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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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