Hi Nick,

Actually youve made me realise I ignored the motion of the object within my subs; I assumed it would be insignificant within the subs length, but at 10secs this isnt really so. I dont think it affects the findings but its marginal.

Scaling from your result I get 10/15*2.3=1.5" which is pretty much 2 pixels. Adding the smear of this in quadrature to the FWHM of the PSF probably doesnt significantly increase the total FWHM but it shouldnt have been ignored. I should have used 5sec subs.

I havent directly measure the FWHMa as Id have to import into IRIS or something which is always painful. 

You may find that youve detected a faint coma that doesnt significantly affect the measured FWHM, as the sharp FWHM of the pseudonucleus is superimposed on a much larger FWHM for the coma - the software effectively ignores the latter.

Cheers

Nick
From: baa-comet@simplelists.com <baa-comet@simplelists.com> on behalf of Nick James <ndj@nickdjames.com>
Sent: 11 February 2021 19:22
To: baa-comet@simplelists.com <baa-comet@simplelists.com>
Subject: Re: [BAA Comets] C/2021 A10 (NEOWISE)- nonstellar
 
Nick,

That is a great piece of work.

This is another NEOWISE discovery and it first appeared on the MPC
Possible Comet Confirmation Page (PCCP) as object N00hurv a few weeks ago:

https://britastro.org/cometobs/2021a10/2021a10_20210122_0609_ndj.html

It has a very cometary orbit but it didn't look very cometary so it
lingered on the PCCP for a long time before it was finally designated a
comet, see MPEC 2021-C25:

https://www.minorplanetcenter.net/mpec/K21/K21C25.html

Peter Birtwhistle at J95 was one of the observers who reported cometary
features.

I attempted to do what you have done last night but I was using 15s
exposures and so the comet was trailed by around 2.3 arcsec. When I
stack the stars so that they trail by 2.3 arcsec over the stack period I
got the same FWHM PSF for both comet and stars.

Your image is another good example of what can be done on a comet using
lots of short exposures with high resolution.

This object will be worth keeping an eye on as it comes to perihelion
just in case its activity ramps up.

Nick.


On 11/02/2021 15:43, Nick Haigh wrote:
>
> Hi folks,
>
> The attached shows a presentation of my images for this object from this morning 11/2/21 0038 to 0305 UT (2hr 3mins of 610 longpass (ie Red+IR) data). 16" newtonian, ASI1600MM, reducer to f2.9.
>
> The first image is stacked on the comet, and the next two images are stars from the 'star stack' of the same dataset with similar peak intensities to the comet.
>
> I do believe its non-stellar. I've subtracted two stars from each other (bottom right, no residual), and a star from the comet, and the comet leaves distinct residuals, even when the central intensity is zero or slightly negative. Slight scaling was used to allow for differences in intensity (done in FITSWORK).
>
> Image scale is 0.65"/pixel. Residual of ~6 pixels across implies a size of 3.9".
>
> I took care to use two example stars from the centre of the FOV  (where the comet ended up)and the edge (near where the comet started in the first images) so any variation in PSF across the sensor could be seen. None is obvious.
>
> I think this is reasonably conclusive....! We have a comet!
>
> Cheers
>
> Nick
>
>
> ________________________________
> From: Nick James <ndj@nickdjames.com>
> Sent: 11 February 2021 07:54
> To: Nick Haigh <happylimpet@hotmail.com>
> Subject: Re: [BAA Comets] C/2021 A2 NEOWISE image.
>
> Nick,
>
> Here's my attempt from last night. Not much evidence of cometary
> activity from this.
>
> Nick.
>
>
> On 11/02/2021 00:44, Nick Haigh wrote:
>> OK Im on it! You know how to press my buttons!!!!!
>>
>> As you say, moving fast, so its bang on one side of my sensor at 0030 and will pass off the top at 0500...should be plenty of time. I assume I can stack this.....I'll find a way.
>>
>> Im using coordinates off NASA Horizons...I assume theyre accurate? Fingers crossed and nothing ventured.....definitely a suitable target.
>>
>> Weather is saying high cloud developing but hopefully not too badly, it usually overestimates it.
>>
>> Cheers! Will let you know what I get.
>
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