An area of concern for me in rebuilding my spectrometer has been the camera focus. The spectrometer's slit provides pseudo-collimated light so the camera lens should be focused on the slit for best resolution. This is more difficult than it might appear. So, I'm posting two sets of observations:
Setup: - My prototype "bench" setup where I can move components mounted on magnets - movable but stable (what's not shown in the photo is the black cloth cover) - A CFL (EcoSmart 5000k 27W at 5-ft) - A very narrow slit (exacto-blade prototype) - The camera positioned immediately next to the dvd grating - The default Syba camera viewer software with resolution set to 1280x1024
1) First, I used the default technique of the kit to adjust the Syba lens for "9 inches" using room light and small-font text on a card -- the DOF is high so this is very hard to do with any accuracy, so it was just my best guess. Then, I adjusted the slit distance while observing the spectrum. The first attached photo shows that the optimal slit distance for this specific default focus is actually close to 4-inches. This is about double the distance provided by the kit.
[Caveat: I did not take the additional step to re-adjust the intensity for each distance to make them all uniform amplitude. This likely is the reason for the general intensity change over this extreme distance range. I expect to re-test and compensate for this. However I still believe there is enough evidence to support obtaining a good focus.]
2) Then, I did a series of lens adjustments based on using a mechanical reference -- the point where the lens "bottoms out" in it's travel. I plotted the adjustment position (# turns) vs distance to the slit where the spectra shows the best resolution. The second attached image shows a simple data plot. Note that a) there are downsides to both ends of the adjustment range and b) the mid-point of the more linear adjustment region turns out to be a 6-inch focus. A very narrow slit helps considerably with saturation and appears to provide the best resolution at 6-inches from the dvd grating.
My own conclusion is that since manufacturing methods for mass produced parts, like the optics for the webcam, are very repeatable, adjusting the lens focus by noting the # turns (or partial turn in this case) from a mechanical stop is a much more precise means for setting focus. As this experiment suggests, setting a proper focus at an optimal distance for the slit of abut 6-inches can be rewarded with better spectral resolution.
With a bit more work, this adjustment can likely be refined and improved but hopefully this is a good first step.