FocussingLightguide

Optimisation routine for lightguide coefficients

End of June 2006 I have asked our Edinburgh workshop to build two of the lightguides as shown as dark blue line in the drawing to the right, each will be 50mm thick. Slight changes like a few added holes suggested by the workshop people do not affect the optical focussing.

To obtain the shape I used a macro routine to actually optimise the lightguide shape. Run the macro from within PHYSICA. The optimisation is one iteration only, check the quality in feeding the out values into a second iteration as input values. For the PDF writeup and the tarball with all relevant files please see:

• 20060709.lightguide.pdf: PDF writeup
• 20060709.lightguide.tar.gz: tarball

-- KlausFoehl - 09 Jul 2006

Prior write-up

Some results on the focussing lightguides. Requirements for the two design options below were a focal plane perpendicular to the disc and a high degrees/mm dispersion.

* compact focussing lightguide:

* curvedx2m54x3m09x4m05b.eps: compact focussing lightguide

The black dots are focal points on a particular ray. Ideally they would all lie on top of one another. The position of a focal point actually does imply the slope of the focal plane in its vicinity. It only can be changed at the cost of imperfect focussing. The focal plane was twisted to vertical orientation to allow photon detectors to be oriented parallel to the magnetic field.

The shorter the focal length, the bigger the focussing imperfections both absolute (in millimetres) and relative (in degrees or beta). The aim was here to get the dynamic range of 20degrees (45-25) into less than 50mm range, so the focal plane could be covered by a standard 50mm or 2" side length PMT.

Focussing polynomial is y= -5.4/60^2*x^2 - 0.9/60^3*x^3 - 0.5/60^4*x^4
the focal plane is at x=77mm, these values can be further optimised.
Focussing imperfections [sigma]:
45deg 0.15mm ; 35deg 0.16mm ; 25deg 0.47mm .

• compact focussing lightguide with two reflections:
  
• curvedx2m54x3m09x4m05m.eps: compact focussing lightguide with two reflections

Including an additional mirror surface into the lightguide one can keep the readout completely upstream or downstream of the disc. It may be easier to accomodate spacial constraints this way than fighting for extra space.

-- KlausFoehl - 05 May 2006

I still think that the Barrel can probably be read out with Time-of-Propagation.

But if photon angle measurement is required, the end of the bars could possibly look like this with a focussing lightguide, and maybe one can get away without correcting the fused silica dispersion:

I am not happy with the focussing (sigma=0.3-0.5mm, quick hand optimisation only), and the vertical focal plane is not the best orientation either, only a compromise propagated from the Disc calculations above. Getting the focal plane perpendicular to the magnetic field may be necessary, which is good news as best focussing is achieved with somewhere between 60-80degrees and not the 90 degrees. No constraint for the focal plane orientation would be even better news...

-- KlausFoehl - 06 May 2006

Photos of first Prototypes

Two focussing lightguides still assembled from polishing sandwiched between thin protective plates. Some polishing material has squeezed into the gaps. Disassembly will take place once one has decided one does not want further polishing.

Each acrylic glass lightguide is 50mm thick.

• Photo 1:
  

• Photo 2:
  

• Photo 2:
  

-- KlausFoehl - 28 Jul 2006

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