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.
> >
> > -----------------------------------------------------------------------------------