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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  

Summary
(Spectroscopy does not get much simpler than this!)



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.


The Technique
(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 producing low resolution spectra.  A similar technique can be used with a diffraction grating, but large diameter gratings (and objective prisms) are expensive and the dispersion produced by even a coarse 100 lines/mm grating is unmanageably high with a typical telescope focal length. 

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)

The Setup



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 independent of the focusing adjustment, I added a blank rotatable filter cell (from an inexpensive crossed polarising filter set) between the lens cap and the Star Analyser. An alternative could be to adapt a blank photographic filter cell the right size for the lens. This could be orientated by screwing the assembly in and out. The small grating aperture produced some vignetting but not so severe that it could not be removed using a flat field.

A typical single 30sec exposure of  Epsilon Aurigae (7th full size) 
Click on the image to view full size (1.5Mb)

Flat field


You need some means of operating the camera without getting camera shake and viewing the images immediately in full resolution to check for positioning, orientation and focus. I  control the camera and view the images on a laptop using a freeware program called  Focus Assist but you will  no doubt have your own favourite method if you are already astro-imaging with your camera.



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 a white balance setting applied)

As well as your target star spectrum you should record  darks and flats 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. (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 Deneb taken using a fixed camera and 200mm lens on a tripod



Once you are happy with your camera orientation, focus carefully on the image of the target star and fit the grating to the front of the camera, taking care not to disturb the focus. Take test shots, rotating the grating until the spectrum is exactly horizontal.

Note It is important to get the spectrum as horizontal as possible as although the image can be rotated in software later, this can produce artifacts, particularly when working with such narrow spectra. 

Check the focus carefully and adjust if necessary, making sure that the spectrum remains horizontal. If there are features visible in the spectrum (sometimes made more visible by deliberately trailing the image by manual adjustments to the drive) it is a good idea to focus on those rather  than the zero order star image but if you cannot see any features, the best focus of the star image will be almost as good, depending on the quality of your lens  (It is here that patience is rewarded with higher resolution in the spectrum.)

Note If you find that you cannot get optimum focus across the whole spectral range and have to chose a compromise setting, this is because of achromatism in the lens you are using. You may also see the same effect in the star image.  

Once you are happy with the image, adjust the exposure time to maximise the brightness  while avoiding saturating  any pixels in the area of the spectrum (It is permissible to over expose the zero order star image) and take a series of at least 20 images, more if your spectrum is faint.

Note It is not a good idea to allow the spectrum to fall on exactly the same pixels in every image as this makes the final result very susceptible to defects in individual pixels and also produces artifacts due to the pattern of colour pixels. If your tracking is good, you may need to introduce deliberate shifts between each exposure (A process similar to dithering) this can also help if your focal length is so short that you are potentially under sampling.


Continue to Processing the Spectra >>>



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