07/02/06

FREQUENTLY ASKED QUESTIONS

What is "Astrometry" and how is it done?

Astrometry is the precise measurement of the position of an object in the sky with respect to a specified coordinate system, at a specific point in time.  For astrometry of NEAs done at the Francisquito Observatory, the procedure goes something like this:

1) A series of at least three CCD images (just like pictures made with a digital point-and-shoot camera) are taken of a small field in the sky where an asteroid is expected (hoped) to be at that particular time.  There is a pre-planned time interval between the three pictures, which depends on the expected sky motion (angular speed) of the target object - this typically ranges from 5 minutes to 30 minutes.

2) Using some pretty clever software, each CCD image is compared to a catalogue of known star positions; stars which appear in the image are assigned coordinates from the corresponding stars in the catalogue.  The star catalogue currently in use at Francisquito is called the USNO A2.0 catalogue (US Naval Observatory) which contains coordinates and brightness magnitudes for over 520 million stars.

3) More clever software compares the objects in each of the three CCD images, and does its best to pick out objects which seem to change position from image to image.  The software usually offers the astronomer a choice to to accept or reject an identified moving object - sometimes "noise" in the image can be mis-identified as an astronomical object.  And with surprising frequency, the software is unable to identify a real moving object in the three images, but a trained human eye can indeed identify it by rapidly blinking the images in a movie-like fashion; this often happens when an object is near the practical faintness limits of a particular telescope and CCD unit, or when the object of interest is moving across a very dense field of background stars.

4) For the moving objects accepted as real astronomical objects, the software calculates the precise position of the object (to 1/36,000th of a degree) by relating its position to the stars in the image which have coordinates (from the catalogue) assigned to them.  The software also keeps track of the precise moment in time at which the object was photographed - to the nearest 1/10th of a second.

What constitutes an observation, and how is it reported?

For astrometry of NEAs, an observation is a single line of data which includes information on which object was observed (its ID number, if it has one), when it was observed (to 1/10th of a second), its East-West position in the sky (to 1/36,000th of a degree), its North-South position in the sky (to 1/36,000th of a degree), its estimated brightness (on an open-ended scale, with increasing numbers meaning decreasing brightness), and the identify of the observatory which made the observation.  For the three-image astrometry process described above, an individual observation would most likely be reported for the object(s) of interest in each of the three images.

Observations for NEAs are submitted to the Minor Planet Center in a specific format.  An example astrometry report to the Minor Planet Center is reproduced here, along with an explanation of what the lines of data mean.

What is the significance of the name "Francisquito" and how do I pronounce it?

Let's tackle the second part of the question first - pronunciation of Francisquito.  Say the name "Francis".  Then say the name "Quito", like the capitol city of Ecuador.  Now put the two together: Francis-Quito, and you've got it!  Congratulations!  Easier than you thought, yes?

The observatory is located at the mouth of Francisquito Canyon, in the Santa Clarita Valley, in northwest Los Angeles County.  The area was originally part of a vast Spanish Land Grant known as Rancho San Francisco.  The name Francisquito in Spanish could be literally translated into English as "Little Frankie".  There is much interesting history in this area - check the following link to the Santa Clarita Valley Historical Society.  On a more somber note, Francisquito Canyon was the site of probably the worst engineering disaster in U.S. History - the St. Francis Dam collapse in 1928.  This event weighs on the mind of one of the observatory co-directors, who is an engineer responsible for the design of structures in soil and rock.