Spectroscopy
 

Spectroscopy is the most important of professional techniques, but a relatively new field of amateur astronomy.  Only in the past decade, with the advent of lower cost CCD cameras and image processing computers, has this field become a practical alternative for amateurs.  Today, with backyard equipment we can record a level of detail that only the largest telescopes could see in the film era.

Unlike many other imaging activities, spectrometry is relatively tolerant of air turbulence, bright sky backgrounds, and changing sky transparency. Like precision photometry, there is a lot of technique to spectroscopy, both in data collection and even more in analysis. 

Instruments: Three spectrographs are mounted on the C-14 to fully utilize the available light: (1) A filter wheel grating, (2) A SBIG DSS-7 “Deep Space” spectrograph, and (3) A SBIG SGS “Self Guiding” high resolution spectrograph.  All three have focal reducers in the optical path.  The DSS operates at about f9.6, the SGS at f6.4 and the grating at f7.

The StarAnalyzer 100 lpm grating is mounted in the ST9’s filter wheel. This creates a very low resolution slitless spectra across the imager for all stars in the field.  Unfortunately, every star in the field is dispersed, making it difficult to automatically extract the results.  Hence this is used infrequently and only for objects too dim for the DSS.

The SBIG DSS-7 uses a 50 micron slit. This gives a resolution of around 400 across the entire visible spectrum with very high efficiency.  Useful spectra of objects as dim as 13th magnitude have been obtained.   During 2008, some 2500 spectra were taken with this instrument in automated operation.  In order to support long exposures, a pellicle beamsplitter diverts 10% of the light to the ST9’s Remote Guide Head.

Shown below is the full DSS solar spectrum from 3900 to 8000A.

The SBIG SGS uses an 18 micron slit and is equipped with the optional 1800 line grating.  This provides an effective resolution of 11,250 at Ha where it is generally used.  A bench image, showing the Ha line of sunlight appears at right.  As this dispersion one pixel represents a doppler shift of 18 km/s.  Three neon calibration lines fall within the Ha centered image allowing very precise measurements.

At this resolution only the brighter stars down to 7-8th magnitude can be observed with reasonable exposures. 

The SGS feeds a front view of its slit to the guide ccd of the attached ST7 so that the instrument can guide during exposure.

The two instruments complement each other very nicely. On the DSS, each pixel is 5.4A and the projected slit falls across three pixels whereas on the SGS a pixel is 0.30A and the slit projection is only two pixels.  Hence the effective resolution is about (5.3/.3)x(3/2)=~27 times higher.  

 

Automated Spectroscopy

A spectrometric instrument cluster

The Optec Perseus selector switches starlight from the Celestron C14 among the SGS/ST7 Spectrograph (R~11000) on the left, the DSS/ST7 (R~400) pellicle guided spectrograph at right, and the SA100 grating (R~50) in the ST9E camera located on the lower port.  The camera also provides CBV&Ic photometry.  The top mounted SSP4 Infrared photometer is fed by an SBIG AO7 tip-tilt mirror. 

A Flip mirror on the SGS port allows visual observing.