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Spectrometer optics

     The WheeTrometer is a miniature UV-vis spectrometer with a crossed Czerny-Turner configuration.

 

A good description of the Czerny Tuner optical bench is provided by B&W Tek.  As shown in the figure to the right, light enters the spectrometer through the slit (bottom) and passes to the collimating mirror which reflects it to the grating.  After striking the grating, the diffracted light is reflected to the focusing mirror and then on to the detector. 

   To build a WheeTrometer, you will need 3D printed parts, an optical package (containing the mirrors, grating and slit), electronic hardware, microcontroller firmware and the user interface.  STL files for printing these parts can be found at our GitHub page.   You can modify the optical geometry by changing the parameters in the OpenSCAD file

Instructions for assembling the optical components are provided in a demonstration video below.

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Availability

Stuff on this web site has limited availability.  We just do not have the cash to stock stuff we may not be able to sell. The kit with the Teensy shield is not yet available. Once we are happy with our prototype we will have some fabricated and we will sell it on this web site.  If you wish to assemble a circuit board yourself, we will be happy to share our most recent design on OSHPark (or another manufacturer of your choice) and you can have the pcb fabricated and sent to you.  If we have excess prototype boards and you contact us, we will be happy to supply them.

The optics kit
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Look for it at this link.  The optical elements used in the WheeTrometer are shown to the left.  They include the optical slit (front, center), collimating mirror (right), diffraction grating (left) and focusing mirror (back). 

A key factor that limits a spectrometer's wavelength resolution is the width of the optical slit: Narrower slits give higher resolution.  Our optical slits have a 0.1 mm width.  If you wish to, you can purchase a narrower slit from a commercial source like this one.  If you decide to go that way, then you will need to modify the slit mounting, but that is something I will help you with.

 

The video below demonstrates assembling the instrument
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The colimating mirror is 20 mm in diameter with a focal length of 100 mm.  The focusing mirror is rectangular, 20 x 50 mm with a 50 mm focal length.  Light dispersion incorporates a 1200 line/mm reflection grating with a 500 nm blaze angle.  I would like to have a greater selection of focusing mirror options, but having them manufactured would take money. 

Modifying the design

The physical dimensions of the spectrometer can be easily changed to meet your requirements: Changing the angle between the grating and the focusing mirror will affect the wavelength range you observe, and changing the grating or the focal length of the focusing mirror will change the dispersion and also the optimal grating angle.  The video to the right shows how to change parameters in the  design geometry using the free CAD program OpenSCAD.  The original design file is found in our GitHub page.

Gratings and Grating Angle

One big benefit of adopting the open source spectrometer is that you can modify the hardware design.  The easiest place to do this is at the diffraction grating.  We use a 1200 line / mm grating with a 500 nm blaze angle. This gives a wide dispersion angle and a maximum efficiency at a 500 nm wavelength.  By choosing a grating with a lower line density, you can take in more spectrum. The grating is what gives you wavelength dispersion.You can change the starting point by changing the angle of the grating, which can be done by changing the inner grating mount.

You can purchase gratings with 300, 600, or 1200 lines per mm and blaze angles for maximum efficiency with light between 300 and 1000 nm.  These can be purchased from Thor labs.  Make sure you purchase a ruled reflective grating that is 12.7 mm square to fit the grating mounts.  You might also want to read the grating tutorial Thor has and adjust the grating angle accordingly.

   

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Fiber optic connections
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Fiber optics are frequently used to connect spectrometers to other pieces of equipment, such as optical cells and light sources.  Many of these use SMA905 connectors.  It is possible to interface the WheeTrometer with other fiber optic elements.  To do this, you need a threaded flange, a lens, a spacer and a mount.  Be aware that the thread on the flange is not a standard thread that you will commonly find in a hardware store. 

  We can provide flanges and fused silica ball lenses. These are appropriate for uv-vis spectroscopy.  We purchase our flanges and lenses from sources on Aliexpress.  The photo to the right shows a fiber optic cable interfaced with the instrument.  The figure  above is a screenshot showing flanges available from a vendor on Aliexpress.