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Photogrammetry

The original GBT photogrammetry was performed with the telescope at the rigging angle of 50.29 degrees. The whole dish could not be seen at once from the camera's position on the crane, so the dish was rotated in azimuth and overlapping photos were obtained. The purpose was to verify that all actuators were within +/-1/4 inch from of the best-fit paraboloidal surface, with the focal length constrained to f=60 meters exactly. 97% of the actuators were within that tolerance. The results are given in the attached file. Of the 2209 actuator targets, 23 were found to be in excess of 3/8 of an inch (0.375" = 9525 microns) away from the target value. (These 23 are listed at the bottom of this page.) The contractor was required to bring these into the +/-0.375" (9525 microns) specification by adjusting them mechanically, but they were never remeasured by photogrammetry. (We believe the adjustment was done by installing shims, but we will confirm this with Harry Morton. It could have been done by sliding them along the slotted grooves, unless the total groove travel was exhausted.) Most of the offending actuators are located on the lower right edge of the dish (as viewed from the receiver cabin). This figure shows the location of the 23 outliers from this viewpoint (by circled numbers). The offset values for these actuators listed in the current actuator zero file assume that the error was completely removed by the mechanical adjustment, but this may not be the case. To investigate this issue, we have examined the traditional holography from Oct 2003 through April 2004. We have estimated the remaining actuator error for each of these 23 actuators, and plotted the results here. Red is the photogrammetry value, green is the holography value, and the y-axis is in microns.

• estimated actuator error from 16Oct2003 holography
• estimated actuator error from 02Apr2004 holography

Outliers

Comparison shots of 660009 = 1.jpg 2.jpg 3.jpg

Shots of 658016 = 1.jpg 2.jpg 3.jpg 4.jpg (shim = 0.4")

Shim thicknesses estimated by tape measure and digital photos.

-- ToddHunter and FredSchwab - 19 Jun 2008

Attachment:	Action:	Size:	Date:	Who:	Comment:
gbtphotogrammetry.txt	action	172738	19 Jun 2008 - 22:02	ToddHunter	photogrammetry table (file dated July 5, 2000)
locationOfOffenders.pdf	action	31339	19 Jun 2008 - 22:07	ToddHunter	location of the 23 outliers on the dish
offenders_16oct2003.pdf	action	135847	19 Jun 2008 - 22:18	ToddHunter	Estimated actuator error from 16Oct2003 holography
offenders_02apr2004.pdf	action	136290	19 Jun 2008 - 22:18	ToddHunter	Estimated actuator error from 02Apr2004 holography
telescopes-1395.jpg	action	66364	19 Jun 2008 - 22:38	ToddHunter	photo of actuator installed on telescope
09360_001.JPG	action	355212	23 Jun 2008 - 22:54	ToddHunter
09360_002.JPG	action	334240	23 Jun 2008 - 22:55	ToddHunter
09360_003.JPG	action	347744	23 Jun 2008 - 22:56	ToddHunter
09360_004.JPG	action	412678	23 Jun 2008 - 22:56	ToddHunter
09360_005.JPG	action	425280	23 Jun 2008 - 22:56	ToddHunter
09360_006.JPG	action	366428	23 Jun 2008 - 22:56	ToddHunter
09600_001.JPG	action	361256	23 Jun 2008 - 23:00	ToddHunter
09600_002.JPG	action	360166	23 Jun 2008 - 23:00	ToddHunter
09600_003.JPG	action	343736	23 Jun 2008 - 23:00	ToddHunter
10600_001.JPG	action	364332	23 Jun 2008 - 23:02	ToddHunter
10600_002.JPG	action	334654	23 Jun 2008 - 23:02	ToddHunter
10600_003.JPG	action	367180	23 Jun 2008 - 23:03	ToddHunter
10600_004.JPG	action	390958	23 Jun 2008 - 23:03	ToddHunter
10600_005.JPG	action	355924	23 Jun 2008 - 23:03	ToddHunter
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18560_001.JPG	action	308846	23 Jun 2008 - 23:13	ToddHunter
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18600_001.JPG	action	321006	23 Jun 2008 - 23:19	ToddHunter
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16620_001.JPG	action	355192	02 Jul 2008 - 22:11	ToddHunter
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19560_001.JPG	action	319410	02 Jul 2008 - 22:16	ToddHunter
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19580_001.JPG	action	1258984	02 Jul 2008 - 22:19	ToddHunter
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25500_001.JPG	action	1257644	02 Jul 2008 - 22:48	ToddHunter
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30435_001.JPG	action	380272	02 Jul 2008 - 22:59	ToddHunter
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42110_001.JPG	action	386782	08 Jul 2008 - 22:58	ToddHunter
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43090_001.JPG	action	383248	08 Jul 2008 - 23:01	ToddHunter
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43090_003.JPG	action	340070	08 Jul 2008 - 23:02	ToddHunter