Since the main
reflector for the
Arecibo Observatory telescope (all 70,000 square meters of it) is actually made up of thousands of individual panels, its
shape slowly warps over time and it must be
resurfaced. Scientists recently conducted a
survey to resurface the dish, using the technique of
photogrammetry.
In order to conduct the survey, a small
retroreflecting strip was placed on the corner of each panel. Surveyers then took multiple
images from
up on top of the platform, and from the top of one of the towers, at
night using a powerful
flash. The resulting images picked up almost exclusively on the points of light produced from the flash reflecting off of the retroreflectors. Unfortunately, the dish is so large that
scientists were forced to
interpolate among the different images to put together a complete image of the entire dish. Using their knowledge of where each of the points where the retroreflectors were located, they were able to
calculate the precise
curvature of the dish, and from there went about deciding which panels needed to be adjusted to restore proper curvature. Crews of workers then descended under the dish to manual adjust the
screws on each panel which had slipped out of
alignment.
The main
difficulties involved in this project related first to the
longevity of the retroreflectors. In the
tropical climate of
Puerto Rico, they were only expected to last a few months at best, meaning the survey had to be completed, the
data interpreted, the adjustments made, and a follow-up survey conducted all within that short time-frame. Second, the
cameras required for the survey were incredibly large and bulky, making it difficult to get them up to the top of the platform or onto the towers. While smaller
digital cameras provided decent results, the
pixelated nature of the images obtained reduced the
precision with which the calculations could be made.