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C/2004 P1 (NEAT)

A faint and distant comet, this object was placed on the NEO Confirmation Page late on 7th August 2004 and announced the next day in IAUC 8383 and MPEC 2004-P23.

In helping confirm objects on the Confirmation Page, normal practice is to produce three positions for each object. A (sometimes large) number of exposures are taken and these digitally combined (or stacked) to strengthen the light of faint objects in order to measure three positions.

To capture this object, listed as mag +19.5 on the NEO Confirmation page a total of 17.5 minutes of exposure were taken with the CCD binned at 2x2 (i.e. the resolution of the CCD was reduced to 3"/pixel in order to increase the amount of light collected in each pixel from the faint object). However, when stacking the images to measure the first position there was a suspicion that it might be slightly diffuse, so to get the last of the three positions the camera was set to operate in unbinned mode for maximum resolution of 1.5"/pixel.

The resulting image shows the comet to be more diffuse than nearby stars of similar magnitude, the stack from all the unbinned exposures appears here enlarged by a factor of three, with the hint of a tail extending to the lower right (in p.a. 235)

As an indication of the comet's diffuseness, the Astrometrica Object verification screen is shown below for both the comet and a nearby field star of similar brightness, as can be seen, the star is significantly smaller than the comet. The comet image was stacked taking into account the motion of 0.24"/minute and the star stacked with the same images and with no motion offset.

Comet C/004 P1 (NEAT) Object verification window showing the comet filling the whole 7 pixel (10.5" diameter) measurement annulus and the PSF-Fit curve to the right showing the rather flat profile of a diffuse disk rather than a centrally condensed starlike image. 

Field star object verification window showing a mag +19.9 star covering ~3 pixels (4.5" diameter) with a well fitted gaussian curve on the PSF-Fit curve to the right, indicating a good fit to a point source of light :

The first orbit published by the Minor planet Center was calculated from 24 positions over the period 5-8 August 2004 and gave the perihelion distance (q) as 5.7 AU, with perihelion passage occurring in May 2006. However, for any orbit, the perihelion time can be fairly indeterminate until observations cover a reasonable proportion of the comet's orbit. For small q comets this can be achieved with just a day or two of observations but for large q comets that are moving very slowly, this can take weeks or months before the perihelion time can be accurately determined.

Early orbit revisions for C/2004 P1 have given the perihelion time as April 2006 (37 observations 2004 Aug 5-9) and then nearly three years earlier on 2003 August (42 observations 2004 Aug 5-20). Further revisions are expected as more observations are made.


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