I've used -4.5 and it gives a number close to the current visual
magnitudes posted observations on the ICQ page.

Chris Spratt
Victoria, BC

--- In guide-user@yahoogroups.com, Bill J Gray <pluto@...> wrote:
>
> 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
>