I managed to image this comet on Thursday morning in a very bright and
hazy sky:

https://britastro.org/observations/observation.php?id=20230817_205409_67bcd83df81cc3cc

I've used astrometry up to August 18 with the constraint e=1 to get the
following orbit for C/2023 P1 (Nishimura) using Findorb:

    Perihelion 2023 Sep 17.624618 +/- 0.0249 TT;  Constraint: e=1
Epoch 2023 Aug 17.0 TT = JDT 2460173.5   Earth MOID: 0.0779   Ve: 0.0426
q   0.22560874 +/- 0.000322         (J2000 ecliptic)
                                     Peri.  116.10237 +/- 0.08
                                     Node    66.93040 +/- 0.050
e   1.0 +/- 0                       Incl.  132.40851 +/- 0.12
 From 56 observations 2023 Aug. 14-19; mean residual 1".30

The attached plot shows the elongation of the comet using that orbit.
The comet has been at an elongation of < 40 deg since the beginning of
May and Nishimura discovered it as it reached around 34 deg rising out
of the morning twilight. For amateur comet discovers searching the
morning twilight sky is a key tactic. The orbit of this comet was almost
perfectly designed to avoid the surveys.

As it approaches perihelion in September the elongation will drop
rapidly and when the comet is at its brightest in September it will only
be 12 degrees or so from the Sun. To see it then you will probably want
to be high up a mountain but we'll see how things develop.

The surveys could have picked it up back in April when the elongation
was much larger but it was then over 3au from the Sun and so would have
been much fainter. I would expect that, once we have a better orbit,
we'll find it somewhere in the survey data.

Nick.

On 15/08/2023 22:16, Nick James wrote:
> CBET 5285 announces the discovery of an 11th magnitude, diffuse comet by
> the Japanese observer Hideo Nishimura on Aug 12.78 UT. The comet is
> currently at a small solar elongation (34 deg) but may be visible in the
> morning twilight. A number of visual observers have reported
> observations in the last day. The solar elongation is almost constant
> through to the end of August then decreases through September as the
> comet brightens.
>
> The comet is currently in Gemini. At the start of nautical twilight at
> 52N tomorrow (Aug 16, 0330 UT) the comet will be around 14 degrees
> elevation.
>
> An ephemeris is available on JPL Horizons. Please send any observations
> to the section.
>
> Nick.
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