Astronomy Principles and Practice Fourth Edition.pdf

Spectra 447
Optical
fibre
Transmission
grating
35mm
camera
Collimator
Light tight box
Figure 24.31. A simple solar spectrometer with a fibre optic feed.
given by
R = 500 × 20
= 10 4
suggesting that it should be possible to resolve the spectral lines in the solar and stellar spectra.
In the case of the Sun, if the whole of the disc is used, the spread of half a degree in the rays
incident on the grating causes a blurring of recorded detail in the spectrum. Consequently, it is unlikely
that spectral lines will be identifiable on a photographed spectrum taken by this simple means.
In order to obtain a solar spectrum for an investigation of spectral lines, subsidiary objects must
be used. First an image of the Sun must be obtained, allowing a small part of the disc to be isolated by
a slit. The light passing through the slit must be collimated and the resulting beam must fall onto the
objective grating camera.
An alternative to this system is to use a fibre optic to collect the light from the Sun simply by
pointing its end in the solar direction, the emerging end acting as a slit of the spectrometer system. The
fibre should make entry into a light tight box housing the spectrometer optics. The light cone leaving
the fibre is f/1 and the collimating lens needs to have a matching or closely matching focal ratio. A
redundant 35 mm camera lens may be used for this. Again, depending on the grating space and the
field of view of the objective grating camera, the latter system may need to be set at an angle to the
original collimated beam. The grating need not be attached to the camera which itself might be offset
to the desired angle. In adjustment, the collimator should be at its focal length from the end of the
fibre so producing a parallel beam and the camera should be focused for infinity. The diameter of the
fibre can be taken as the width of an equivalent slit—the height of the recorded spectrum depends on
this same diameter with a magnifying factor given by the ratio of the focal lengths of the camera and
collimator lenses. A schematic layout of the required equipment is depicted in figure 24.31.
For stellar sources, the grating and the 35 mm camera can be used directly but it must be made to
follow the particular star or star field, the exposure perhaps lasting a few tens of minutes. In order to
give the recorded spectrum some height, it is necessary to arrange the ruled lines to be parallel either to