American Association of Variable Star Observers (AAVSO)
Mon, 10/17/2022 - 20:51

I am wondering whether some of you do Echelle spectroscopy. I would be interested in getting real world impressions about echelle spectrographs, particularly the limiting magnitude reachable with an 8" scope, how difficult calibration is and any other information by users.

Thank you very much


American Association of Variable Star Observers (AAVSO)
Some Echelle Thoughts

Hello Gianluca,

This may be of limited value in terms of information, but for what it is worth I wikl tell you about my experiences using smaller Echelle spectrographs. Larger, professional observatory spectrographs are a different process, but similar.

I use an early model Shelyak Echelle unit, their eShel system.  It has a telescope head which essentially has a port for monitoring the field stars and pickoff region for capturing the target's light. The light is then fed via a fiber optic feed to the spectrograph which houses the CCD imager. External to all this is a power supply system and a calibration lamp system for both a tungsten lamp (flats) and ThAr for wavelength calibration. 

Software is one of a couple of choices. They suggest, as do I, their latest Demetra system. It can both control the spectrograph and also perform data pipeline work to calibrate the images, and extract normalized spectra across all the various orders.  One gets multiple orders of spectra, not one connected spectra. They overlap a little bit in wavelength. The resolution is very high, but this comes at a cost. Spreading out the target's light so greatly requires longer integration times, a large aperture or both.  I started with a 10" RC scope and was able to do bright stars easily enough. Getting past the magnitude 2.0 region required a larger scope: 16".... then to get most of the variable I want, we went to a 0.7m scope.  Guiding is sensitive. The target is on a hole through a mirror that is used to guide using a second camera. Like all guiding situations, this can be finicky.   With an 8" scope, and say 10 minute integrations, you should get most of the stars that define constellations.  Going fainter will require longer integrations and/or stacking for a good S-to-N ratio. Your sky darkness will also determine your limits. Where I work (not good for this) we are limited to 15 minutes before we get notable unwanted signals from Hg and Na lamps. :-(



British Astronomical Association, Variable Star Section (BAA-VSS)
Echelle experience

Hi Gianluca,

Joan Guarro Flo could be a good contact. He has built several telescope mounted echelle's based on a design by Tim Lester (which for efficiency would be advantageous over fibre feed for your limited proposed 8 inch aperture) and runs them remotely. He has over 2000 spectra in the BAA spectroscopy database which you where you can check the equipment and exposures used for each spectrum. He routinely discusses his design, operation and results in  dedicated threads on the ARAS forum