Compact Astronomical Spectrometer
(Don Davies)

The spectrometer is a slit spectrometer, i.e. the telescope forms an image of the object to be analyzed on the entrance slit of the spectrometer, and only the light which goes through the slit (i.e. a thin strip of the sky) gets into the spectrometer. Inside the spectrometer the slit is at the focus of a collimating lens which takes the diverging light from the slit and makes it into a parallel beam. This beam strikes a diffraction grating, which reflects the light back, but each wavelength gets reflected back at a different angle. This light goes back through the lens and is refocussed near the slit. However, since the different wavelengths re-enter the lens at different angles, they are focused at different positions.

Spectrometer Diagram

The spectrometer will work all the way from the violet region around 400 nanometers to beyond 800 nanometers in the photographic infrared. Because the resolution of the spectrometer is fairly high, only a small piece of that wavelength interval is accessible at any one time.

Another way to look at how this spectrometer works is to think of a camera, for example a 35mm camera. Divide the film plane into a left and right half. In the right half, put the entrance slit with its long axis perpendicular to the plane of the drawing. In the left half, put a CCD focal plane. Focus the camera lens at infinity. On the outside of the camera, cover the lens with a mirror. The light that goes through the slit will go through the camera lens, be reflected back through the lens by the mirror and be refocussed to an image of the slit on the CCD focal plane. If the mirror is now replaced by a grating, tipped at the correct angle, the image of the slit will now be dispersed into a spectrum of the light going through the slit. Each point along the slit will be dispersed into a separate spectrum separated on the CCD in the direction perpendicular to the plane of the drawing.

Normally the collimating and re-focusing functions in a spectrometer are done with mirrors with complex, off-axis, non-spherical surfaces. Even with these mirrors, there are only two surfaces to work with to reduce the aberrations in the system. The 35mm camera analogy is important. Camera manufacturers have spent decades developing lenses that have large focal planes (provides room for CCD and slit), superb resolution (gives very sharp spectra), and fast f# (allows use even with fast Dobsonian telescopes). The spectrometer uses an old lens from a non functioning 35 mm single lens reflex camera.

The spectrometer uses a 135 mm focal length f/3.5 telephoto lens (vintage 1968), a 1200 or 600 grove/mm 2 inch square grating (Edmunds, experimental grade approx. $30 for the 600 g/mm, more for the 1200) and the Cookbook CCD Camera. With an entrance slit of 25 microns, the performance of the spectrometer is essentially perfect, i.e. the spectral lines show the profile expected from a 25 micron slit and a perfect optical system. I am quite happy with the performance of the system and encourage others to consider building similar instruments. The cost of the spectrometer, excluding the CCD camera, is very small.

With that note, let me mention some things that need to be considered:

• Do a geometric ray trace. The space around the slit and focal plane is tight. Even using the diagonal mirror, I had trouble with the large housing on the Cookbook Camera, and I had to extend the cold finger to move the focal plane as close as possible to the window.
• Think about what adjustments will need to be made to the positions of the components in order to align the spectrometer. The focal plane rotational orientation needs to be aligned with the slit, and the grating dispersion direction needs to be aligned with one of the CCD axes.
• Watch the weight. I built my telescope with intention of putting an instrument near the secondary, so I could accommodate some weight there. A critically balanced telescope might have a problem. (I just toss another 4 pound diving weight into a tray behind my primary mirror if its a bit out of balance. I've got 20 #s there now...)
• To estimate the spectral coverage/resolution, just scale from the numbers for my system : 135mm focal length lens, a CCD chip that is 6.4 mm in the dispersion direction, and a 600 g/mm grating gives approx. 77 nanometers spectral range, with 0.306 nanometers/ picture element at 252 pixels/line.
• You might want to make provision for having the slit and the focal plane a little bit above or below the centerline of the lens. Depending on the lens coatings and the shape of the lens surfaces, light reflected off of the lens surfaces could be reflected back to the focal plane. By placing the slit and CCD off-center (in the direction perpendicular to the dispersion direction), reflected light misses the focal plane.
• I found that with the cooled CCD chip very close to the window, condensation on the window was frequently a problem. I solved it by mounting a small fan (one used to cool computer CPU chips) on the side of the spectrometer, blowing air across the window. The air going into the fan comes through a small duct with a resister in it to warm up the air slightly.

One last point: stellar spectroscopy is one of the few areas where living in the city is not a major handicap. When the skyglow is dispersed by the spectrometer it is not a limiting factor. I routinely integrate for 8 minutes and see no adverse effect in my spectra. This even under my observing conditions - sidewalk in front of my home in the metropolitan Los Angeles area, two obnoxious streetlights right across the street and best limiting visual magnitude of about 2.5 i.e. I can barely see Polaris with my naked eye!

(11/99) I have modified the spectrometer to give my computer control over the grating position. This is accomplished with a linear stepper motor controlling the angle the grating makes with the optical axis of the collimating lens. With this set-up, I can conveniently get measurements between 430 and 780 nm. with 0.15nm resolution with the 1200g/mm grating. The spectra of Polaris and Saturn on my "Data" page were obtained with the new setup.