Tuning Spectral Distribution - only able to see a portion of full spectrum

[LaurenM]

We are currently only able to view a portion of our spectral distribution, and when we adjust the grating we can see that the spectrum continues as we pan over (using the cage knobs to adjust). What might be the reason for this? So far we've tried recalibrating the position of the 2nd lens in the lens group (AC254-050-A), the grating angle, and the position of the camera. Each variable changed features of the spectrum, but we haven't found an adjustment that brings the entire spectral distribution into view.

I attached an image showing what we thought could be happening with the 2nd grating.

[Luc]

Hi LaurenM,

I don't see the attached file. Could you upload it again?

One possibility (to be checked) which happens often is that you might be looking at the 2nd order of the grating which doubles the dispersion of the grating (the spectrum therefore spans on twice the range). You can already check that you didn't accidentally rotated the grating too much by stepping back in the rotation.

I'll be able to assist better once I have the image

[LaurenM]

Hi Luc,

Unfortunately my files aren't uploading, I'm getting the error that my "board attachment quota has been reached." My coworker is also on this forum, and may be able to upload later (potentially a permission thing?). But we tried your suggestion of rotating the grating, and are still not able to see the full spectrum within the window, it didn't seem to make a difference. After trying that, we adjusted the grating using the lower knob on the cage (closest to the base). Doing this pans horizontally over on the spectrum, allowing us to see that approximately 1/3 of the grating is being cut off at any given time.

Not sure if this is a related issue, but we are not getting a response from rotating our edge filter either (adjusting it does not remove any of part of the spectrum).

Thanks!
Lauren

[Luc]

I updated some of the forum settings; please try again uploading the files it should work now

a view from the top of the spectrometer is also useful for debugging

[LaurenM]

Hi Luc,

We think we figured it out. Here are the details...

Not seeing a response form our edge filter: We realized we were using a 567nm dichroic mirror instead of the 550nm (which was sold out at the time). The spectrum that the edge filter would remove was likely already cut out.

Not seeing the full spectral distribution: Our hypothesis is that the system is actually performing as expected (that is, we are able to see the spectrum that this setup is intended to resolve), and there is additional spectral data that exists, but we are not focusing on it given the specifications of this setup. I've attached 3 images - one is the overview of the setup, the one labeled "far left" is when the grating is adjusted to see the leftmost portion of the spectrum, and "pan right" is after we have adjusted the grating to pan right (you can use the high intensity spectral line as a point of reference). If we were to continue panning right, there would be additional spectral data, but our thought at this point is that it lies outside the specification of this system. Interested to know if you have any thoughts / if you agree with that assessment!

Thanks!
Lauren

[Luc]

There is already some good news.

Generally speaking, the span required for Raman spectroscopy at 532 nm is much smaller than the total spectrum of most calibration sources like neon or even the fluocompact lamp. So it's not an issue if you don't image the complete spectra of these lamps. Here, you'll only need about 110nm to cover the emissions from 500 to 3500 cm-1.

I see that you are using the big grating holder part from thorlabs. I stopped using it because it blocks (vignette) a big portion of the light. You'll find on the website a small custom part to hold the grating with a bit of epoxy glue and that solves this issue.