Astronomy Principles and Practice Fourth Edition.pdf

286 Detectors for optical telescopes
in the production and accumulation of silver ions wherever the light was incident on the emulsion.
Exposure of the emulsion to optical radiation does not alter its appearance to a visual inspection.
The distribution of the illumination of the exposure over the surface of emulsion is not immediately
apparent. Any images which have been recorded are, in the first instance, latent. The emulsion must
be processed before the image is available for analysis.
In order to produce and retain any image in the emulsion, the exposed plate must undergo two
processes: the images must be first developed and then fixed. This is carried out away from the
telescope in a photographic darkroom. Development is performed by immersing the emulsion in a
liquid chemical reducer which converts the silver ions to individual silver grains. After development,
the emulsion contains a distribution of silver grains, matching the images that were imposed on it,
and the remaining silver halide crystals which have been unaffected and which are, of course, still
capable of responding to any further illumination. Fixation of the images is, therefore, obtained by
the removal of the remaining sensitive silver halide from the emulsion, and this again is achieved by
using chemicals in solution. Before the emulsion is dried, all traces of the fixer material are removed
by washing in water.
There are many kinds of photographic plate available and it is important that the correct type
should be chosen according to the image information which is to be recorded. The chief parameters
that have to be considered are those of spatial resolution, speed and spectral sensitivity.
18.5.2 Spatial resolution
If an exposed plate is examined under a microscope, it will be seen that the images are made up of
individual blackened grains. Any seemingly sharp edge in the image is fuzzy on the scale which
depends on the grain size. Microscope views of star plates reveal that each image is made up of an
agglomeration of grains with a central condensation; the edges of images are not well defined. It is
the grain sizes in the emulsion which normally determine the limit of any spatial resolution. The large
choice of emulsion types embraces plates covering a wide range of graininess and, hence, resolving
power. Quantitatively, the resolving power of any particular type of plate is determined by forming
the image of a grid which has numbers of black and white ruled lines on it at different separations. The
resolving power, R, is defined as the highest number of ruled lines per mm that can be resolved. The
various emulsion types give a range in R from about 50 to 1000. In any photographic observation, it
is important that the value of R is chosen so that all the detail which is provided by the telescope and
other optics is recorded without any deterioration.
Because of the large plate formats that are available, the photographic process scores well when
large amounts of spatial information need to be recorded. Each collection of grains within an area
equivalent to ∼1/R 2 acts as a resolved picture element or pixel. Thus, for a plate measuring 100 mm ×
100 mm with a value of R = 100, the plate contains
100 × 100
( 1
100 )2 = 10 8 pixels
an exceedingly good number in comparison with other detectors.
18.5.3 Speed
The time of exposure required to obtain an image of a certain strength of a particular astronomical
object with a given telescope varies according to the emulsion type that is used. A plate or film is
said to have a certain speed, designating the amount of energy that needs to fall on to the emulsion to
produce a good image.