Nick,

Thanks. That's interesting.

The attached is a measurement of the PSF of the comet and a star of
equivalent brightness using Astrometrica. This is using the images from
iTelescope 72 last night. Astrometrica reports a smaller FWHM for the
comet (!) but the fit is not as good and there is some evidence of
"tails" in the distribution. One problem is that I was using 60s
exposures and the comet is moving at 1.2 arcsec/min. I don't think there
is any evidence here for this being cometary.

In other news, I managed to image 3I last night using a 90mm refractor
in my garden in Chelmsford (the main telescope can't see that far
south). That shows that this interstellar "comet" is currently available
to many observers.

Nick.

On 03/07/2025 12:02, Nick Haigh wrote:
> Final stack (!) with a subtraction of a nearby star, showing a faint halo ~10" across around it. Both stacked astrometrically in ASTAP, using the ephemeris for the comet, so instrumental PSFs should be identical. Subtracted star was scaled such that the residuals showed a dip in brightness in the middle to ensure I was fully subtracting the nucleus, hence it being a vague doughnut.
>
> Cheers all
>
> Nick H
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