AN MY D IO NO NO O S R P T H S E A R E L A C N E O N I T T E A R N A R Y EC R IB TO O OBSERVA March 2001, Number 32 Photo: David Parker, 1997/Science Photo Library Observing Proposal Reminder: The next proposal deadline is June 1, 2001. Please make a note to get your proposals for observations using the Arecibo Observatory facilities submitted by that date. Details can be found at our web site http://www.naic.edu/vscience/proposal/proposal.htm. Resetting the Arecibo Primary Reflector Surface Paul Goldsmith lthough not strictly considered part Aof the Arecibo Upgrade project, the surface of the 305 m telescope plays a critical role in the overall system per- formance, particularly at the higher fre- INDEX Resetting the Primary Reflector ..... 1 Radio Astronomy Highlights ........... 4 Observing with the Upgraded Arecibo Telescope ....................................... 5 Space and Atmospheric Sciences .. 12 Planetary Science ........................... 16 Computer Department News ........ 16 Employee of the Year 2000 ............ 17 A School Science Project ................ 17 Visit from Congressional Staffers . 19 Colloquia since the last Newsletter 19 Comings and Goings ...................... 19 Figure 1: An image of the errors in the main reflector surface processed from Lynn Baker’s photogram- metry data by Germán Cortés. Blue indicates positive devation from an ideal surface and red/yellow means negative deviation. The unweighted rms is about 15 mm. (Courtesy Germán Cortés). The NAIC is operated by Cornell University under a Cooperative Agreement with the National Science Foundation. quencies that is one goal of the Upgrade. of 0.25 K/Jy at 10.67 GHz. And this was combine the angles to a given target from The surface was last surveyed and ad- before the contractors started redoing the three or more viewing positions, you can justed about 15 years ago, and a lot has surface for us! The mini-Gregorian il- solve for the three dimensional location happened since. The main cables sup- luminated only a small fraction of the of the target. This technique has been porting the surface are held to the ground total surface, but the derived surface rms refined and turned into a commercially by about 2000 cables which connect to from those measurements was 3.3 mm— available combination of hardware and concrete blocks sitting on the ground be- not too terrible, but higher than one software by a company called Geodetic neath the reflector. Soil motions thus can would like for efficient operation. Services Inc. NAIC has been working directly impact the shape of the reflec- with the president of GSI, Mr. John Previous campaigns to set the prima- tor surface. Brown, since 1994, and last year we fi- ry reflector were based on optical sur- nally were able to get an order in for the Based on monitoring by surveying, veys with theodolites. In this procedure, special equipment needed. A somewhat José Maldonado (NAIC) had indications the location of targets located above the different version of this same approach that these motions had been significant, main (North-South) cables was deter- was used to measure the secondary and particularly in the southeast quadrant of mined by triangulation based on mea- tertiary reflectors—the main difference the reflector. That part of the natural surements made from several points is for those relatively small reflectors, a “sinkhole” in which the reflector was around the reflector rim. The locations CCD camera was used. built had been filled in extensively with of these selected points could be mea- dirt and construction material from oth- sured and adjusted to an accuracy of ap- For the measurement of the primary, er parts of the bowl and elsewhere. This proximately 1 mm rms. However, the we have to use a large-format film cam- area was less stable than the rest of the panels are only 1/4 the size of the spac- era. Part of the reason why is evident if ground, and it was not surprising that it ing of the main cables, and each of them you compare the number of pixels in a 6 should be more subject to gradual move- can be adjusted. In the approach used inch by 8 inch piece of film, versus even ment. Puerto Rico is in a fairly active earlier, the positions of panels between the biggest “megapixel” CCD. My crude seismic zone and there are tremors that the main cables were “interpolated” be- estimate is that we get at least a “gigapix- produce small motions—in particular, of tween the measurements of the widely- el” format with the film camera. This is this not very well compacted portion of spaced targets on the main cables. It was necessary if you want to measure a tar- the ground under the reflector. thought that the overall rms was on the get 500 m away to an accuracy of 1 mm. order of 2.5 mm, only slightly less than In addition to the subsidence, the What happens in practice is that the implied by the X-band measurements upgrade work itself was quite traumatic camera is taken up to the top of one of mentioned above. for the reflector surface. There were the towers. It is accompanied by several many panels damaged by items dropped To perform really well, one needs the intrepid NAIC staff members, typically by construction crew working on the overall rms surface error to be less than Lynn Baker, Felipe Soberal, and some- platform and the feed arm. Also, there 1/20 wavelength, which translates to 3 times others. From the tower top, they was one large cable that was dropped, mm rms at 10 GHz. The panels them- take a number of photographs of the dish and when this hit the dish surface, it de- selves are thought to have an error of surface—several photographs are neces- stroyed over 100 panels, and broke some approximately 1 mm rms, and the sec- sary to cover the entire area, and they of the cables that support the dish sur- ondary and tertiary reflectors contribute also take photographs with the camera face. These panels and cables have long smaller errors. So it would be desirable rotated by 90 degrees to be able to iso- been repaired, but the process may cer- to get the primary surface adjustment late any distortion in the camera’s imag- tainly have contributed to a deterioration error below 2 mm rms. It was judged ing system, and take views from two of the accuracy of the reflector surface. impractical to reach this level using the different positions on each tower top as technique employed previously. In as- well. The illumination is provided by a Since one of the major goals of the sessing options, we decided to adopt powerful strobe lamp, which together upgrade was to raise the upper frequen- optical photogrammetry. with the retroreflective properties of the cy limit of operation to 8 GHz or higher target, guarantees that the targets stand (wavelengths less than 4 cm and hope- For this approach, reflective targets out with good contrast relative to the fully as short as 3 cm) it was evident that are put on the panels; these targets are 3 general dish surface. It also means that we would be pushing the accuracy of the inch diameter disks of retroreflective the effective exposure time is very short, primary reflector. Some very limited material. Using a special camera, pho- minimizing any mechanical vibrations, tests using the mini-Gregorian carried tographs of the dish are taken from the etc. Getting all the required equipment out by Phil Perillat (NAIC) in April 1991 top of each of the towers. You can imag- to the tower tops is no mean feat, and we showed that things were not terrific in ine each photograph as yielding the an- have to admire those who carried out this this frequency range, with a sensitivity gular coordinates of the target. If you difficult and sometimes hazardous work. March 2001, Number 32 2 NAIC/AO Newsletter After the photographs are taken, the Cortés—NAIC) is shown in Figure 1. installed by José Maldonado’s team, and film is developed, and then each image Blue means high and red/yellow means they have not yet been adjusted. The is digitized using a special scanner that low. The big surprise is that the un- largest errors outside the center are on is located in lab adjacent to Tony Ace- weighted rms is about 15 mm! This is the order of 100 mm! This is even big- vedo’s (NAIC) office. This scanner is a worse than had been determined in 1991. ger than José Maldonado had expected, close relative to plate measuring ma- The obvious conclusion is that all the and shows how much that part of the dish chines used by astronomers; it measures upgrade work (plus the passage of the surface had sunk. the centroid of each of the “spots” pro- intervening 10 years!) severely degrad- While the first round of photogram- duced by the targets to an error of no ed the surface accuracy. It is difficult to metry was going on, José Maldonado more than a few microns. These cen- compare this photogrammetric rms di- and his crew were undertaking to refur- troid positions are entered into a data file rectly with that derived radiometrically, bish a lot of the panel support hardware on the PC controlling the scanner. because the Gregorian system does not that had corroded since installation in illuminate the entire surface, and large After all the photographs are 1974.