Astronomy Principles and Practice Fourth Edition.pdf

Image photometry 321
a small area of any selected part of a plate can be illuminated uniformly. The size of this circular
patch is controlled by an iris diaphragm. After the beam has passed through the plate it is directed to
a photomultiplier. A chopping system allows the strength of the beam to be compared with a standard
beam which is obtained from the original lamp. The strength of the beam, after passing through the
photographic plate, depends on the size of the diaphragm and the density distribution of the area of the
plate which has been isolated by this aperture. When a star image has been adjusted by movement of
the carriage to be at the centre of the aperture, a balance between the compared beams can be achieved
by controlling the size of the diaphragm.
It should be obvious that an image produced by a bright star will be very dense at its centre and
will have a comparatively large disc. In order to allow a certain amount of light through this image
to provide the balance, the diaphragm must have a large diameter. At the balanced or null condition,
the diameter of the diaphragm is read off a scale. A calibration curve must first be obtained, by using
standard stars, before the magnitudes of other stars may be determined. The uncertainties of any
determined magnitudes are typically ±0·05 mag but, in some cases, it is possible to improve on this.
The ease of working the photometer is improved in larger instruments when the part of the plate under
investigation and the image of the diaphragm are projected on to a screen, allowing the star field to be
identified quickly. The X, Y and diaphragm diameter scales may also be projected and an electronic
record of these parameters may be available for automatic dispatch to a computer. Automatic reduction
instruments, as already described in section 19.2, may have a facility for determining brightness values
as well as positional coordinates.
19.8.2 Photoelectric photometry
In the discussion of the photomultiplier (section 18.11), it was pointed out that this detector, in its
simplest form, responds only to the total amount of energy which falls on to its sensitive area. If a
brightness picture is to be built up of an extended image or series of images, it is necessary to look at
each picture point in turn. If the stars in a field are to have their brightnesses measured photoelectrically,
it is necessary to look at each star one by one and, for this purpose, a diaphragm is placed in the focal
plane of the telescope. This allows any single star to be isolated and keeps to a minimum the amount
of background sky light entering the photometer.
Whereas in the case of photographic observations the plate is placed in the focal plane of the
telescope, an in-focus image must not be allowed to fall on the photocathode. Any photocathode has
a detecting area which is non-uniform in sensitivity and any image wander over its surface caused
by poor telescope following or image motion due to seeing will generate fluctuations or noise in the
output signal of the cell. To prevent such noise, a field lens or Fabry lens is provided so that the
collector aperture is focused on to the cathode. A lens is chosen so that the whole of the sensitive
area of the cathode is filled with the light for detection. It can be seen from the ray diagram of the
simple photometer depicted in figure 19.10 that by imaging the collector aperture on the sensitive
area, movement of the telescope’s direction related to the star or agitation of the star’s position caused
by seeing does not produce movement of the patch of light over the cathode. In fact, there should
be no change in the detector’s output until the deviation of the telescope’s direction is such that the
diaphragm in its focal plane begins to cut off the star image. Intensity scintillation, giving fluctuations
of the illumination of the telescope aperture, is not removed by the expedient of the field lens and is
apparent in the cell’s output.
Provision is usually made for the insertion of colour filters which limit the spectral passband.
For ease of operation, retractable viewers are usually provided before and after the diaphragm so
that the star field can be studied and particular stars chosen for measurement. As we have seen
in section 18.11, the output from the photomultiplier is usually monitored by DC amplification or
photon counting. A series of measurements involves recording the output signal over short but regular
integration times