wpo - amateur spectroscopy - areas for experiment

astro-optics page text & images [c] Maurice Gavin 1988/2000 - BAAJournal -1988 Dec 9; v98;#4 Observers' Forum

Although the instrumentation described here 12 years ago remains valid today, the modern CCD has revolutionised this field to greatly improve the sensitivity of the spectrograph to record much fainter stars, for example.
The adjacent sketches show how simple stellar and solar spectroscopy can be carried out with modest instrumentation.  Because starlight is so weak only low spectral dispersion is possible and even then is preferably recorded by camera onto fast black-and-white film, such as HP5, Tri-X or Kodak 2415.   For maximum efficiency, use a prism before a camera of 50 to 85 mm focal length.  Using an unguided camera it is possible to record the main spectral classes (O to M) of stars to about magnitude 2.   Arrange spectral dispersion in declination (up and down) so the images will be widened as they trail across the film-plane.  This reveals the absorption lines better.   The stellar spectrograph, with starlight focussed onto a slit, cuts down on sky fogging, permitting lengthy exposures and maximum data.

For solar work there is plenty of light, I even for the eye, so the grating is preferred for its high spectral dispersion.  Many fainter solar lines are still unidentified and may provide the dedicated amateur with rewarding work.  To see the Sun's white-light image transformed into hydrogen light is fascinating.   Careful baffiing is needed throughout these projects with all non optical surfaces painted matt black.

Stellar spectra via gratings and prisms

 1 - Grating spectrograph for Newtonian. (See Sky and Telescope, Feb 1974.) This is the classic Cassegrain spectrograph adapted for the Newtonian via a Barlow lens to give a slower f-ratio to the system.  Science can be conducted with an instrument like this if well constructed.

 2 - Field spectrograph for Newtonian. This uses a small piece of transmission grating set between clear glass before the telescope focal plane. As the star image (zero order spectra) is also recorded, it is offset from centre so one of the 1st order spectra falls onto film.  Little of the light goes into the spectra so exposures are lengthy but limited by sky-fogging.

 3 - Objective prism spectrograph. This simple but classic device uses a right-angled 45o prism [i.e. diagonal] set in a lenshood before a regular camera.  A narrower-angled prism will serve just as well.  A large star field can be covered with low spectral dispersion to within 3 to 4 magnitudes of the camera's normal limit.  Skyfog limits exposures.

 4 - Objective grating spectrograph. This repeats item 3 but uses a reflective grating instead of a prism. With higher dispersion, less faint stars will be recorded before the film is sky-fogged.
 


Solar spectroscopes using reflective gratings

 5 - Single lens spectroscope with needle slit. (See BA A Journal - April 1981.)  This uses a single lens for all imaging (except eyepiece) with an ordinary sewing needle as a reflective slit.  The effective slit width is only a fraction of the needle diameter at f/30 i.e. is dependent on the f-ratio of the imaging optics.  The top left mask is placed inside adjacent to the lens.

 6 - Stigmatic reflective spectroscope. (See Modern Astronomy - July & Sept 1973.)  This uses opposing portions of a single spherical concave mirror as collimator and viewing telescope respectively - with the grating on the optical axis and the slit and eyepiece/detector both equally off-axis.  A simple lens concentrates the Sun's image onto the slit to brighten the spectrum.  A small flat (or star-diagonal) is used to direct the final image out at 90o otherwise the observer's head would obstruct the slit.

 7 - Spectrohelioscope/ graph.  This instrument is used to view the Sun's disk in a particular wavelength usually Ha.   Although of simple construction it must be built with care on a rigid base if success is to be expected.   It uses a small (20-mm faces) right-angled prism to laterally reverse the single reflection off the grating.   Thus the two slits S1 and S2 are carried in unison on a single oscillating platform to scan the Sun's image.  By tilting the prism slightly, in any of the three axes, small misalignments of S2 can be accommodated.

The Ha image is very dim and must be viewed with low power in near darkroom conditions.  Image resolution will be low due to the small scale of the instrument.  Simple plano-convex or meniscus lenses will do for OG and autocollimator, if of good quality [free from spherical aberration], as the light used is monochromatic. The optical axis of the OG coincides with S1; the autocollimator axis is midway between S1 and S2.

Inexpensive diffraction gratings.   My sample of these £15 replica gratings, seen advertised recently in astronomical journals, measures 25 mm x 25 mm x 2 mm  and is on glass with a bevelled edge.  It is ruled with 600 lines/mm and blazed to reflect about 8O% of the light into one 1st order spectum for efficiency.   Although quoted of B quality on the base of about 1-wave accuracy,  they represent excellent value and are ideal for a simple solar or stellar spectroscope.  Paton Hawksley Electronics Ltd, Wellsway, Keynsham, Nr Bristol can supply gratings/details including transmission types.

[c]Maurice Gavin - Worcester Park - Surrey - UK