[dsg-hallb_rich] Mirror reflectivity source around 400nm

[Tyler Lemon]

Hi Marco Mirazita,

Over the past week, I've tried to get measurements with the reflectivity test station at lower wavelengths so we can get data around 400 nm for the RICH mirrors.

With the source we have, we can measure ~400 - 600 nm wavelengths if the integration time of the spectrometers is increased. However, as a consequence of the higher integration time, the spectrometer CCD is saturated above ~600 nm. Also, the counts observed around 400 nm is still relatively low. Attached is a plot (NEW reference-AVG.png) of the spectrometer counts of the reference light showing this.

This behavior is due to the nature of the quartz-tungsten halogen (QTH) lamp used for the source. While the source does output some light (as specified) down to 360 nm, it has the highest intensity around 850 nm.

Looking back at the data from RICH-1, we see the same behavior of the old test station, which also uses a QTH lamp. Attached is a plot (photodiode A by mirror and date.pdf) of current measurements from the control photodiode (the equivalent of the reference light in the new test station) for four mirror measurements taken in 2017 and we see the same general shape as the data from the new test station.

With this, it's apparent that the light source in the new test station is comparable to the light source in the old test station.

To be able to get more counts at lower wavelengths, we could try getting a filter that only passes the lower wavelength light so we can use higher spectrometer integration times resulting in more counts. Something like the Thorlabs 450-nm cutoff shortpass filter<https://www.thorlabs.com/thorproduct.cfm?partnumber=FESH0450> or Newport 450-nm cutoff shortpass filter<https://www.newport.com/p/10SWF-450-B> would be compatible with our source's built-in filter holder.

-Tyler
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