Technical Meeting #16: Optical layout for PDR
This meeting took place at 09:30 on Tuesday 24 May. Attendees: DFB, MF, CAH, ADR, EBS, XS, DMAW, JSY
- Common baseplate pin position - thermal expansion issues
- MF 2011-04-20: "This fits in with our original idea of pinning the base-plate near to the camera. Can you place some numbers on the defocus and shift assuming the base plate changes by 5 degC and the lens doesn't and then add the extra shift due to the lens changing by 5 degC (i.e. catching up with the ambient temperature change)"
- ADR 2011-05-18: "I've run David's new layout that he proposed yesterday through ZEMAX to see what happens. Assuming a temperature change of 5 degC, a coefficient of thermal expansion of 20e-6 /K, and a pinning of the baseplate under the second fold mirror, then the image moves by less than 0.02 micron. This is a very small value because all optics (notably the lens) on the baseplate move together in this very particular manner (i.e. they all move away from each other as the baseplate expands, and all angles remain constant). The magnitude of the motion of each element in this case, however, is on the order of tens of microns.
This motion of the image assumes that the baseplate is uniformly heated, and there is no movement of the optics in their mount as a result of this temperature change; the mounts are just carried by the expanding baseplate to a new location. The camera has not been moved at all as a result of this change in temperature.
In contrast, should e.g. fold mirror 2 move independently in its mount by ~0.3 micron along the line normal to its surface, with no movement of other optics, then we see the image move by ~0.3 micron also. This is the movement that I was referring to in the meeting on Monday."
- Optical layout for PDR - need to allow for connections to camera and its enclosure, and also clamping of camera base to optical table
Conclusions - baseplate
- Happy with concept for baseplate, kinematic supports and pin position under final fold mirror
- Final beam fold does not need to be a right angle
Conclusions - layout
- Would be more convenient if camera could be moved parallel to line of holes for initial focusing, but this is not a show-stopper
- Possible alternative lens design with 70mm shorter focal length (and 3mm thicker), but worse encircled energy. Encircled energy exceeds criterion off-axis.
- Shorter focal lengths to be avoided because everything gets more difficult e.g. better centroiding needed
- Right-angle layout unsuitable even with shorter f lens
- Would need to use almost all of 70mm to catch first line of holes, hence still problem with camera cable
- Hence adopt 2011-05-17 layout
- If it turns out we need more space for cable clamping, there are two fallbacks:
- Widen the baseplate (would need to add a notch) so final beam fold is at a more acute angle (preferred)
- Reduce lens focal length by <= 70mm
- MF/XS: Investigate possible ways of clamping camera cable and assess any extra space needed
- XS: Make 3d drawing for NMT based on 2011-05-17 layout showing changes to space envelopes that we are requesting