Using the Rainbow Optics Star
Spectroscope for Visual
Observing
Paul Bakke
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
Standard
eyepiece
Diffraction
grating cell
Cylindrical lens
Grating cell mounted
to eyepiece
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
Longer
eyepiece
focal length
Wider
spectrum
Shorter
spectrum
Increases length or
“dispersion” of
spectrum
Grating
mounted to
star diagonal
My setup:
10-inch, f = 10 SCT
25 mm eyepiece
Photo taken with hand-held
Canon Power Shot at ¼ sec,
f3.5, 1600 ISO
Source: Jack Martin, London, England
12-inch Dobsonian with Rainbow Optics Star Spectroscope (2-piece model)
Photo adapted from http://www.starspectroscope.com/
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
Paint Shop Pro
Bonus:
Use graphics software to make
intensity vs. color histogram
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)
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
More challenging
Wolf-Rayet stars (type WN, WC)
– Emission lines (blue)
– HD 192163 in Cygnus (illuminates Crescent
Nebula, NGC 6888), mag. 7.7, type WN6
– HD 192641 in Cygnus, mag. 8.2, type WC7
Fact: ~½ of all Wolf-Rayet stars brighter
than magnitude 9 are in Cygnus!
More challenging
Carbon stars: absorption bands in blue &
violet due to carbon molecules
– Y Canum Venaticorum (“La Superba”)
– U Hydrae
– 19 Piscium
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
HAVE
FUN!