changing rap in comet calculation lsst_comet_activity.py

[mschwamb]

Once #4 is merged, please go in and change rap=df["seeingFwhmEff"] back to the default value of 1 arscescond. We can live with that as the value as anyone can now go edit this and pip install it or supplement with their own way of doing cometary activity.

From email conversation between Steph and Colin S.

In theory the Afrho measure of activity is independent of aperture size (rho) for a comet in steady state, but obviously that isn’t true for an aperture that is larger than the physical extent of the comet, which might easily be the case at very large distances. Also, I think that the simulator gives PSF magnitudes rather than aperture ones, so what we need is the equivalent PSF magnitude for an object with a non-PSF profile at a given brightness.

Because the rho parameter goes into the magnitude calculation you should get sensible magnitudes within an aperture for any value of rho that is not larger than the comet actually is (so this will be fine for rho < 100,000 km, typically). That is 1.4” at 100 au, so for any reasonable comet detection (30 au likely to be a realistic maximum distance even for a very big and bright one) the coma should be larger than the seeing disc, even in poor conditions. I guess a weakly active comet could have a coma smaller than 100,000 km in radius, but this wouldn’t be bright enough to detect at anything like those distances, so we are going to be OK using the seeing FWHM as rho for calculating.

The problem will be when converting this aperture magnitude to a PSF magnitude. If you just calculate the comet magnitude within an aperture radius rho = seeing FWHM, you will include more flux and get a brighter magnitude as the seeing gets worse, which means the simulator will tell you it is easier to spot comets in bad seeing, which isn’t true. Perhaps if it treats the magnitude as a point source and then accounts properly for seeing (so this point source is more difficult to detect in poor seeing) this cancels out, but I’m not sure. I’ve been trying to think of a good way to define the equivalent PSF magnitude for a non-PSF profile with a 1/rho profile, but I don’t have an answer yet. Perhaps you have a good idea :)