CBP Calibration Hardware

Note: discussion of some of this hardware and different options is also here: https://confluence.lsstcorp.org/display/LTS/Readout+and+Timing+Options+for+CBP+Calibration

Plan to replace solar cell with lens + photodiode

Powerpoint with initial version of hardware design: CBP_large_lens_calibration_system.pptx

Updated version: CBP_large_lens_calibration_system_v2_060923.pptx

lens: https://www.ebay.com/itm/184986621155?chn=ps&_trkparms=ispr%3D1&amdata=enc%3A1G08XAgSuSmibxPFCG1ZFlQ82&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&mkscid=101&itemid=184986621155&targetid=1587268788377&device=c&mktype=pla&googleloc=9002000&poi=&campaignid=19894961968&mkgroupid=148855406073&rlsatarget=pla-1587268788377&abcId=9307911&merchantid=113505756&gclid=CjwKCAjw-IWkBhBTEiwA2exyO38BmKS59kDX7yC1bDVaryyAKyGwXfi_9vPYCjG1mPkSEO2YcfymaxoCHIoQAvD_BwE

Should the focused beam fit on a photodiode?

Theoretical answer: Yes, comfortably. d = 2*w0 = M^2 * k(A, D) * lambda * f/D, where k is ideally 1.27, usually between 1.5 and 2, M is the beam quality, which should be pretty high, f=700 mm, D = 260 mm

So for lambda = 1 um, d = 5.38 um

Then the power through an aperture P(r, z = 0) = P0 (1 - exp(-2r^2/w0^2)), if want P/P0 = 0.999, then r/w0 = sqrt(-1/2 ln(.001)) = 1.86, and P/P0 = 0.0001 has r/w0 = 2.145. This beam diameter is still under 15 um, so it very easily fits within the diameter of the photodiode.

Chromatic aberration

We want to calculate chromatic and spherical aberration

We have K9 glass, according to http://www.ygofg.com/products/Colorless_Filter_Glass/119.htm, 

Note, if we assume f = 700 mm at 546 nm, this means at 405 nm f = 685 mm and at 1.5 um f = 726 mm.

Calculated the expected aperture diameter needed to capture a percentage of beam power as a function of wavelength, assuming the focal plane of the sensor is set at 700 mm. Calculations here: Chromatic Aberration.ipynb

This is somewhat concerning...

spherical aberration: I think this is less of an issue with a plano-convex lens, but we may have to measure it. CWS estimates it's about 1mm of image blur

How about this as a sensor: 

around $1000 from Digikey. We'd have to measure its QE relative to NIST diode. Also now that I think about it we will want a temperature sensor on it as well, and measure QE vs. temperature at red edge of response curve. He's the claimed response curve :
(0.5 A/W at 600 nm is 100% QE)

Notes from  meeting on 6/9/2023 with Chris and Chuck

• biggest concern for whether the spot fits on the photodiode is chromatic aberration for a spherical lens
• build access panel/hatch instead of having a tube connector
• lens covers should be metal and they should be actuated
• whole assembly should go on the TMA, either at the top of they pylons or by the primary mirror
• should use black rubber coverings or honeycomb aluminum instead of sheet metal rings
• need to design an adapter from cylinder to rail for the lens holder (cylindrical to rectangular)
• consider using 3-4 aluminum rings to stabilize the tube
• for mounting the lens
  • use 3 100 mil teflon pads as inserts
  • at same location drill 1/4" through holes in radial direction
  • squeeze RTV through those holes
  • use CTE of glass, RTV, and aluminum to determine the appropriate size of the gap to make everything athermal
  • machine aluminum clips to dimension of known gap from CTV calculation
• For the logic timer - if we need to switch out the Keithley 6514 for the 6517B we can
  • check timings - see if that's a viable option
  • could make new CSC if necessary - figure out if it is

Measurements of beam spot size 

Tried to measure beam spot size through the two 300 mm lenses on 9/7/2023.

Setup: Collimated an LED with one 300 mm lens and then focused it with another. Used a red LED to perform these measurements. (First I used a broadband LED but there's obvious chromatic aberration when you do that).

General observations: spherical aberration seems larger than expected, but it is much worse when the flat side of the lens faces the collimated beam. Qualitatively that is what we expect (see e.g.  https://www.comaroptics.com/technical/spherical-aberration). However, doing calculations based on the link page implies that spherical aberration should make the beam on the order of 1mm in size, but it didn't quite fit in the standard NIST hamamatsu photodiode (it almost did, but not quite).

But doing a simple sum over the whole spot taken with the canon camera, the total flux from the spot is identical to far less than a percent (or even one part per thousand) CBP_calibration_spot_flux.ipynb. This is extraordinarily weird to me because the glass should reflect 4% of the light.

For an estimate of spot size (very, very crude - based on the above notebook, but I apparently failed at focusing even just on the fiber so take this with a grain of salt): spot size = 40 pixels (by eye this is an overestimate) * 5.25 um = 210 um. With an 800 mm lens that corresponds to an angle of .21/800 = .26 mrads. If we're 3 or 4 meters away that's (say 4): ~1mm. Huh. That is definitely not what I observed as a spot size...  

Things built in shop:

Version for photodiode holder: 9/27/2023 Photodiode_holder_design.pptx

9/27/2023 sent this to shop: pinhole_holder_2 v6.step

Items that have been built in the shop: Rail_Connector.step Tube_End.step Tube_End_Drawing.pdf Rail_connector_drawing v3.pdf CBP_Calibration_Tube_end_drawing v3.pdf