2004 GC19 (Imaging a fast
moving 20th mag. Apollo in a rich star field with Astrometrica)
Discovered by LINEAR early on the 15th April 2004 as an 19th
magnitude object moving North at about 10"/min, 2004 GC19 was confirmed
from Great Shefford later the same day and followed up from four other
observatories before the discovery circular was issued at 4pm UT on 16th April (see
MPEC 2004-H06).
2004 GC19 was already within about 22 Lunar Distances (LD) at
discovery and still approaching the Earth, reaching its closest on 21
April 2004 at a distance of 13.8 LD (or 0.0354 AU) by which time it was
moving at about 26"/min, was magnitude +19.2 and starting to fade fast.
Positions were obtained from Great Shefford on six nights
following the confirmation night and it was last detected shortly after midnight on
25th April 2004 when it was moving at 19"/min and predicted to be
magnitude +20.2. At this time it was in Draco, just 5° from the northern
border of Cygnus and in a rich star field.
To image the object that last night to get astrometry, but without
the object trailing significantly, individual exposures were limited
to 10 seconds duration. A total of one hundred such exposures were taken between
02:06 UT - 02:46 UT on 25th April 2004.
When these images were stacked together using
Astrometrica the fast moving asteroid was
immediately visible (see below in a stack of all 100 images) but the
many nearby star trails interfered with the centre of light of the
asteroid and meant that no precise measurement of position
could be obtained.
100 x 10 second exposures stacked for motion of
18.74"/min in p.a. 96°. Field size is approx. 25'x25'. Note the
satellite trail crossing the upper left hand corner of the field (also
visible in the frames below)
However, because of the many short individual images taken, those
where the asteroid was passing close to field stars could be left out to improve the quality of the final image.
In the image below, all 100 exposures have again been stacked, but this
time with zero motion. Because the faint asteroid's light is now dispersed
across a long track rather than being concentrated in one spot it isn't
visible directly in the image below. However, the path of 2004 GC19
has been marked, together with tick marks against the stars that had
interfered with the image stacked to compensate for the asteroids swift
motion.. The image numbers of relevant images are also marked, so stars
can be seen to have interfered on images 26, 40, 48, 55 and 72.
100 x 10 second exposures stacked for motion of 0"/min.
Field size is approx. 25'x25'.
The images were restacked but this time leaving out those where stars
interfered (and also those to either side of those images, again
accounting for the asteroid's motion to concentrate its light into a
single spot.
The before and after effect on the asteroid's image can be seen below
in screen shots of the Astrometrica object verification window -
a very measurable target has been obtained from a set of images that on
first appearance might have looked unlikely to provide an accurate
position.
Above: The Astrometrica Object Verification form from the original
stack of 100 images, showing the vague concentration of light at the
asteroid's location.
Here 16 images have been left out of the stack and now the asteroid
stands out from the streaked stars.
(Note that leaving out the 16 frames has changed
the mid-time of the stack of images from the stack of 100 images, so the
indicated RA & Dec positions are not exactly comparable).
The full picture from the stack of 84 images now looks like this, with
the asteroid standing out in better contrast:
84 x 10 second exposures stacked for motion of 18.74"/min
in p.a. 96°. Field size is approx. 25'x25'.
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