My recipe for the high resolution spectrograph

The spectrograph was built with inspiration from Christian Buil's rich and inspiring website, where it is described how one can assemble a standard type grating spectrograph from ordinary camera lenses, and with any material you have at hand for the "optical table"! This was appealing from the viewpoint of cost. A number of other inspirators have been important as well, and these can be found in the links via the main menu. A speadsheet is supplied on Christians page for the calculation of different design parameters for the spectrograph with the given lenses. A goal was to design a spectrograph with sub-Angstrom resolution (of R= lambda / delta lambda = 3000 - 6000 or so) so that Doppler shifts of many objects can be measured (such as spectroscopic binary stars).

I had a 28-90 mm Vivitar series 1 zoomlens lying around, that I used for collimator, and a 55-200 mm Canon zoomlens that I used for a camera lens. The collimator is used at 90 mm focal length at f\3.5, and the camera lens in the range 100-200 mm depending on desired wavelength range (and at f\5.6 and above). I purchased two Edmund optics 30x30 mm reflective gratings (from England) with 600 and 1200 lines per millimeter (at about 100 euros each) with an adjustable mounting frame, and a good quality Thor Labs slit (from the US) with a micrometer adjustment screw. This was more expensive, but worth every penny for the sake of increasing the spectral resolution dramatically.

The slit is placed at the telescope focal point, and at the collimator focal point, and the collimator lens is set to infinite distance so that the emerging rays (from a point source/star) are parallel at the grating (the collimator lens is run "backwards" with light going from the primary lens and out through the backside of the objective). The camera lens is also set to infinity, and the slit is then in focus, as seen when back illuminated (and when the grating is used at zero order where it acts as a mirror, reflecting the slit image into the camera lens).

All the parts were assembled with 12 mm (stiff) plywood that was cut and fitted using saws, drillbits and screws (only a carpenter needed: no machining necessary). My Canon 450 D camera, and an old-timer but high quality ST6 CCD camera, can be mounted into the spectrograph. The end result is a "hi-fi IKEA-type spectrograph"!

I used about one month to build the spectrograph, in March 2013.


Left: Fully assembled "IKEA-spectrograph" with flip mirror and ocular (no slit at this stage). Black cardboard lids (light weight) are used to prevent stray light. Right: Ocular for use with flip mirror for cenetring the object. The tube to the right is a standard that fits in the ocular holder of any (?) telescope.


Left: The Edmund Optics 30x30 mm grating placed in its holder, which can be rotated to center any wavenelgth range on the camera lens (including the zero order reflection). Right: The Thor Labs slit is in place, and I made an illuminated LED-and-plexiglass device with a guide line (cut with a sharp knife) to be used so that the object can be centered on the slit with the aid of the ocular and flip mirror. This slit is not reflective (preventing guiding on the reflected light that does't enter through the slit), but the telescope tracking is good, holding the image centered for several minutes in the 10 inch fixed mount telescope.

"The thing" has been tested in the 10 inch Newton home made computer controlled telescope, and a 16 inch Schmidt-Cassegrain. The spectrograph is pretty heavy (4-5 kg), but the instruments have stood up so far, with extra counter weights.... Weight could be substantially reduced by using more simple lenses with the minimum achromatic two-component configuration. Commercial spectrographs should be extra light-weight compared to this, but those are expensive, and perhaps not as flexible when it comes to modification and tailoring to the specific needs.

Apart from that: Self made (can be) more fun (for some), and failure and regret is now a non-dramatic option!


Left: Testing on the 10 inch before finishing the spectrograph. Right: The author and his drill making a slit holder, listening to space music of course.


Fully assembeled QCSPEC, and testing on the 16 inch Schmidt-Cassegrain: It does not tip over, and the motors do not catch fire (yet)! An autoguider is mounted on top of the telescope, which is of great help keeping the object on the slit. This has also been tested on comets, tracking comet Panstarrs in 2013.