Re: (IAAC) Supernova observations
At 12:36 AM 4/22/98 +0100, you wrote:
>Although subsequent examination of a net image of the supernova showed this
>was the correct location, Gary Popyner of the BAA variable star section told
>mew the SN was about 16 magnitude, too dim for my 20". What I probably saw
>was a condensation in the galaxy.
Good job with the supernovae, Nick! I have had some clear nights here, and
you are putting me to shame - I have been shooting minor planets with a CCD
and not getting enough photons on my retina.
But I have to take issue with your statement that 16th magnitude is too dim
for your 20", unless you meant on that particular night (because I know you
have quoted 6th magnitude naked eye limits in the past). Even under light
polluted (aurora counts as light pollution in these circumstances) 4th
magnitude skies, 15.5 magnitude stars are visible by a casual observer
using an 18" telescope, and with additional magnification stars of 16.3
have been easily seen under such conditions.
Under a moderately dark sky, such as one with a 6th magnitude limit, 17.3
is pretty easy in an 18" that I use occasionally. I don't offhand have my
'record' for that instrument, but I am sure it is down around 18th
magnitude somewhere. Not as great an achievement as a 14.6 magnitude star
with my 4.5" scope, to be sure - but still a lot deeper than 16th
magnitude. With your additional 2" of aperture, you should be able to
exceed even that.
This is one area where I think we amateurs are a little bit snowed by a
combination of factors. First is the relative lack of availability of deep
photometric sequences with which to test our limits. Second is the
crippling condition of using fifty to one hundred year old limiting
magnitude theories to set our own supposed limits (in fact, some of the
oldest magnitude limit theories still quoted today were devised before
Pogson ever quantified the scale - quite useless to us today, I should
imagine!). And third, in my opinion, is the refusal of many amateurs to
recognize that there is significant variation in visual acuity and
sensitivity between observers.
No amateur today should be using the old tables compiled by Pogson,
Steavenson, Sidgwick, Pickering, and Maunder, which have been reproduced
with and without attribution in literally hundreds of books. Nor should we
be mucking about with that travesty of a formula that tries to derive a
telescopic limit from the naked eye limit. There is only one useful (though
still not perfect) theory of limiting magnitudes, which takes into account
all quantifiable variables; that is the one devised by Bradley Schaefer and
published in 1990PASP..102..212S.
Better yet is determining our own magnitude limits empirically. There are
still few reliable sources for deep photometry. The GSC and USNO A1.0 (or
its abridged counterpart, SA1.0) are useless for this purpose. Even the old
Everhart sequences published based on Chou's work in an early 1980's S&T
have been shown to suffer from systematic error. And standard stars like
Landolt just don't go deep enough to determine limits for anything but
small finderscopes. The only handy source of reliable deep photometric
sequences that I am aware of is on my web page at:
I also have a link to Schaefer's paper there, if you want to read it. It is
much better than the watered down version appearing in S&T a while back.
There are plenty of sequences still in development, so be patient with me -
one comes out every month or so; but each one has full references to the
original source. If anyone has any other handy URL's for this purpose, let
me know and I will add a link. Fooling with a slightly abridged version of
Schaefer's theory here suggests that at best his theory allows for a 20" is
somewhere near 20th magnitude (with a Gaussian shaped uncertainty curve
spanning a magnitude on either side) - of course, that assumes all
variables are optimal, and we know how often that happens....
At any rate, unless your 20" has one of those old speculum metal mirrors, a
16th magnitude supernova should be a very do-able, though somewhat
challenging, target for you - challenging because of the bright galaxy
surrounding the target. The proof, however, is in the pudding: in a series
of images of this galaxy made the other night at Junk Bond Observatory, the
only bright knots in evidence are on the far NE side of this galaxy pair
and in the center of the western object. The POSS seems to concur that the
supernova is in a fairly uncluttered area. If I were you, I would look
again, but I would still chalk your observation up as seeing the supernova.
The question here is not whether it is possible to see it, but whether you
managed to resolve it out of the bright background.
Jeff Medkeff | Acting Assistant Coordinator
Rockland Observatory | Association of Lunar and Planetary
Sierra Vista, Arizona | Observers, Solar Section
On the web at http://shutter.vet.ohio-state.edu/