Date: Fri, 22 Aug 1997 12:51:44 -0700 To: dd From: "Scott C. Texter" Subject: OTG Measurements at the XRCF Status: R Content-Length: 4907 Hi Dan, Two questions you could answer to help with this: i) Why is the 5.24 mm additional offset there in the first place? Answer: Let me just review the bidding for a moment. (1) EKC's GSE placed the OTG's such the distance between the CAP CL and the Xg=0 point should have been 60.613 inches. (2) We installed 0.400 inch thick shims that moved the OTG's closer to the HRMA such that the distance between the CAP CL and the Xg=0 point should have been 60.213 inches. (3) We measured the distance between the CAP CL and the Xg=0 point and found it to be 60.028 (HETG) and 60.024 (LETG) (4) Therefore the difference between where we think the OTG's should have been (on average) versus where we measured them to be was 60.213 - 60.026 = 0.187 inches = 4.75mm (5) Hence, I'm not sure where you got the 5.24mm. Now, let me try to answer your question..... First off, I don't really know the answer, but I can speculate as to some possible sources: (1) I'm not sure how EKC verified, back at their facility in Rochester, that their structure was built such that the OTG's would have been where they were supposed to be when installed. My guess is mechanical tolerancing. If this were the case, and given the complexity of the interface between the HRMA and its off-loaders, and the support structure, I'm not sure how well this could be controlled. Specifically... (2) The Aft HRMA GSE structure, which bolted onto the HRMA Support Structure, had a pretty loose fit. That is, the bolt holes in the aft structure were pretty big. We actually took advantage of this fact to "wiggle" the aft structure around when we installed it on the base structure to get the aft HRMA contamination cover and aft contamination door to line up so we could attach them. Also.. (3) When the HRMA was hanging on its offloaders, we rocked it in theta-Y quite a bit to align it to the OTG's. We had to do this since there was no theta-Y adjustment capability on the OTG support structure (Actually, as Mike will tell you, we shimmed the HETG to get the to OTG's co-aligned and then rocked the HRMA into alignment with both the OTG's) (4) Having said this, I should point out that mechanical toleranceing should have done a good job of putting both OTG's at the same location wrt the HRMA. Now, 0.5mm is a lot and I'm not really sure if all our adjustment could have moved the HRMA/OTG's wrt each other by that amount, assuming EKC had the GSE built correctly in the first place, but I don't think its unfeasible. ii) You listed a "phase delay through XARM" term that is pretty large: should there be such a term for either of the OTG measurements? Ok, we had the theodolite behind the HRMA stably mounted to the DETB out in the Clean Room looking at the back of the HRMA. The OTG ARM's have their reflective surfaces on the -X surface of the glass, so we were getting a return directing from this surface without the beam traversing anything but air. However, the HRMA XRCF ARM is a piece of 50mm thick BK-7 with its front (+X) surface coated, which means that the beam had to transit this thickness before reflecting off the front surface and returning. (Note: the back is AR coated) Now the device works by comparing the outgoing and incoming wave phases. Its essentially an interferometer which measures the optical path length through air. In fact, its built so that it has 2*pi phase change at 99.999m of air. You can make distance measurements greater that this, but there're all modulo 100m (we never did this). Now, if you stick a piece of something in the beam path with index of refraction greater than that of air, then the wave phase changes more rapidly than it would in air and the thing you're ranging off appears to be farther away than it is. The relationship is linear, has a slope related in the difference between indices of refraction and should have a zero offset. Well, being good engineers rather than theoreticians, we calibrated the relationship with pieces of polished, AR coated BK-7 of various thickness, and found the result to be OPD = t*0.532 - 0.076 mm with an error of about 0.5mm. With a 50mm thick glass, that results in 26.5mm correction. Now, I have to admit, that we never did the calibration with the reflective target right on the glass. There could be some strange interaction going on there. Also, I have to admit, that measurements through glass with the far side reflective coated had to be done very carefully and the instrument alignment had to be tweaked to get repeatable results. However, the fact that the two OTG measurements agree with each other certainly suggests that they were in the same location wrt the HRMA, even if both axial measurements had systematic errors. Hope this helps, Scott ============== ======================== Scott Texter TRW MS R9/1869 One Space Park, Redondo Beach, CA 90278 (310)813-1904 Voice (310)813-6352 Fax