I am trying to make an educational echelle spectrograph using easily available materials, basically in a cardboard box. An echelle grating (36 lines/mm) needs another prism to do a cross dispersion and generate a two dimensional spectrum like the following (taken from a astro website).
The design I have is a very "crude" version of the following taken from Astro website. The key difference is that I want to use a slit and a fluorescent lamp as a light source instead of a fiber and the camera is an ordinary digital camera rather than a fancy DSLR. http://www.astrosurf.com/buil/echelle/first.htm
One can readily generate this type of spectrum using any white LED without any collimation. Simply illuminate the echelle and view it via prism. The problem starts with any other light source such as a fluorescent lamp. The requirement to generate a successful echellogram is to have collimated light.
What is special about LED that it does not need collimation? Even a cell phone light produces a beautiful echellogram, exactly as pictured above.
As stated above, problem is with other light sources such as a fluorescent lamp or street light. Does anyone know of a good trick for decent collimation of light from slits made in a cardboard box? Light spreads out a lot from the slit as a wide cone of light.
If we make a simple slit with the help of blades on a cardboard (approximately 0.5 mm wide, 0.8 cm long) and if we place a fluorescent lamp very close to the slit and a lens with its focus at the slit, collimation is poor. The lens is a 25mm Dia. x 100mm Focal length, Achromatic Doublet Lens. By poor collimation, I mean that the echellogram shows repeating images which is a sign that the light is not well collimated.