Astronomy Principles and Practice Fourth Edition.pdf

448 Practical projects
right ascension or declination with tracking of the camera which drifts in declination or right ascension
respectively. If a sufficiently fine grain film is used and care is taken, some stellar spectral lines will be
resolved.
The spectra will appear on both sides of the undeviated and undispersed images, the latter being
used to identify the stars in the field.
When colour film is used to record stellar spectra in this way, blue stars (A and B types) generally
provide three brightness peaks in the blue, yellow/green and red, hinting as to how the colour recording
process is effected. Bright red stars (K type) give only a weak contribution to the blue part of the
spectrum and the stars can be easily spotted on the film.
24.11 Michelson’s stellar interferometer
The largest known apparent angular diameters of stars are of the order of 10 −7 radians and for the
condition of minimum visibility of fringes to occur with the Michelson stellar interferometer, the
separation of the apertures needs to be of the order of a few metres. An exact simulation of this
experiment would be very difficult to achieve in the laboratory but the principle can be demonstrated
by using a variation known as Anderson’s method. Simple apparatus can be used to measure the angle
subtended by an illuminated pinhole.
For the laboratory method, 1 the apertures are in the form of slits shaped like cats’ eyes, at a fixed
separation. They can be made simply by cutting three pieces of brass shim and glueing them together.
The slits and the layout of the equipment are illustrated in figure 24.32.
The double-slit arrangement needs to be placed behind the ‘telescope’ at a distance within the
focal length of the lens. The effective separation of the slits can be altered by moving the slit system
along the optic axis. Suppose that without the double slit in the beam, the image of the artificial star is
at a distance, S ′ , from the objective. If the slits are set at a distance, D, from the image position and
the separation of the slits is a, the effective slit separation, a ′ , as seen from the star, is given by
a ′ = aF D
where F is the focal length of the objective. The effective slit separation may, therefore, be controlled
by varying the distance, D.
Set up a travelling microscope so that it lies on the axis of the optical bench at its end. Without the
double slit in the beam, focus the microscope on the image of the artificial star formed by the objective.
Adjust the tilt of the objective by means of the three screws until the best image is seen with minimum
flare.
Check that by sliding the double slit along the optical bench, the slit face can be brought into
focus without readjustment of the microscope.
Place the double slit on the optical bench close to the objective. When viewed through the
microscope, the artificial star should now appear as a band of light with a fringe pattern across the
band, the fringes appearing much the same as Michelson would have seen them with his interferometer.
Adjust the position of the double slit in the holder until the sharpest fringe pattern is observed.
On moving the slits progressively towards the microscope, the visibility of the fringes will
decrease until they disappear. Continuation of the travel causes the fringes to reappear but with less
separation. After a little practice in controlling the fringe visibility, find the position for minimum
visibility, corresponding to an effective separation of a
min ′ and note the position of D min of the double
slit on the optical bench. Alter the double-slit position and repeat this part of the experiment several
times to obtain a mean value of the slit position which gives minimum visibility. Determine this
1 The idea for this experiment has been taken from Palmer C H 1962 Experiments and Demonstrations (Baltimore, MD: Johns
Hopkins Press).