VIS Spectrometer – Collimation

If you didn’t read my previous article, start here first: VIS Spectrometer – Hardware

After assembling the hardware of the spectrometer, the next step is to adjust the collimation of the device. Easier said than done.

In theory you could try to shine a light through the optical slit and project the collimated beam onto a surface and change the distance. If the beam stays the same size at all distances, then it’s collimated. In practice you’ll quickly notice that it’s all a blurry mess and only serves as a rough starting point.

Luckily for us we also have a focusing lens with a sensor attached. This enables us to use the following workflow:

  1. Remove the collimator and grating assembly from the focusing lens.
  2. Set the focusing lens to infinity focus by pointing it to something far away and adjust the focusing ring until the image is as sharp as it gets.
  3. Assemble the spectrometer again.
  4. Put a CFL bulb in front of the spectrometer and adjust the collimation until the spectral lines are in focus.

This should get us a very good collimation.

For adjusting the infinity focus it’s helpful to fix the parts into place, in order to keep everything steady.

spectrometer focusing assembly

Then start up the pyCCDGUI and set the capture mode to continuous. After your “image” is in focus it could look something like this. (Yes, I forgot to take a screen capture and this is the only image I found.)

image taken with a line sensor

Take a photo of the position of the focusing ring. You may need to move it during assembly. In my case the image was in focus when the 2m marker lined up with a screw and you could use this as a starting point.

position of a focusing ring

Reassemble everything, turn on your CFL bulb and adjust the screws of the optical slit assembly until the peaks of the spectrum are as high as you can get them. There are 3 screws for increasing the spacing and 3 screws for decreasing it. These are opposing, so if you want to tighten one set you’ll need to loosen the other. Make sure that all screws are (lightly) tightened in the end though.

Now you can finally cover the space between the slit assembly and the collimator with black electrical tape.

In the next two posts I’ll go over the wavelength calibration and the relative intensity calibration.