Astronomy Principles and Practice Fourth Edition.pdf

The photographic plate 285
lowering the threshold of visibility. These changes take about half an hour before the sensitivity settles
to its improved value and, in this condition, the eye is said to be dark-adapted.
There are two causes which give rise to dark-adaption. One of them is due to the automatic
expansion of the pupil, so allowing a larger collecting area for the incoming radiation. The other,
providing the greater effect, is due to the biochemical changes occurring in the retina itself.
Under dark-adapted conditions and with a little practice, it is apparent that the eye’s sensitivity
depends on the direction of the object. The minimum of sensitivity occurs when the eye is looking
straight ahead and, by using averted vision, it may be possible to detect faint objects which disappear
when looked at directly.
Dark adaption also gives rise to changes in the spectral sensitivity of the eye. The peak in
sensitivity moves by approximately 500 Å towards the blue end of the spectrum and the eye loses
its sensitivity to red wavelengths. This change is known as the Purkinje effect and the dotted curve in
figure 18.1 illustrates an average spectral sensitivity under dark-adapted conditions.
Prior to the application of the photographic plate, photoelectric detectors and CCD devices, the
eye was used to judge brightness differences between astronomical objects. The values of differences
which might be detected depend on several factors such as the colours of the objects for the comparison
and their absolute brightnesses. With practice, brightness differences of a few per cent can be detected.
It is not really convenient to assign a quantum efficiency to the eye but by considering its ability just
to be able to detect a sixth magnitude star, it can be said that it is necessary to receive a few hundred
photons per second to register a star image. The limiting magnitude of the telescope–eye combination
has already been discussed in section 17.4.
It is difficult to give a hard and fast rule for the resolving power of the eye as it depends critically on
the type of observation which is being attempted. However, under ordinary circumstances, the average
eye is able to resolve angles of one minute of arc. This figure corresponds (see equation (17.6)) to the
resolving power which might be expected by an aperture of the size of the pupil ∼2·5 mm, a typical
diameter when operating in an everyday environment, and to the sizes of the detector elements which
make up the retina. Under special circumstances, the eye has special properties of high vernier acuity
and symmetry judgment allowing even smaller angles to be resolved. It is capable of resolving a
break in a line which might correspond to an angular difference of ten seconds of arc. This ability
is put to use in measuring instruments whose settings are read off a vernier scale. Although such
instruments are now no longer used on a telescope, they may still be found on ancillary equipment
which might be employed to analyse astronomical data recorded, say, on a photographic plate. The
continuing advance of new techniques and automation, however, will eventually displace the use of the
eye for all data reductions. As well as taking advantage of the eye’s symmetry judgment in general
measuring instruments, it was this ability which made the eye useful for making classical double-star
measurements on the telescope.
18.5 The photographic plate
18.5.1 Introduction
As soon as the photographic process was developed, it was immediately applied to the skies and has
been one of the chief ways of recording brightness and positional information associated with starfields,
galaxies and spectra since the middle of the 19th century. With the advent of solid state detectors (see
later), its role has diminished by a large degree in the last 20 years but continues to be used in some
areas of study particularly when large areas of the sky are being surveyed (see the Schmidt telescope—
section 20.4). It may be noted that research on the production of improved emulsions continues.
Photographic material consists of an emulsion of silver halide (mainly bromide) crystals in gelatin,
which is attached to a glass plate or celluloid sheet in a thin uniform layer. The exact mechanism
occurring within the emulsion when it is illuminated is not fully understood but the interaction results