<p class=”wp-dark-mode-bg-image”>I’ve been doing some astrometry on 3I/ATLAS and have been surprised how much scatter in the magnitude that I’m getting day to day. The plot of magnitudes from MPC astrometry data also shows a huge scatter. This data doesn’t have a consistent photometry aperture or filtering but, for most observers at the moment 3I/ATLAS is a point source so it is surprising. The scatter may be due to measurement errors in the crowded star field but it is worth making estimates of the magnitude if you can.
Might the scatter arise in large part from the multitudinous fainter field stars which are too faint to show up clearly in the images but the photons from which will create a very noisy sky background. The noise will add both to the counts at a given a pixel and will vary strongly from pixel to pixel.
Have you tried PSF-fitting photometry, using DAOPHOT for example? It would be interesting to see whether you get significantly different results. DAOPHOT is famous for working well in crowded fields. It is a shame that using it is often a fiddly pita in practice.