Simple Low Resolution Spectroscopy of Bright Stars
Using a Digital SLR Camera
Note: This describes the Star Analyser 100 used with a telephoto lens. A similar result can be obtained using the Star Analyser 200 with the shorter focal length zoom lens supplied as standard with most DSLR. More about the SA200 used in this application here
This technique uses a diffraction grating mounted in front of a camera fitted with a long focus lens.
It was developed as a simple way to improve the resolution of low dispersion spectra of bright objects using the Star Analyser diffraction grating, compared to the standard configuration.
In practise a resolution of typically 15A can be obtained, some 2-3 times better than the alternative simple configuration with the grating mounted in the converging beam between telescope and camera.
The peculiar eclipsing binary star Epsilon Aurigae was the original application, (Click here for high resolution spectra of Epsilon Aurigae) however I suspect it has significant potential as an educational tool, providing the simplest possible introduction to practical astronomical spectroscopy and the spectral classification of stars.
(Modified 12 May 2009 to include results using a fixed camera on a tripod)
Using an Objective Prism (a wedge prism placed in front of the
telescope aperture) is a long established way of
can be used with a
by even a coarse
Using a camera lens instead of a telescope however gives a more reasonable dispersion and the smaller objective size of a typical camera lens means small affordable but efficient 1.25 inch diameter gratings such as the Star Analyser can be used.
Of course using such a small aperture restricts the technique to brighter objects but because the light from the star is already collimated, it potentially can give significantly improved resolution (typically 3x) compared with the alternative simple technique of placing the grating in the converging beam between the telescope and the camera.
Making the most of this potential increase in resolution requires a higher dispersion than is normally used with the Star Analyser, which means a larger camera detector if the full spectrum plus zero order is to be imaged. (Useful to aid focusing and calibration, particularly for beginners). A large format monochrome astro camera would be an ideal but expensive option. Alternatively Digital Single Lens Reflex cameras such as the Cannon 350D etc perform well under astro imaging conditions and can be used successfully in this application, though with restricted wavelength range towards the IR unless the internal IR blocking filter has been removed or replaced.
The choice of lens focal length depends on a number of factors:
The dispersion increases proportionally with focal length but so does the size of the star image so the resolution is largely independent of focal length.
A shorter focal length means a shorter, more concentrated spectrum and therefore fainter objects can be recorded.
With too short a focal length however, the star image will be undersampled (ie the star image will be smaller than a single pixel, remembering that for a colour camera like a DSLR, the effective pixel size is larger than a single pixel due to the Bayer pattern of pixel colour coding). Undersampling can produce severe artifacts in the spectrum, particularly with colour cameras.
A short focal length also means a greater area of sky is imaged giving a higher sky background brightness and an increased risk of interference from other stars and their spectra.
(Note that using this technique with short focal lengths can be useful for recording spectra of fainter diffuse objects such as comets as seen here bottom of the page. In extreme cases it also be used to cover a wide field at ultra low resolution as in this meteor spectrum)
Too long a focal length and it will not be possible to fit the star (zero order) and spectrum in the frame. Tracking also becomes more critical at higher focal lengths.(Indeed, with modest focal lengths it is possible to produce "drift spectra" with a fixed camera)
For these tests I used a 100 lines/mm Star Analyser grating with a Canon 350D camera fitted with a 75-300mm zoom lens at 200mm focal length which gives a dispersion of about 3.5A/pixel. It was piggyback mounted on my main telescope. ((It is also possible to used a fixed camera on a tripod, orientating it so the star drifts perpendicular to the dispersion direction.)
To mount the grating, I cut a hole in a lens cap and
screwed the Star Analyser into it. To make it easier to
rotate the grating
the focusing adjustment,
I added a
blank rotatable filter
filter set) between the lens cap and the
alternative could be to adapt a
cell the right
size for the
This could be
orientated by screwing the
and out. The small
produced some vignetting but provided the calibration
reference star is placed at the same position as the
target this is not a
(Note flats cannot be used in the conventional way with slitless
spectroscopy like this. See here for some tips on how
to avoid flat field defects in Star Analyser
A typical single 30sec exposure of Epsilon Aurigae
(7th full size)
Recording the Spectra
If you are using a DSLR camera set it to record RAW images.
(unlike jpegs, these are uncompressed, have a greater bit
depth and do not have
As well as your target star spectrum you should record darks and at least once (and ideally every session), a calibration star spectrum (a bright spectral type A star is best eg Vega, Altair, Castor, Regulus, preferably at similar elevation to your target). It is a good idea to take the calibration star spectrum first, using it to get the focus and grating orientation correct and then move to the target without disturbing the settings.
Start by recording an image without the grating in place. Position the star in the centre left of the frame, leaving enough room to the right for the spectrum. Try to place the star at the same position in the field each time to minimise flat field effects (A zoom lens is handy here as you can locate the star and position it roughly before zooming in.) Check that there are no faint stars close to a line running horizontally from the star in the region where the spectrum will fall and no bright stars on the same line within the frame or within a frame either side which could potentially produce spectra overlapping with the wanted spectrum. If there are, reorientate the camera so that the horizontal line is free from such potential interference.
Note I f you are using the camera on an undriven mount, orientate the camera with the horizontal axis of the camera field pointing at the celestial pole. (ie orientate the spectrum along the Dec axis). That way any star trailing will occur perpendicular to the direction of the spectrum. If there is potential interference from other stars, you will be unable to rotate the camera to avoid them but you could try running the spectrum from left to right instead by rotating the grating 180 deg.
A "drift spectrum" of Altair taken using a fixed camera and 200mm lens on a tripod
Continue to Processing the Spectra >>>