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(IAAC) Re: Revised NGC / IC


Hi Lew,

You quoted Brian Skiff, who commented:

<<Nope, [this is a] misunderstanding [of] surface brightness.  The units are
mag/square arcminute (usually), not a simple magnitude.  If you do a lot
of observing under constant conditions, you'll find that the change in
threshold surface brightness is in fact only a very shallow function of
telescope aperture.  Thus the detection limit on galaxies results from
both the total brightness and mean surface brightness _in_combination_.
Thus using my 70mm Pronto, I can see IC 1613 and IC 2574, which have
mean surface brightness around muV=14.5 only because they have total
brightness around 10th.  But I simply cannot see a galaxy at total mag.
14.5 even at the very highest surface brightness (even a star!).>>

Well, it's not actually surface brightness (SB) that troubles me. SB says a 
great deal about the contrast of a galaxy against the relatively black 
background sky (which actually varies a lot with observing location and 
weather conditions). SB (whether stated in units per square arcminute or per 
square arcsecond, as preferred by Roger Clark) is just one more predictor of 
what a diffuse object will look like - if you can see it all. But total 
integrated visual magnitude is also important, as Brian points out in his 
example with the 70mm Pronto.

<< Again, read the stuff at Bartels' site, and also Roger Clark's book,
so that you can start to figure out how the eye works at low light levels.
It's complicated, but particularly using the techniques outlined in Roger's
book, you can get a feel for things with a modest amount of effort.>>

I had the good sense to buy a copy of Roger Clark's book, Visual Astronomy of 
the Deep Sky, when it was first released back in 1990 . Unfortunately, the 
book is no longer in print, although there is some talk of a reissue. It is 
one of the best, and most frequently consulted, texts in my astronomy 
library. His explanations of human vision, as related to astronomy, are the 
most thorough I've ever seen, even if they are a bit complex in places. Dr. 
Clark points out the critical importance of attaining both optimum contrast 
and angular dimension in the eyepiece when trying to observe a faint object. 
The magnification required to reach these points varies with each aperture 
and each object being observed.

No, what really confuses me is the range of source information various 
authors evidently are using to create their databases. Brian may be correct 
about RC3, the THIRD REFERENCE CATALOG OF BRIGHT GALAXIES (RC3), by G. and A. 
de Vaucouleurs, H. G. Corwin, R. Buta, G. Paturel and P. Fouque (1990). But 
if it is the "Bible" of references, it should certainly be granted broader 
use. As it is, the inconsistencies from reference book to reference book are 
rather significant. As an example, Brian sights IC2574, a member of the M81 
group of galaxies. A check of six references I have handy show this galaxy to 
have a magnitude ranging from 10.4 to 13.0 and an angular size of anywhere 
from 9.0' x 4.0' to 13.5 ' x 8.3'. Some references, including Brian's own 
"Observing Handbook and Catalog of Deep-Sky Objects" don't even mention it. 
The problem, therefor, is trying to predict an object's visibility from its 
various published magnitudes and angular dimensions.

In another post you mentioned that professional astronomers don't spend much, 
if any, time actually observing visually - as amateurs do all the time. 
Having a professional astronomer in the family, I can attest to that! So I 
suppose it's little wonder that reliable references for visual observing are 
so scarce. 

Oh well, none of this has in any way dampened my love of observing, using all 
kinds of equipment. What we really need is someone to explain our frustrating 
New England weather!

Best regards,
Doug Stewart 


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