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March 12, 2022

While successful, implementation of collimated beam projector to date has two issues: 1) dynamic range mismatch between monitor diode (wants nA of current) and astronomical instrument (100,000 electrons per pixel max), and 2) not directly monitoring beam at exit pupil, due to size mismatch. 

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At 808 nm: 

If angle of incidence is 10 degrees, reflected beam rotates by 20 degrees. If beam of diameter D rotates by theta, distance it must travel to not have incident and reflected overlap obeys tan(theta)=D/L so L/D=1/tan(theta)

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notional system diagram: 

RACBP Spring 2024 Parts List:

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We note the reflective designs in this paper which document a 4-element system that adequately* preserves the initial polarization / phase of light.

Status updates: 

An initial version of the RACBP was constructed in the benchtop lab setup on Feb. 22.

Initial testing with a 532nm laser unfortunately showed strong and tightly spatially located double-bounce propagation through the system, indicating that the 30-arcmin wedge on the wedge windows purchase would be insufficient for canceling double-bounce issues. 

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Consequently, plano-convex lenses of the fastest f-ratio were ordered and installed the following week as replacements to the wedge windows; initial observation reveal that this modification  appears to successfully cancel out double bounces, representing a step closer to taking controlled flux measurements through the system at this proof-of-concept stage. 

Current status of the project is temporarily described in this document.

May 15, 2024

We have an idea to improve our ability to redirect refracted light to a photodiode. We will use a wedge prism that has a wedge angle similar to the refraction angle of light through the glass, as governed by its index of refraction. As a result, the refracted light should travel down the length of the prism and exit normal to the opposite side of the prism, whose face is orthogonal to the long axis of the prism (not wedged). On this backside of the prism, we will graft on a condensing lens that should cleanly focus the light onto the photodiode. This results in more precise management and control over the light we will use for referencing to our photodiode.

Orders have been placed for: 

x1   Optical wedge, the 15 deg. Nom. Uncoated, N-BK7 Wedge Prism.
x1   Condenser lens, 25mm Dia., 0.25 Numerical Aperture, Uncoated, Precision  Aspheric Lens.
x1  2" clear aperture adapter, AD2-CA - Ø2" OD Adapter for Ø1" Optic with Clear Aperture, 0.28" Thick.
x1 2" to 1" center adapter, AD2 - Ø2" OD Adapter for Ø1" Optic, 0.25" Thick.

I was only able to find a 1" BK7 wedge (25.25 degrees) that fit the calculated refraction angle of BK7 / UVFS, which ranges from 26-28 degrees for BK7 and 27-29 degrees for UVFS over the range 0.2 - 1 micron. 

Alternatively, we could probably use a 30-60-90 prism, but I was unable to find a size larger than 1" made of BK7/UVFS, that was available off-the-shelf.

Since one surface is now N-BK7, I have ordered a 2" N-BK7 plano-convex lens to accompany it to ensure we don't introduce fractional polarization asymmetrically.

Reference:

(Galvez, E. (2001). Achromatic polarization-preserving beam displacer. Optics Letters, 26(13), 971–973. https://doi.org/10.1364/OL.26.000971)

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References

Large optical wedge vendor, example: https://www.precisionoptical.com/precision-optics/optical-flats/unmounted-reference-flat/

plano-concave high quality mirror blanks, up to 50mm diahttps://www.lambda.cc/product/plano-concave-mirror-blanks-pccm/

polarization-preserving configuration with 4 reflections ol-26-13-971.pdf