The G300 Spectroscope
Written by Dave Grennan   
Monday, 16 April 2012 22:53

I began spectroscopy a couple of years ago after purchasing a wonderful spectroscope.  The L200 is an IKEA type instrument (produced by Ken Harrison) which the end user assembles at home.  It comes with a 600 lines/mm grating which makes it a medium resolution spectroscope.  It's great for looking close up at profiles within objects.  However I also wanted a '"one shot" instrument capable of capturing a full spectrum on one frame of a CCD camera.  Thus after lots of thought and 'back of envelope' drawings, I came up with the idea for the "G300".  I soon realised that in fact this was no new idea and that several similar instruments had been built by others.  The G300 is in essence a 'Classical' spectroscope in design.  The classical design consists of two lens, an entrance slit to 'sample' the light from the object and a diffraction grating which does the real work of breaking up the incoming light into its component colours (the spectrum) to be recorded on the camera.

 

Fig 1.1 - The Optical Layout of the G300

(Click Image for Full Size)

Light enters from the telescope and is focused on a narrow slit.  As the incoming bean diverges past the slit, it meets a folding mirror which re-directs it in the path shown.  The diverging beam is turned into a paraller (collimated) one by a 135mm collimating mirror.  It is important that the collimating mirror is focused on the slit by being placed in position and then having its position adjusted until the slit is seen in focus while viewed through the lens with another objective focus at infinity (specifically a telescope finder).

Now that the beam is collimated it can now meet the actual diffraction grating.  I chose a 300 lines/mm grating (25mm x 25mm) from optometrics.  This is by far the most expensive component of the instrument itself.  The grating breaks up the light into its component wavelengths.  The angle of the grating needs to be adjustable so that both the spectrum itself and the un-diffracted light (the zero order) can be centred on the camera CCD chip.  The final focus lens then focus the collimated beam onto the CCD camera.

In principle the construction was relatively straightforward. I am lucky to own both a metalworking lathe and a well appointed 'Milling' machine.  Put together these give me the ability to machine aluminium parts to a very high tolerance.  This of course opens up a whole new world of possibilities for producing home made 'kit'. However I do not beleive that the lack of such tools should but any budding DIY'er off undertaking such a project. Basic tools such as a 'Jigsaw' maybe some HSS hole saws and a can-do attitude can also go along way.  Where the machine  tools made it a lot easier centred around making the lens holders, grating support etc.  Also cutting holes in 5mm aluminium to a very high tolerance made fitting T-Thread rings at the entrance and exit apertures so much easier.

The grating support is a shameless ripoff of Ken Harrison's idea for the 'L200'. It consists of a circular base with a centred hole into which is press fit a small bearing.  The rotating part is a smaller disc with a protruding piece which tightly fits into the base bearing.  A small disc of nylon sheet is sandwiched between to make the two parts rotate easily with respect to one another.  On top sits an L section onto which the grating is fixed carefully with nothing more than double sided tape.  The adjusment mechanism is rather crude.  Basically another bearing is fixed above a hole in the base a short distance from the grating holder. Through this bearing, a small knob and shaft is push fit.  A small 'disc was then placed over the shaft to hold it in place and a small rubberised band was stretched around the upper grating support and the small pulley disc.  It works ok but ocassionally slips.    Therefore I have plans to replace this with an 'armature' mecanism to which I can attach a small motor for more precise positioning  and remote operation.

With the components built and assembled testing began.  Thats when I saw a problem.  All the spectral lines of a test neon reference lamp showed a lot of 'flaring' to one side.  I suspected the optics were not lined up properly.  A small home made laser was centred on the optical path so that the alignment process would be easier.  However the flaring just would not go away.  I eventually figured out what I was doing wrong.  What was going on was compensation errors in the alignment.  Although on the face of it alignment looked ok, small errors in the placement of the collimating mirror were being 'corrected' by unintentionally making compensating errors in the position of the folding mirror and hence the errors moved through the whole optical path.  Once I realised this it was much easier to solve the problem.

So how much did it cost to build?

  • Metal €30
  • Grating €110
  • Lenses €15
  • Various ancilliaries such as bearings and screws €10

A few other items such as the t-threads were scavenged from t-rings etc donated by friends. (Thanks Dave Jackson and Alan Costello)

So around €165 to build.  A similar instrument commercially may have some additional features but may also cost several thousand euro.

Fig 1.2 The "G300" - Note the baffles to prevent stray light.  Simply cardboard wrapped in non shedding black cloth.

One other thing worth noting.  The folding mirror was cut from a larger first surface mirror bought on e-bay.  Although this mirror is not advertised as optically flat, I can assure you it does a fine job.  You gets lots of it to use in other projects too and its easy to cut with a standard glass cutter.

The parts were spray painted flat black after a good primer coat.

The last thing to mention is that mounted in the lid is a small port where a tube containing a neon bulb can be inserted to produce a reference spectrum to assist in calibrating the target spectrum.

 

Some sources for components which I used.  I have no connection with these suppliers they are simply were I bought this stuff.

Grating from Optometrics

Metal from Forward Metals UK

Bearings screws etc from RS Components (IRL Link) (UK Link)

Lenses and slit from Surplus Shed (US)

 

 

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