Hi Thomas,

Best I can suggest, at least right now, is to use the following
logic: this object is in the ballpark of mag 2 now, fifteen magnitudes
brighter than what you'd get with the current H value. So change H
from 10 to -5, and you'll get a passably correct current magnitude.

Which is about all we can hope for. We don't know what this object
will do in the future, so the best we can do is to "brute force" the
magnitude to something close to its current value.

So. Use Extras... Edit Comet Data, and select P/Holmes. You'll
see the H value given in the resulting dialog. Change it to -5, and
click OK.

The comet will then appear _much_ larger in Guide, with a five-degree
tail.

It occurred to me that H=-5 is a _very_ bright comet. (It means that,
were the object one AU from the Sun and you were standing on the Sun,
the comet would be a mag -5 object... sort of a measure of how intrinsically
bright the object is, with the effects of distance from the sun and observer
removed.) The closest in recent times was Hale-Bopp, with H=-2, making
this object (at least briefly) three magnitudes bigger and brighter than
Hale-Bopp. Gives you an idea as to just how enormous an outburst this is.

-- Bill