The lens may then be also dispensed with, and the whole collimator becomes unnecessary if the luminous source is narrow and at a great distance, as for instance in the case of the crescent of the sun near the second and third contact of a total solar eclipse.
If a source of light be placed behind the auxiliary slit a parallel beam of light will pass within the collimator and fall on the slit the width of which is to be measured.
The platform carries the two slits, the collimator and camera objectives and the prism-train.
The spectroscope is then moved parallel to itself, admitting to the collimator slit light from all parts of the sun's disk.
The collimator of a spectroscope should be detached, or moved so as to admit of the introduction of an auxiliary slit at a distance from the collimator lens equal to its focal length.
The collimator has a vertical slit at its outer end, the width of which may be regulated by a micrometer screw; in some instruments one half of the slit is covered by a small total reflection prism which permits the examination of two spectra simultaneously.
In grating spectroscopes both plane and concave gratings are employed in connexion with a collimator and observing telescope.
Suppose a fixed image of the sun to be formed on the collimator slit of this spectroscope, and a photographic plate, with its plane parallel to the plane of the solar image, to be mounted almost in contact with the camera slit.
The slit of the collimator confines the light to a nearly linear source, the beam diverging from each point of the source being subsequently made parallel by means of a lens.
In diameter on the collimator objective, as its focal length is 60 in.