Spectroscopy of astronomical objects is a relatively unexplored field for many amateur observers, as truly compact, inexpensive and effective spectrometers have been unavailable until recently. This situation is now changing and spectrometry is becoming popular as a way to extend the scientific value of amateur observations and to add a new dimension to the hobby. The SX Spectrograph is our answer to the need for a very compact and high performance unit that is easy to use.
The SX Spectrograph is only 125 x 140 x 75 mm and weighs just over 1 Kg. It is solidly constructed in a machined aluminium enclosure and so will not flex during use. The optical design is based on a highly corrected, 550 groove concave reflecting grating, that provides a spectral length of 31 mm from 340 to 900 nM. The entire spectrum is accessible by adjusting a sliding camera carrier, using a threaded drive screw, but most of the visible spectrum can be seen without adjustment, when using an SX694 imaging camera. A 6 position slit wheel, with various slit widths and lengths, is provided and will allow the sensitivity and resolution to be optimised for the user’s project. The resolution R factor when using the smallest slit width (20 microns) is approximately 2000.
The SX spectrograph design incorporates a Lodestar X2 guide camera, which observes the image field via a 10% / 90% beam splitter cube. The slit is not directly visible in the field, but its position co-ordinates are provided and marked by a cursor box, generated by the PHD2 tracking software. It is therefore quite easy to move the object of interest into the slit cursor box and then to select any convenient guide star in the Lodestar field for tracking.
The spectrograph also includes a battery powered spectral line source for calibration. This source uses a small gas discharge bulb, which is filled with a mixture of neon and argon. The result is an array of spectral lines that covers the entire visible spectrum and which provides a way to determine the wavelengths of the lines from any external source. The calibrator light is powered by an alkaline PP3 radio battery and may be switched on by operating the small toggle switch, located above the Lodestar sockets.
The optical input to the spectrograph is via a T2 female ring adaptor. There is also a 48 mm female ring option, available on request. The back focal distance from the slit to the front of the T2 ring is approximately 40 mm mechanical (37 mm optical) and the clear aperture is 11 mm. The small hole is for access to the slit wheel lock screw and is not part of the optical path.
The T2 thread is part of a removable barrel that is normally locked into a fine-focus sleeve by an M4 setscrew (accessible when the box cover is removed). The fine focus sleeve is itself driven by the large, fine focus ring, which is seen surrounding the T2 adaptor. One rotation of this ring, moves the camera in or out by 0.75 mm, and permits the accurate focussing of the spectrum onto the camera sensor.
As the spectrum is too long to be completely imaged with a typical CCD sensor, the focus assembly may be moved along the spectrum, by rotating the drive screw that is seen at the right hand end of the cross slide. The three knurled thumb screws are slightly loosened to permit the slide to move, and re-tightened once it is in the correct position. As the grating is designed for a compact optical system, the available back focal distance for the imaging camera, is somewhat limited. It is typically about 20 mm maximum, and this is sufficient for most compact astronomical CCD cameras, but
not for a DSLR. We recommend using either the SX-694 or SX-814 mono cameras for high resolution imaging with good sensitivity. Nice colour spectra may be recorded with an SX-M25C. The camera is screwed onto the T2 adaptor and the correct spectral orientation achieved by loosening the focus lock screw and rotating the T2 adaptor. The approximately correct focal position can also be achieved by setting the T2 adaptor extension with the lock screw.