The following is a paragraph in a paper on visual comet magnitudes that I've written for the BAA Journal.  It is a fairly general paragraph (as an introduction to the concept) and perhaps needs expanding - this may happen in a different paper.  VEM = visual equivalent magnitude.  I think it is a reasonably easy guess who the one observer is!  Note that in this sense bias does not mean an error, just a systematic difference from the mean.

Bias corrections, both for observer and instrument effects, can be computed when there is a really well observed comet.  Such comets are unfortunately few and far between, with the last to have over 1000 observations being 2014 Q2 (Lovejoy).  For that comet there was only one observer reporting a significant number of VEM magnitudes.  2017 T2 (PanSTARRS), which reached perihelion in 2020 was less well observed, with just over 500 observations, but was observed by several observers reporting VEM magnitudes.  Considering those observers who made more than 10 observations of this comet, the mean bias correction for all but one observer was less than 0.5 magnitudes.  That particular observer observes from a very good location and has very sensitive eyesight and almost always reports brighter than the mean.  The standard deviation of the observations contributing to each bias is however higher for the VEM magnitudes than for the visual magnitudes.  The bias standard deviation is slightly reduced when a correction for reported coma diameter is also applied.  This confirms that the amount of coma that is recorded is a significant factor in how bright a comet appears.

I'll wait until the end of March before making any predictions on the likely brightness of 2020 R4 as there aren’t enough observations yet.

Regards,

BAA Comet Section visual observations co-ordinator
https://www.ast.cam.ac.uk/~jds/

-----Original Message-----
From: baa-comet@simplelists.com <baa-comet@simplelists.com> On Behalf Of jjgonzalez jjgonzalez
Sent: 19 March 2021 19:33
To: BAA Comet Section List <baa-comet@simplelists.com>
Cc: Juan Jose Gonzalez Suarez <jjgonzalez@telecable.es>
Subject: Re: [BAA Comets] C/2020R4 ATLAS is morning 17th March

Peter, Thomas abd friends,

Many thanks for the supplementary information and data.

Up to now we have these COBS recent estimates for C/2020 R4 (ATLAS) :

Visual (7) :
Mar. 17.19, 9.6, 3' (C. Labordena, Culla, 1000 m, Spain, 0.2-m Schmidt-Cassegrain) Mar. 16.31, 10.0, 2' (J. de Souza Aguiar, Campinas, Brasil, 0.27-m Newtonian reflector) Mar. 15.79, 9.9, 3' (M. Mattiazzo, Swan Hill, near sea level,  Victoria, Australia, 25x100 binoculars) Mar. 15.30, 10.0, -- (J. de Souza Aguiar,  0.27-m Newtonian reflector) Mar. 14.31, 9.8, -- (J. de Souza Aguiar, 0.27-m Newtonian reflector) Mar. 13.82, 10.4, 2' (P. Camilleri, Australia, 0.406-m Newtonian reflector) Mar. 13.21, 8.4, 6' (J. J. Gonzalez Suarez, Alto del Castro - Aralla, 1720 m,  Spain, 0.203-m Schmidt-Cassegrain)

CCD - CMOS (3) :
Mar. 17.21, 10.1, 4.7' (P. Carson, remote, Fregenal de la Sierra Spain, 580 m, Spain, 0.315-m Cassegrain reflector) Mar. 15.15, 9.9, 10' (T. Lehmann, remote, Hakos, 1830 m, Namibia, 0.2-m Newtonian reflector; 0.26-deg tail in p.a. 265) Mar. 09.78, 10.7, 1.5' (M. Mobberley, remote, Siding Spring, 1160 m, Australia, 0.5-m Ritchey-Chretien)

When comparing these two lists, there is an interesting result :
most estimates ( 6 visual, 2 CCD - CMOS ) are close to m ~ 10.0 ...
... but they correspond to clearly different diameters !  --> ( 2' - 3' visual, 4.7' - 10' CCD - CMOS ).
This is somewhat peculiar ...

Though I have a master's degree in Astrophysics ( Universidad Complutense, Madrid, 1970 - 1976 ), I am not a photometry theorist ... only an old style visual comet observer, but  ,,, From my long observing experience, I know that for a detailed comparison of comet estimates data ( m1, coma diameter, DC, tail length if present ) and drawing lightcurves, in relation with the conditions of the observing site, we must know at least two factors not included in the "classic" abbreviated comet report formats :
- Night sky brightness.
- Elevation of the observing site.
These factors are especially important for diffuse comets with faint outer coma, located at low altitude over the horizon.

My estimate for C/2020 R4 ( m1=8.4, Dia.=6', DC=3 ) is consistent with the wider observed diameter from a dark location at 1700 m in the Cantabrian Mountains.

As Peter say, lets hope my prediction of a m1 ~ 7.0 mag comes to be ...

Best regards and clear skies,

J. J. Gonzalez Suarez

P.S.: Disclaimer : I'm not an specialist on comet magnitude predictions ...

-------------------------------------------------------------------------------------

> On 18 March 2021 Thomas Lehmann wrote :
>
> I have observed the comet a few days ago remotely under pristine skies
> of Namibia using a 20cm telescope, with CMOS camera and green filter.
> I measured the total brightness within a large aperture of 10'
>   2021-03-15.15 UT, m1=9.9, coma dia. 10'
>
> Thomas
>
------------------------------------------------------------

> On 18 March 2021 Peter Carson wrote :
>
> Hello Juan Jose,
>
> My measurement of C/2020R4 was made from Fregenal de la Sierra in Extremadura at an altitude of 560m under skies that were SQM 21.8 at the zenith with low humidity. My image records a coma dia of 5’. The Comphot photometry tool used a measuring aperture of 4.7’ so measured virtually all the apparent coma. I’m confident my result is consistent with all my other photometry in COBS.
>
> Lets hope your prediction of a m1 7.0 mag comes to be.
>
> All the best
>
> Peter
>
----------------------------------------------------------------
> On 18 March 2021 J. J. Gonzalez wrote :
>
> Nick, Peter, and friends,
>
>     In my condition of "bright outlier", I kindly disagree when you say "It probably won't get much brighter through March and April".
>
>     COBS recent visual estimates around m1~10.0 are associated to a observed coma diameter in the 2' - 3' range.
>
>     From dark mountain skies the comet shows a wider diffuse outer coma. In my recent observation :
>
> C/2020 R4 (ATLAS):
> 2021 Mar. 13.21 UT: m1=8.4, Dia.=6', DC=3, 20 cm SCT (77x).
> [ Altitude: 12 deg. Mountain location, very clear sky.
> Sidgwick method. Tycho-2 comparison stars. SQM: 21.0.].
> ( Alto del Castro, Leon, Spain, alt. 1720 m; SQM 21.5 at zenith ).
>
> This estimate is in good agreement with the formula :
>
> m1 =  7.5 + 5 log delta + 10.0 log r
>
> providing m1~7.0 at Earth's close approach ( delta = 0.46 AU on 2021 Apr. 23 ), located high in the sky on the Hercules–Corona Borealis border.
>
>     It will be an interesting photometric evolution to follow ... weather permitting.
>
> Best regards and clear skies,
>
> J. J. Gonzalez Suarez

----------------------------------------------------------------------
>
> > On 18 March 2021 Nick James <ndj@nickdjames.com> wrote :
> >
> >
> > Peter,
> >
> > Thanks for the image obtained under difficult circumstances.
> >
> > Martin Mobberley got 10.7 on March 9.78 and visual estimates on COBS
> > are all around 10.0 now with one bright outlier. It probably won't
> > get much brighter through March and April. At least it will become
> > easier for us to observe from the UK over the next few weeks.
> >
> > Nick.
> >
> > --------------------------------------------------------------------
> > ---------------
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