Telescope Reviews: Comet C/2011 L4 (PANSTARRS)

One thing the low elevation of C/2011 L4 does remind me of is whether or not the ICQ extinction tables/method needs updating.

There are tables provided for altitude to every 0.5 km to account for Rayleigh scattering and the 0.016-mag/air mass extinction for ozone is essentially "baked into the cake".

It's the aerosol extinction, though, that perhaps needs a makeover. There are three tables to account for various aerosol loads ("average", "winter", and "summer"), with the "average" set at 0.120-mag/air-mass and the winter and summer values set at 70% and 130% of that, respectively.

There are multiple issues I've got with this approach. (1) The "winter" extinction table can be used year-round at desert locations and most of Australia, and even that doesn't account for how clean the air can be in these locations. Likewise (2) some locations with humidity and high industrial emissions rarely have clean enough air to justify the "average" extinction table, and (3) the aerosol extinction is assumed to decrease logarithmically with altitude. Normally that's the case, but there could high-altitude haze (e.g. forest fire smoke, volcanic ash) that might be absent from the lowest 10,000-ft altogether.

Aerosol extinction, in other words, simply is what it is for a particular location and time. The only way to really account for it in your estimate is to compare the brightness of stars low along the horizon with those near overhead and to back-calculate the aerosol (and, thus, total) extinction if--IF--an estimate needs to be corrected for atmospheric extinction. It'd be very cumbersome to put it in tabular form, but a simple spreadsheet program with a few inputs would easily suffice.

Obviously an experienced comet observer should go out of his/her way to seek out suitable comparison stars at similar altitudes as the comet to avoid the use of extinction tables altogether, but some comets take away that option from the get-go. And I'd concede that the methodology of the observer can easily introduce as much error and uncertainty as atmospheric extinction to begin with. It's just something that's bothered me for some time, and the low elevation of C/2011 L4 at its brightest just reminds me of it. This isn't meant to be a slight to Dr. Green's very important and well-researched 1992 ICQ article regarding atmospheric extinction; quite the opposite. Maybe it's time to update to procedure a bit by tailoring the equations to specific conditions and locations.

Clear Skies,
Phil