[dsg-hallb_rich] Mirror 5C d0 tests - better d0, but images still distorted

[Tyler Lemon]

Hi Marco,

Mindy and I took three additional measurements of spherical mirror 5C in the horizontal position this morning. Each measurement changed something in the set up, as recommended by your last email. All of the data from today has been moved to the ifarm directory below.

"/w/hallb-scifs17exp/clas12/mirazita/RICH/Mirrors/CMA/RICH-2/2021-10-21"

In that directory, there are three subdirectories, one for each measurement.

"04_fiber-polished"
We cut the fiber with a Xacto knife and then used the supplies found in the large cleanroom to polish the end. For the cutting and polishing, we used the brass block that holds the fiber to the CCD stand as a guide by cutting the fiber flush to the opening in block, moving fiber slightly forward, and then using block to hold the fiber as it is polished. After polishing, we had to lower the CCD exposure time to 0.5-ms to avoid any saturated pixels.
The image at d0 was still not a uniform circle and the image 4 mm closer to the mirror was distorted and not the shape of the mirror.
However, the d0-fit result was slightly smaller at 2.18 mm.

"05_other-ccd"
We swapped CCDs for the second one we had on hand. We used the same 0.5-ms exposure setting and z-positions as the previous measurement (CCD counts were only ~20 counts different than with other CCD after focusing by eye).
The image at d0 was still not a uniform circle and the image 4 mm closer to the mirror was distorted and not the shape of the mirror.
The d0 result was again slightly smaller at 2.05 mm.

"06_cover-edges"
We cut a square out of a sheet of blackout cloth and draped it over the mirror stand so only the center portion of the mirror is visible (photo attached).
With this set up, we did have to raise the exposition time from 0.5 ms to 1.2 ms. This is expected since less light is reflected back to the CCD from the source.
For this measurement set, again, the image at d0 was still not a uniform circle and the image 4 mm closer to the mirror was distorted and not the shape of the mirror. However, both images in this situation are more uniform than the previous two.
This measurement also resulted in the smallest d0 yet at 1.27 mm.

Attached is a PDF with three pages, one for each measurement set. Each page has the d0-vs-Z plot, the image where d0 is a minimum, and the image 4 mm closer to the mirror than from where d0 is at its minimum.

>From these three measurement sets, I think it is evident that the distortion in CCD images at d0 and a few millimeters away from d0 are caused by the quality of the mirror. From the last measurement set with the mirror edges covered, it does look like the edges of the mirror are in worse condition than the center portion.

-Tyler

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