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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Note that this is also somewhat dispersive, which is actually a plus.
Source needs to be on-axis for spherical reflector, otherwise we get off-axis aberrations (coma, in partticular).
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Fused Silica index is < 1.5 for lambda > 275 nm. So concave-convex FS lens has 1/f=(0.47)(1/r) and f=2.12 R.
A more elegant implementation:
Considerations-
Need to rotate the axis of reflection each time, to balance out polarization effects upon reflection.
best to keep angle of incidence below 10 degrees.
From https://refractiveindex.info/?shelf=glass&book=BK7&page=SCHOTT
At 808 nm:
| angle (degrees) | Rp | Rs |
|---|---|---|
| 0 | 0.041365 | 0.041365 |
| 5 | 0.040948 | 0.041784 |
| 10 | 0.039693 | 0.043069 |
| 15 | 0.037593 | 0.045301 |
| 20 | 0.034640 | 0.048630 |
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)
| theta_incidence | theta_refl | L/D |
|---|---|---|
| 5 | 10 | 5.7 |
| 10 | 20 | 2.7 |
| 15 | 30 | 1.7 |
Purchases
Lambda optics 2 inch concave mirror blanks, polished both sides.
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plano-concave high quality mirror blanks, up to 50mm diahttps://www.lambda.cc/product/plano-concave-mirror-blanks-pccm/
| title | link | |
|---|---|---|
| Theory of the optical wedge beamsplitter | The_Theory_of_the_Optical_Wedge_Beam_Spl.pdf | |