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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge

Supplement: Build Yourself a Simple Hand-Held Spectrograph


Completed "MkIV" Hand-Held Spectrograph

In class during Lecture 25, I mentioned how I had constructed a simple hand-held spectrograph using one of the diffraction grating slides and a cardboard tube, and then used that to examine various light sources to determine how they worked.

Below is a set of instructions for how to construct your own hand-held spectrograph.

WARNING!!!

NEVER, EVER POINT THE SPECTROGRAPH AT THE SUN!!!
THE SUN IS DANGEROUSLY BRIGHT, EVEN AT WAVELENGTHS YOU CAN'T SEE, SO YOU COULD SERIOUSLY DAMAGE YOUR EYES.

Materials

The materials you'll need to make the simple spectrograph are as follows. All are readily available.
  1. A 1.5-inch diameter cardboard tube. A small mailing tube works best, but in a pinch the tube from an empty role of paper towels will do. The thicker the walls the better, since it will make for a sturdier spectrograph. I used a mailing tube that came with a small poster, about 19-inches long.

  2. A roll of black masking tape or black electrical tape.

  3. A piece of holographic diffraction grating film mounted in a 2x2-inch slide. You can buy these from Edmund Scientific for about $10 for package of 15:
    scientificsonline.com (item #3054510)

  4. A thick 3x5 card

  5. A sheet of thick cardboard to use as a cutting pad.

  6. Some Tools:
    1. #2 pencil
    2. Sharp pair of scissors
    3. Metal straight-edge ruler
    4. Fine-blade xacto knife or other very sharp razor knife.

Assembly Instructions:

Step 1
Mount the grating on one end of the tube using tape. Note the direction of the long axis of the grating, this defines the "dispersion axis" of the system. The pictures below show how the grating is mounted:

Step 2
  1. Set the 3x5 card face down on a table, and place the open end of the tube against it and with a pencil draw a circle around the tube on the card.

  2. Place the 3x5 card onto the sheet of cardboard to protect the table. In the middle of the circle you drew, cut slit 1-inch long and 0.5mm wide in the paper, using a sharp razor knife and a metal ruler to make the edges as clean as possible.

    Note, this may require a few tries to get it right. My current spectrograph took 3 cuttings before I got it clean enough.

  3. Cut out the circle with the slit

  4. Aligning the long axis of the slit with the *short* axis of the grating, tape the slit to the open end of the tube, covering as much of the aperture with black electrical tape.
Finished spectrograph slit

Using the Spectrograph

WARNING!!!

NEVER, EVER POINT THE SPECTROGRAPH AT THE SUN!!!
THE SUN IS DANGEROUSLY BRIGHT, EVEN AT WAVELENGTHS YOU CANNOT SEE, SO YOU COULD SERIOUSLY DAMAGE YOUR EYES.

To use the spectrograph, point the slit at a light source like an incandescent light or a fluorescent light and look through the grating, with your eye a few inches away from the grating. Align the slit up/down with the source so you can see it clearly through the slit, then look either to the left or right about 30 degrees, you'll see the spectrum of the source.

Examples of spectra (best pictures I could snap, I haven't designed a proper camera rig for it yet) are attached:

Fluorescent Lamp

Emission-line spectrum from a standard flourescent light fixture. This shows the sharp, bright emission lines of Mercury plus broad bands from the organic phosphors coating the inside of the tube.
[Details: The lamp is a Philips TL70 ALTO, a low-mercury lamp using Philips' proprietary HI-VISION triphosphor coating.]

Incandescent Lamp

Continous spectrum emitted by a 100W incandescent light bulb, showing a continuous spectrum from the hot tungsten filament.
[Details: The lamp is an Osram/Sylvania 60W/120V SoftWhite Incandescent bulb with CC-8 Tungsten Filament. The "SoftWhite" coating is a thick coat of white silica powder.]

Street Lamps

Compare the spectra of streetlights that use high-pressure sodium (HPS) and low-pressure sodium (LPS) lamps. HPS lamps are found in many downtown areas, and appear whitish-yellow in color and are very bright. LPS lamps are used in mainly in residential areas where very bright lights are not needed, they have a reddish-yellow color and are not as bright. LPS lamps also tend to be long and thin, while HPS are round. The International Dark-Sky Association has some online slides illustrating many aspects of light pollution and light sources.

[Note: I have long since lost the original source of the graphic above. If it is yours, please contact me so I can put on a proper citation (or take it down if you do not want it posted on this website.]

Other Sources of Light and Information

Try looking at neon signs of various colors, an LCD or CRT computer monitor, a plasma or LCD TV, etc. What kinds of lines can you see?
Representative Emission-Line spectra of various elements

Emission Spectra from various light sources. This is a wonderful collection by Ioannis Galidakis, including details on his double Amici prism spectroscope.

History, Science & Technology of Light Sources by M.F. Gendre.

Again NEVER LOOK DIRECTLY AT THE SUN WITH THE SPECTROGRAPH! I can't emphasize this enough.

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Updated: 2007 April 20
Copyright © Richard W. Pogge, All Rights Reserved.