My spectrograph does not have a reference spectrum and does not have a means of determining the precise angular position of the grating. Hence, it can be difficult at times to determine the wavelengths of lines, corresponding to their pixel positions. Even spectrographs which have micrometer-controlled grating angles would not be precise in their estimates, though they are useful for giving very close estimates.»
The image may be that of a slit, onto which an extended object has been focused, or simply that of a star when the spectrograph is used in slitless mode. The former would be used for the Sun (carefully, or off a cloud or other reflective surface), the moon, planets or nebulae. A slit would also be used for testing and calibrating the spectrograph by using an artificial light source such as a Hg or Ne lamp.»
This is an inexpensively-built spectrograph for insertion into the focussing tube of a moderately-sized (amateur’s) telescope to give quality spectra of stars and nebulae.
The best website I have found for this is http://www.astrosurf.org/buil/us/stage/calcul/design_us.htm, “Theoretical Parameters for the Design of a Classical Spectrograph”.
The aim has been to build and use an inexpensive spectrograph. A very useful website is http://astrosurf.org/buil/ which includes plans and advice for building the spectrograph, useful information about the Canon EOS 350D (Rebel) and its subsequent upgrades, and technical data about the spectra that can be obtained.»