Good morning Charles,

Many thanks for your posting. I agree absolutely that we should always
urge caution with observations such as this and any potential "fragment"
should be confirmed over multiple nights by multiple observers.

Any comments from a person with your level of experience must be heeded
but I disagree that these observations should only be made with metre
class telescopes. Within the BAA, Nick Haigh, Denis Buczynski, Richard
Miles, myself and others have imaged the fragmentation of C/2019 Y4 with
telescopes in the 0.26-m to 0.35-m range and provided astrometry to the
MPC. Have a look at my sequence here:

http://www.nickdjames.com/Comets/2020/2019y4_20200405_ndj.jpg

and many of the images in our C/2019 Y4 archive here:

https://britastro.org/cometobs/2019y4/thumbnails.html

You can compare images taken by multiple observers on the same night and
see common features. The quality of comet imaging with modern systems
has come on significantly in the last 10-15 years.

With regard to the new suspected fragment I am as surprised as you are.
It wasn't on my image of May 1st but it is on three stacks of 15 minutes
each taken last night (May 2nd). The animated GIF attached shows stacks
used for astrometry (the star trails are around 26 arcsec long). The
object in question is the small dot around 22 arcsec ahead of the bright
remaining fragment B and moving with the comet. It is rather faint, 19th
mag, but it is measurable. On the plane of sky 22 arcsec corresponds to
around 15,000 km at the current distance so, assuming a break up around
March 25, a relative velocity of 4.5 m/s in the plane of sky would suffice.

Regarding the fragment lettering I agree it is a mess and it must be
really hard for MPC/JPL to keep track of these based on astrometry
alone. The lettering evolved during the breakup and was different for
different observers but I think generally we agree that the remaining
bright fragment is "B". This corresponds to the official MPC designation
(C/2019 Y4-B). At the time on comets-ml I suggested that it would be a
good idea to come up with a standard approach early on but, in reality
it is the MPC who does this. It was certainly not HST since they only
released their images towards the end of April.

Nick.

On 03/05/2020 05:54, Charles S Morris - cometguy3783 at yahoo.com (via
baa-comet list) wrote:
> I appreciate the fact that you are asking for confirmation of your observation.  I can say that there is no such object on my image taken on May 2.196 UT with my 41 cm SCT + CCD.  I processed my image every possible way and ran brightness profiles through the coma.  Nothing was there.
> I have no idea what is on your image.
> Now for a reality check - I apologize for this Nick, but a few weeks ago everyone, regardless of instrument size, was claiming to have imaged "the comet and fragment ."  I threw cold water on that and finally, that ended.  Now I have to end this before it starts again.
> 1) It would take months or years for a fragment to move that far in front of (or any direction from) the main nucleus.  I saw the fragments from Comet West many months after the break-up (through John Bortle's 12" L) and they were still in a tight little cluster.  The differential velocity between fragments simply isn't that great and we are looking at vast distances between the center of the comet and the edge of the coma.
> 2) If it was real, there would be a string of fragments and not just one out there by itself.  I suspect that all the fragments are still very close to the center of light in the coma.
> 3) Unless you have a meter class instrument, you are not going to be able to resolve the fragments.  Everyone, PLEASE stop looking for things that can not reasonably be at the edge of the coma.  You will find them.  I doubt that they are real, but you will find stuff.  Heaven knows people did a few weeks ago.  And none of it was real.
> 4) Prior to suggesting fragments, I propose that such reports be ignored unless 1) the observers are using meter class or larger telescopes, 2) they have two nights of observations with astrometry proving the unknown object is connected to the comet, or 3) the observations are with HST!
> A separate question - who has assigned letters to the various fragments???  I hope it is the HST folks that clearly had 11 fragments.  I have seen at least one assignment of fragments and their distribution of fragments did not agree with the HST image - a small problem.  So who did the assignment?
> Sorry to be straightforward and undiplomatic, but I don't want to get people's hopes up that they are going to image something that isn't really possible.
> I will be posting my recent images on FB and probably on the comet images group.
> Regards,Charles