Blair MacDonald and I observed the Near-Earth asteroid 2005UY55 the other night simultaneously at "2011-11-09T01:55:00 UT" from two sites in Nova Scotia just 11.2 km apart. The goal was to try to detect parallax and detemine a distance to the asteroid.
We used the tried-and-true 3-2-1-GO! method via cell phone to synchronize the exposures. The exposure lengths were 30 seconds in each case as controlled by a shutter timer. Blair used a Canon Rebel DSLR and 4" APO refractor in his back yard and I used an SBIG ST8XME CCD and Celestron C14 at my backyard Abbey Ridge Observatory. He was outside in the cold... I was not!
Blair analyzed the two images by scaling his images to match mine and registering them using the stars recorded on the image. He corrected the end points (start/end times) of his image by the measured delay caused by the cell phone network (about 0.75 seconds one way). The measured parallax was 6.9 arc-seconds. The pixel sizes are 1.47 arc-seconds for Blair images, so it would be hard to be more accurate than that, so I'll call the measurement 6.9" ± 1.5" (5.4" to 8.4" range).
This parallax is a projected parallax that is only correct if the object were overhead, which it wasn't.
The azimuth of the object (247°) was close to the same angle of our geographic separation (257°), so the projected site separation need only be reduced by the sine of the object's altitude (42°=0.67x) which turns 11.2 km into 7.5 km.
By using the projected separation of 7.5 km, the parallax range of 5.4" to 8.4", and simple trigonometry, the asteroid's distance is determined as follows:
- Distance 1 = 7.5km / tan (5.4"/3600"/degree) = 286,500 km
- Distance 2 = 7.5km / tan (8.4"/3600"/degree) = 184,200 km
The JPL Horizon's ephemeris at the time of observation gives a predicted distance of 346,642 km, which corresponds to a parallax of 4.46". Our results under estimate the distance, but not overly so given the uncertainties and small angles and geographic separation involved.
Dave Lane Halifax Centre