[Dsg-rich] reference mirror reflectivity measurements after test station's move to cleanroom

[Tyler Lemon]

Hello, 

On Monday, we took reflectivity measurements of the reflectivity test station's reference mirror. 

When taking the measurements, we noticed that the area of the cleanroom where the test station is located is not completely dark because of the light from the exit sign. This is reflected by the CCD measurements being much greater than the previous reflectivity meaurements. 

To try to compensate for this, we also took measurements without the test beam with the CCDs in the calibration position and measurement position. These measurements could be considered the "background light noise" in the clean room. As is the procedure for the D0 tests, we then subtracted the background from the measurements before calculating the calibration constant and reflectivity. 

The attached excel sheet contains the raw data from the CCDs ("raw CCD data"), the CCD measurements with the background subtraction ("background subtracted data"), and the results from the reference mirror measurement before the test station moved ("pre-move data"). Attached is also a plot that contains all three reflectivity values together for comparison. 

For most of the wavelength range tested, the raw CCD data and background subtracted data resulted in a reflectivity measurement of ~94% which is a ~1% difference from the pre-move reflectivity result. 

However, from 350nm - 450 nm, the background subtracted data is closer to the pre-move data with ~2.5 % difference. The raw CCD data has ~7% difference in this range. 

In light of these results, should we also be taking background measurements when taking reflectivity measurements of the spherical mirrors? 

If so, we could start taking the background measurements for all future reflectivity tests. 

Best regards, 
Tyler 

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