1. Effect of Spherical Aberration on MSE Filters December 5, 2005 Steve Scott & Jinseok Ko MIT PSFC Cambridge, MA Thanks to Fred Levinton, Nova Photonics File: spherical aberration.ppt

2. Effect of Spherical Aberration on MSE Filters or “Oh, never mind” December 5, 2005 Steve Scott & Jinseok Ko MIT PSFC Cambridge, MA Thanks to Fred Levinton, Nova Photonics

3. q Optical Assembly at MSE Filters Lens Filter Detector Fiber optic 4.5 x 6 mm 70 mm  = o ( 1 – (N 1 /N 2 ) 2 sin 2  ) 0.5  = 0.15 nm for o = 658,  = 2.45 o., N 1 = 1., N 2 = 2. Oven spherical aberration causes excessive focussing at the edge of the lens 

4. Thought Process (or: why I got excited) The narrowband filters used by MSE shift to the blue if the incident light is not normal. 5 degrees  0.63 nanometer shift. We use a lens to collimate the light from the fiber bundle onto the filter, to direct nominally collimated light onto the filter. Because the lens height is comparable to the focal length, it will suffer significant spherical aberration, particularly for rays that hit near the periphery of the lens. Ray-tracing for a fiber bundle at the focal point of the lens, including the its real size (4mm x 6 mm), indicates a mean shift of 1.2 nm and some rays experience a 10 nm shift, i.e. enormous.

5. There is a clever solution to this problem Shift the fiber bundle closer to the lens. From geometric optics, the emerging rays will diverge (virtual image) The amount of divergence can be chosen to approximately cancel the ‘overfocussing’ effect of spherical aberration. Can realize reductions of factor of 100 in angular spherical aberration using this approach – if light source is a point source on the optical axis. Even when a distributed light source (4mm x 6 mm) is considered, can still realize factor ~10 reductions in aberration.

6. But Fred Levinton already thought of it In the design of the PMT/filter housing implemented by Fred, the fiber bundle was moved forward by ~16 mm relative to the focal point. This is exactly the optimum shift suggested by my ray tracing. When the ray tracing calculations include the 4mm x 6 mm size of the fiber bundle, they suggest that we might see shifts of 0.2 – 0.5 nm, i.e. not necessarily negligible. We need to know the ‘illuminated area’ of the filter, which is determined by the angular spread of light hitting the fiber bundle at the dissector. Previous measurements of the filter function by Bill Rowan found blue shifts of 0.05 – 0.2 nm compared to the filter specs.

7. Conclusions We should close out this analysis by Ray-tracing the MSE optics to compute the angular distribution of rays striking the fiber bundle. Measure the size of the image spot on the filter or collimating lens. But it looks unlikely that spherical aberration is causing a serious problem for MSE.