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.
A reflective design can take advantage of the known index of refraction of glass to have a good theoretical value for attenuation. Typical uncoated optics reflection is around 4%.
We can estimate how much attenuation we need by comparing the integrating sphere areas aubtended by output aperture and monitor diode. Full optical fiber input energy is distributed on surface of 2 inch diameter integrating sphere. Area is 4piR^2, or 8.1e3 square mm. For a 10 micron output pinhole, fraction of energy that emerges is A1/A2 = pi*2.5e-3^2/A2 = 2e-5/8e3 ~ 2.5e-9 of optical fiber energy. Wow and ouch.
ratio of photodiode area to pinhole area is (5mm/5 microns)^2 ~ 1e6.
When we run current CBP with 5mm output aperture, photodiode to aperture ratio is about 1:1.
For 200 micron fiber to subtend 1 arcsecond, we need a focal length of at least 200e-6/FL=5e-6 so FL=40 meters. That won't work!
Use 10 micron pinhole, need 10e-5/FL = 5e-6 so 2 meter focal length. A telephoto configuration is then 1m long. Sounds OK.
Assume beam we need is around f/10, that means 200 micron fiber needs to be 2mm away from pinhole. That would be fairly efficienct, compared to existing configuration.
CVI laser makes UVFS 3 inch diameter wedge with 3 degree angle. Successive-reflection beams emerge 6 degrees apart, with slight displacement. Sin(6 deg) = 0.1, so for 1m length the separation is 10cm = 4 inches. That's about right!. The Thor Labs rectangular beamsplitting wedge is 5 degree angle.
Purchases
Lambda optics 2 inch concave mirror blanks, polished both sides.
Your Lambda Research Optics order has been received.pdf
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/
| title | link | |
|---|---|---|
| Theory of the optical wedge beamsplitter | The_Theory_of_the_Optical_Wedge_Beam_Spl.pdf | |