1. Using the Rainbow Optics Star Spectroscope for Visual Observing
Paul Bakke

2. Items to discuss:
How Rainbow Optics Star Spectroscope works for visual observing
Optics used
Objects observed
First attempts using a camera with the Rainbow Optics Star Spectroscope

3. Standard eyepiece
Diffraction grating cell
Cylindrical lens
Grating cell mounted to eyepiece

4. Optical Considerations
Spectrum width proportional to Exit Pupil
Best: 2 to 3 mm (useful range 1.5 to 4 mm)
Spectrum length controlled by:
Eyepiece magnification
Distance from grating to eyepiece lens
Exit Pupil = Eyepiece Focal length
Telescope f-ratio
Wider spectrum
Longer eyepiece focal length
Shorter spectrum

5. Increases length or “dispersion” of spectrum
Grating mounted to star diagonal

6. My setup:
10-inch, f = 10 SCT
25 mm eyepiece

7. Photo taken with hand-held Canon Power Shot at ¼ sec, f3.5, 1600 ISO

8. Source: Jack Martin, London, England
12-inch Dobsonian with Rainbow Optics Star Spectroscope (2-piece model)
Photo adapted from

9. Method:
Center spectrum in field of view
Turn off siderial drive
Take “long” exposure photo (< 30 sec.), just enough to “smear” the image
Rotate and crop using software

10. Use graphics software to make intensity vs. color histogram
Paint Shop Pro
Bonus:
Use graphics software to make intensity vs. color histogram

11. What to observe First-magnitude stars (using cylindrical lens)
Main-sequence (class V) stars are best
Giants & supergiants have less prominent absorption lines,
Cooler spectral types are good even at larger luminosity class
Spectacular: Vega (A0 V), Sirius (A1 V), Betelguese (M2 I)

12. Absorption lines Hot (type A, B) stars:
Hydrogen beta, gamma (blue) are easy
Hydrogen alpha (red), delta (violet) are difficult
In type G & cooler stars: G (blue – CH molecule), D (orange – Na), b (green- Mg)
Molecular bands (TiO) in M stars

13. More challenging Wolf-Rayet stars (type WN, WC)
Emission lines (blue)
HD in Cygnus (illuminates Crescent Nebula, NGC 6888), mag. 7.7, type WN6
HD in Cygnus, mag. 8.2, type WC7
Fact: ~½ of all Wolf-Rayet stars brighter than magnitude 9 are in Cygnus!

14. More challenging
Carbon stars: absorption bands in blue & violet due to carbon molecules
Y Canum Venaticorum (“La Superba”)
U Hydrae
19 Piscium

15. Rainbow Optics Star Spectroscope
Spectroscopy at its most inexpensive & “low tech”
Pleasure of visual astronomy
Simple photographic possibilities
Activities in light-polluted or moonlit sky

16. HAVE
FUN!