SPECTROSCOPY ON A BUDGET

In this age of kilobuck devices and gadgets, it's easy to forget that
many scientific instruments that will help us understand the world
around us in principle are easy to build and afford. Presented here is
a design and operating instructions a  spectrograp, the total cost of
which can, as it was in the author's case, be less than $10.

-=-=-=-
Overview...

Spectroscopy is one of the most enjoyable pursuits in astronomy and
physics. The lure of the intense pure colors of spectra and the secrets
about the light sources hidden in the absorption and emission lines
beckon those of a curious bent into a new field of study. Upon
embarking on such a study, one quickly learns that spectroscopy seems
to be a rich person's sport. But in fact, it doesn't have to be that
way. If one has just average skills with hand tools, a perfectly
serviceable astronomical spectroscope/graph can be constructed for a
nominal cost.

First it helps to know a little bit about spectra and associated
instruments. What is presented here is only the most brief introduction
and the interested reader should peruse the literature listed at the
end of this paper.

Spectra are formed when light passes through any wedge of transparent
material (for that wavelength) that is a different refractive index
compared to the surrounding medium (air, water etc.), or when it is
diffracted around an intervening object or reflected off a reflective
diffracting medium. The most common means of making spectra is to pass
light, sunlight for example, through a prism. This will produce a
rainbow of colors in a band from red to violet for most eyes. Note the
intensity of the pure colors.

While prisms will make this colorful band or spectrum (note that a
single one is a "spectrum" and many are "spectra") for a large object
like the Sun, this spectrum is a smear of individual spectra from every
minute point that makes up the object. If you make a spectrum of the
Sun this way and project it on the ceiling in a darkened room it will
simply be this bright band with a red tint on one end and a blue on the
other. Interpose piece of paper with a slit, some small fraction of an
inch in width, cut into it or use a small hole and the bright band will
now be a spectrum. If you have made a good slit you may even see a dark
line or two running perpendicular to the 'dispersion'. Your device is a
spectroscope, albeit a very crude one. The term "scope" denotes the
ability to observe the spectrum with the eye. If you find a way to
attach a camera it becomes a spectrograph and you have begun to study
spectroscopy.


Objective Prisms...

The most simple form of astronomical spectroscopy is objective prism
spectroscopy. This consists of putting a large prism in front of a
telescope and looking at the sky through that. Everything going into
the telescope will first have to pass through the prism so that all you
will see are spectra. Instead of making the slit or pinhole mentioned
above with the Sun we will rely on the tiny size of the stars to act as
a pinhole.  It works very well and is a  technique used by professional
observatories for decades, in one form or another. The professional
telescope with the most of these objective prisms is the Burrell
Schmidt Telescope owned and operated by Case Western Reserve Univ. on
Kitt Peak. It has 5 prisms, four of which can be used in various
combinations, all of which can be used singly. The dispersion or
spreading out of the light into the spectrum, is done in a north-south
orientation or along a line of right ascension. When using this as a
spectrograph the widening of the spectrum can be done by varying the
telescope drive rate.  It is this kind of spectroscope/graph that is
the easiest to make and use.




Building an objective prism spectrograph...

A very effective spectrograph of this type can be made by any amateur
astronomer with reasonable abilities with hand tools. The items needed
will be:

1 camera and telephoto lens (~250-500 mm) as small an f/no. as possible

1 large prism that can cover the lens of the telephoto.

misc. wood and metal bits that can make your mounting.


The first and most difficult part for most is getting a prism. For this
you will have to prowl the surplus houses either on the Internet, in
magazines, or in your local town. AstroMart, C&H Sales and EBay are all
good places to start your search for a prism on the Internet. It need
not be a thing of beauty, which will help to keep the cost down. I got
my prism from a surplus house for $2. It is a right angle prism with
2"x2" faces and has a chip in it, hence the very reasonable price.
Right angle prisms are the most inexpensive.  This chip has no effect
on the performance in this application. Most objective prisms have very
slight angles and rely on the long focal length of the telescope to
spread the light out into a nice, well resolved spectrum. We don't have
the luxury of a long focal length here so we need more dispersion to
spread the light out. This means a greater wedge to the prism. So we
will use one half (actually a bit more) of this prism making use of one
of the 45 degree angled halves to be our objective prism so you want
these smaller faces to be a little bigger than the diameter of the
front lens of your camera.

I used an old Praktica camera body and attached it to a 270 mm achromat
aligned and mounted in a piece of PVC tubing. The lens cost about $8
and the camera body was free, a throw-away that was given to me since
only the "bulb" position worked on the shutter. The camera and lens
should be mounted to a board and the prism fashioned in some manner so
the hypotenuse is towards the camera lens entrance with one of the
smaller faces of the prism facing the sky. As fig.1 shows, you will
want to mount your camera a little higher than the prism so all the
light goes into the lens. Keep things away from the front of the prism.
Your spectrograph will be looking off at about a 23o angle as shown in
the figure.

What is shown in all the figures with this paper are schematic only.
You will have to use your creativity and materials to best advantage to
accomplish this project. The photographs of this spectrograph first
built in 1984, show what I did, but perhaps you have a better way or
have different materials.

Mount the spectrograph to a telescope mounting. There are as many ways
to do this as there are telescopes. Keep in mind the pointing
difference between the spectrograph and the telescope. Mount it so the
dispersion or spread of the spectrum runs north-south.  You may wish to
mount the device so it physically points in the same direction as the
telescope but then you will be constantly making the offset for the
prism. I found it much more convenient to make a piggy back mounting on
a large hinge that was adjustable so I could set it to be looking at
the same object as my telescope. The advantage is that I always know
what is in the spectrograph and when.


Now to work...

The first thing you have to do with a new spectrograph is determine the
focus. While you may know the infinite focal position for the camera
lens,the introduction of the prism will change this to some degree.  It
will likely require some test exposures to accurately determine the new
infinite focal position. An excellent star for this test is Vega as it
has good strong balmer lines. I recommend using a film like T-Max 400
and processing it for good contrast (8 min @ 70 deg. F, with 5 sec.
agitation every 30 sec.). Take short exposures, and shift the focus and
telescope pointing between them so you have a series of unwidened
spectra on your frame. You will notice one thing right off as you
examine them under magnification. One position will work best for the
red end and the other will work better for the blue end. (The red end
is generally the brighter end, even with a blue star like Vega because
of film sensitivity.) Select the one that as the best focus in the
middle or a little to the blue end, between H-Beta and H-Gamma. If you
don't know the hydrogen series, you might want to get familiar with it
at this point. Just learn which lines are which, where the "Balmer
Jump" is and what the series looks like. Also take time to note where
the H&K lines of calcium are. They become important when determining
spectral class.

Next you should do this same focus test but this time widen the spectra
by an amount that will make them half a millimeter or so wide on the
negative. This time you will determine focus by looking at the lines of
hydrogen in the spectrum. You will now plainly see the effect mentioned
above, the best focus will move across the spectrum as you change
focus. Again, pick the setting with the sharpest H-beta or H-gamma
line.

Now that you know where the good focus is, set your lens to best focus
and take exposures of a number of different stars. Try to find a good
selection of bright stars like Vega-Deneb-Altair and use the sidereal
drift of the sky to widen the spectra. Then select some fainter ones of
varying spectral type too.  (For fainter stars you will have to use an
offset R.A. drive speed from the sidereal rate.) The object here is to
get a selection of sample images for the first roll of film, from which
you can make a time/brightness/spectral type/exposure/widening plot so
every image will be a useful one in the future, that's the goal. You'll
undoubtedly fall short of this and it will take some work to get it
right the first time. After your first roll of film you should have a
selection of stars from 0-3rd mag. or so. This will be enough to
determine your best exposure time. You should plot the results in some
manner for use at the telescope later. I did a plot of magnitude versus
time with separate plotted lines for the B & A stars and another for
the M and later stars. Between these two curves are the F & G stars and
above it the O stars. This plot covers only the best exposure for an
unwidened spectrum. That should be all you need to know. Determine what
the width of the stellar image is in your camera and use that number as
your step size in widening the spectrum. Thus when looking in the
eyepiece as you widen, take an exposure of the star in one position,
say 20 sec., then step over the predetermined distance and take another
and keep repeating this procedure until you have the required width.
Another time saving technique is to do a portion of the widening at the
telescope and stack that spectrum several times in the darkroom or on
the computer. This will also help to avoid sky fogging.

With my system, a 270mm lens (f/6.2), I find that Vega is perfectly
exposed using sidereal drift alone. But Arcturus needs 5 sec. exposures
at each step across the film. I keep widening to 0.1-0.3 mm depending
on the object with my longest exposure being around 10 min. total. You
will have to make very sure that your system has no sag in it. This
would be disastrous to detecting spectral lines. Red stars, at least
with this film and my optics, take about 3-5 times longer to expose
well as the Vega/Antares comparison indicates. Carbon stars, like Y CVn
may take even longer.

A First Program...

Now that you have the technical details out of the way you will want to
do something of value with this contraption. One of the first things
you should do is take images of every bright star you can shoot with an
eye towards getting a couple from each spectral class.  You will need
an atlas of such spectra and there are some available on the web, found
by searching for "stellar spectral classes" or some permutation of that
phrase. Some of these are from university courses in astrophysics and
thus well suited to this task.  This project will likely keep you busy
for some time, but you need to make a good spectral atlas with your
instrument, using your techniques, at your location. This can then be
used for comparison with unknown objects in the future.

Learn how to do the step widening with your system and try to do some
widening at the telescope and some with software in the computer by
replicating blocks of your spectra. This will give you a good looking,
easy to read spectrum. You will soon learn the pitfalls of poor
signal-to-noise as you try to pull a workable spectrum out of a poorly
exposed image. This will be good experience.

Your ability to record faint objects will only be limited by your sky
and patience. There is no reason why you could not spend hours making
one spectrum if your sky were good enough.

After you have assembled a good catalog assemble the images into a form
that shows the different spectral types, their features and
intercomparisons. If you get good at this you may even be able to
discern different luminosity classes, but be forewarned, this usually
takes a slit spectrograph. Take time and do this well because it will
be your reference source. A mistake or sloppiness here will lead to bad
results later.  You will find that this process of making an atlas,
will never end. As you discover unusual objects on your images
(intentional or otherwise) you will constantly add to this database.


The sky's the limit!

Now you can either go on a search for particular objects or sit and
wait for them to come to you.  When you hear of a bright nova, get a
spectrum. Comets like Hale-Bopp can present a wonderful object for
study. The emission spectrum of this object, as with many gassy comets,
changed from night to night.  Take spectra of these objects from night
to night and stack them into a time sequence to show changes.  This can
also be done with brighter variable stars or those that are bright for
at least part of their cycle.

Once you have a device like this in your astronomical arsenal, you will
begin to look at the sky in a whole new light!





GLOSSARY...

Absorption lines - The dark lines that perpendicularly cross an
absorption spectrum.

Absorption spectra - Spectra where the lines are black on a bright
spectral background (continuum)

Continuum - The bright background made by a prism or grating when
looking at a normal incandescent light bulb. This is often the
background on which spectral dark or bright lines are laid when looking
at things like the Sun or Moon (dark lines) or a fluorescent light
(bright lines).

Continuous spectra - Spectra where there are no lines, only a smooth
gradation through the colors from red through violet. Most easily seen
in the spectrum of an incandescent light bulb.

Dispersion - The direction of the spreading out of white light into a
spectrum.

Emission lines - The bright lines that perpendicularly cross an
emission spectrum.

Emission spectra - Spectra where lines are bright against a dark or
weak background of spectral colors.

Grating - A piece of glass or plastic on which lines of a particular
shape are scratched or laid on such that they break light up into the
spectrum.

Prism - A large piece of glass in one of a number of geometrical shapes
that can, in certain orientations, break up light into a spectrum.

Spectra - The rainbow of colors produced by a prism or grating (a CDROM
acts like a grating). One is called a spectrum, many are spectra. There
are no such things as spectras, and spectrums.



*This paper is copyright (c) Richard E. Hill 2000
    
                                    
                                    
                              BIBLIOGRAPHY

Birney, D.S., OBSERVATIONAL ASTRONOMY, Cambridge Univ. Press, New York,
1991. [Best, easily available book for information on spectral
classification.]

Cutting, T.A., MANUAL OF SPECTROSCOPY, Chemical Publishing Co., Inc.,
N.Y., 1949.

Hearnshaw, J.B., THE ANALYSIS OF STARLIGHT - One hundred and fifty
years of astronomical spectroscopy, Cambridge Univ. Press, New York,
1986.[An excellent book on the history of spectroscopy and spectral
classification.]

Houk, N., and Newberry, M.V. A SECOND ATLAS OF OBJECTIVE-PRISM SPECTRA,
Univ. of Mich., Ann Arbor, MI 48109-1090 ($6) [A superb reference
source at a very reasonable price!]

Sawyer, R.A., EXPERIMENTAL SPECTROSCOPY, Prentice-Hall, Inc., 1946.
(Reprinted by Dover, 1963.)

The prism assembly in it's raw state.



More images of the finished spectrograph.








This is an image showing a pre-brightening spectrum of Delta Sco. What can be seen here is how the hydrogen lines are mostly filled in by emission in the early stages of the "outburst".