manifest where there are sudden changes in the density of an
image, causing distortions of density values. It is known as the Eberhard effect and it is illustrated in
figure 18.5.
Positional measurements are obviously affected if there are geometric distortions in the gelatin of
the emulsion and this can occur in the development process. During development, the gelatin becomes
tanned and the extent of the tanning process increases where the images are strongest. The effect of
tanning is to make the emulsion dry quicker and this sets up strains as the tanned areas shrink. In the
simple case of a double star measurement, the shrinkage causes the star images to move together and,
if the effect is not allowed for, systematic errors may result.