Astronomy Principles and Practice Fourth Edition.pdf

328 Astronomical optical measurements
PM
Antichopper
Photographic
plate
Neutral density
wedge
Lamp
Figure 19.14. The schematics of a microdensitometer shows two light beams, one passing through the
photographic plate, the other passing through a neutral density wedge. The beams are alternately detected by
a photomultiplier. As the plate is scanned, any difference in intensity of the two beams causes movement of
the neutral density wedge to restore the balance. The movement of the wedge is drawn out as a graph with the
horizontal axis matching the movement of the plate.
be converted to intensity values. There are several laboratory instruments which enable analysis of
spectrograms to be made and the brief description of one of them, the automatic microdensitometer,
is given here.
The microdensitometer has an adjustable carriage onto which the photographic plate is placed
(see figure 19.14). A projector lamp, followed by an optical system, illuminates a small area of the
plate and light passing through the plate is collected by a microscope. The image produced by the
microscope is projected on to a slit and light accepted by it is made to fall on to a photomultiplier. The
current produced by the cell, therefore, corresponds to the transparency of the area of the plate which
has been selected by the slit.
The same projector in the instrument also provides a beam of light which is made to pass through
part of a neutral density wedge. This wedge is made of glass having a density which increases
linearly along its length. After passing through the plate, this beam is made to fall on to the same
photomultiplier as above.
By a chopping mechanism, the photomultiplier is allowed to see each beam in turn. If the
density of the part of the wedge through which the second beam is passing does not match that of
the investigated area of the photographic plate, an error signal is generated. This is made to control a
servo-system which drives the position of the wedge until a match is achieved between the two beams.
Thus, the density of the investigated part of a plate can be read off according to the position of the
density wedge.
Automatic spatial surveys, such as scans through the length of the spectrum, can be effected by
driving the carriage supporting the photographic plate and linking the motion to a second table which
is made to pass under the density wedge. A pen attachment to the wedge allows the density variations
to be drawn out automatically on a graph carried by the second table.