Astronomical Spectrographs

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Spectrographs - Gratings to Slit

Spectrographs can be as simple or as complex as you want to make them. Like amateur telescopes the performance will vary with design.

Resolution

A key parameter for a spectrograph is spectral resolution - the ability to resolve features in the spectrum which lie close together (like absorption lines) . Resolution, R is defined as wavelength/lambda wavelength where lambda wavelength is the smallest wavelength difference between features which can be seen.

Example: If we can clearly see the two Sodium lines at 5895.92A and 5889.95A - a separation of 6A, then the resolution, R = 5890/6 = 980

Obviously the higher the resolution, R, the finer the detail we can record in our spectrum.

Spectroscopy 101

It is recommended that novices starting their journey in spectroscopy begin with a low resolution transmission grating - this is "Spectroscopy 101".

You won't break any new ground using a grating, but it will provide valuable experience in obtaining and processing spectra. Once you have gained this introduction to spectroscopy, you can decide whether or not to invest the time and money upgrading to a "serious" high resolution slit spectrograph which can allow to you make some meaningful contributions to the science and also to the frequent ProAm Campaigns.

Transmission Grating Options

An astronomical spectroscope can be as simple as a prism, or grating placed infront of a camera lens - "Objective grating/ prism" and an exposure taken of the night sky.

Placing the grating in the "converging beam" of a telescope will provide some interesting low resolution spectra, R=100

Slit spectrograph Options

To further improve the spectral resolution a slit spectroscope mounted to a suitable telescope is used. This has an entrance slit added at the front of the spectrograph. This slit provides a narrow gap on which the target star is focused. Depending on the spectrograph design resolutions of R=10000 can be achieved. There are Commercial Spectrographs available from Shelyak and Baader which can provide very capable results.

The alternative is to build your own. The basic design options are discussed -Classical/ Littrow / or Other Designs. One significant issue to be addressed with astronomical slit spectroscopes is the effective acquisition and guiding of the target (star) on the entrance slit gap. Many DIY builds fail to accommodate guiding and never suceed in practise.

Design Spreadsheets

We have an update of Christian Buil's spreadsheet - SimSpecV4 which allows the user to enter all the parameters and evaluate the capabilities and performance of the spectroscope being assessed. There's also a brief tutorial for SimSpec here.

There is also the TransSpec spreadsheet to evaluate grating arrangements, both "in the converging beam" and as objective gratings.

How to make a Cassegrainian (- and why not to)

In Ingalls' classic "Amateur Telescope Making, Book one" there's a brief chapter with this heading. Basically it says that it's very difficult to make a Cassegrainian telescope, demands a lot of work and the end result may not be worth the effort. Think about it seriously before you start....

It's a bit like that with DIY spectrographs.

First the good news, there have been DIY built spectrographs which perform very well and are capable of very useful work, unfortunately there are many more which still languish in the workshop - unfinished........

If you want to experience the benefits of a slit spectrograph and spend your time processing useful data then buy a commercial instrument. (You currently have a choice between Shelyak and Baader. All the others, Optomechanics - Model 10c, SBIG - Self Guided Spectrograph (SGS), Deep Space Spectrograph (DSS7), Elliot Instruments - CCDSPEC and the Spectra-L200 have dropped by the wayside.)

"I want to fly!" - do you design your own airplane or just buy a Cessna?? The guys in China seem to be keen! Building a precision instrument like an astronomical spectrograph demands a working knowledge of optics, skill and attention to detail. You don't need a college degree in physics, mathematics or engineering, but you do need to be practically minded, appreciate cleanliness and accuracy - if you don't know the difference between a wrench and a micrometer, Allen headed socket screws and Philip heads, I'd pass on attempting a spectrograph DIY build.

I'm not being a pessimist, just a realist!

You will need a reasonable collection of functional workshop tools (and the experience to use them safely!) but I can assure you - what you will learn about how spectrographs function and the satisfaction of finally having built a capable instrument at a cost much less than the commercial units will be well worth the effort.