Re: (IAAC) Re: (SNES) How Bright are the Galaxies - Really?

[Albireo54@aol.com]

In a message dated 12/6/01 1:38:11 PM Eastern Standard Time, 
dedalus@latrade.com (Lew Granmer) writes:

<< Very experienced deep-sky observer Doug Stewart posted the following to the
 "Southern New England Stargazers" email list recently...
 
 (Interested folks can find the full text of Doug's initial post at:
     http://snegazers.brainiac.com/snegazers/archive/0112/msg00006.html )
 
 >[T]he popular star charting program, SkyMap Pro, is now available. Version 8
 >features a number of advances and refinements... One of the most interesting
 >changes in version 8 is the inclusion of the revised New General Catalog
 >(NGC) database, published earlier this year...
 >
 >I find the numbers for M81 to be of particular interest (6.9 vs. 7.8), since
 >so many reports have surfaced of observers seeing this galaxy naked eye. I 
was
 >skeptical about these claims when M81 was listed at mag. 6.9. I am even more
 >so with the new figure of 7.8 - a value which jibes much better with my
 >observations over the years, using everything from binoculars to large
 >telescopes. Overall I find the new higher magnitudes to be much more 
realistic.
 >M33, as an example, makes much more sense at magnitude 6.2 than at 5.7 - for
 >only under the very darkest FDO skies have I ever seen it with the naked 
eye.
 >I'd be interested in hearing comments from other observers.  >>

My response to lew today is as follows:

Hi Lew,

I appreciate your having taken the time to respond so thoroughly to my two 
posts - about M81 and Comet Linear WM1. Your audience is a wide and 
experienced one, so hopefully we'll see some helpful and possibly thought 
provoking feedback!

Regarding the comet, and its visibility, it appears that the current 
magnitude of 5.6 or so is as bright as we northerners can expect to see it. 
(It may reach the high fours while "down south". ) At that level it will 
remain a binocular object for probably 95% of your readers who observe from 
home - as it will be in my back yard in Webster. From Frosty Drew 
Observatory, however, we expect to be able to see it naked eye tomorrow night 
- provided that the skies clear on schedule. I am especially interested in 
observing its tail in a variety of scopes - the Astronomy Picture of the Day 
showed it very well this AM in a shot taken two days ago.

The battle over M81's naked eye visibility is one that's raged for many 
years. Whatever the galaxy's true "visual" magnitude, I suspect that a few 
select observers have indeed seen it without optical aid - under outstanding 
(Bortle class 2) skies. Even then, as Brian Skiff points out, you have to be 
intimately familiar with the star field, and to have gone through a good 1/2 
hour or more of dark adaptation to have any chance of seeing it. As we both 
noted, even if the brighter 6.9 magnitude is accepted, we are still talking 
about a diffuse object that is nearly a third the area of the full moon!   

I agree that the topic of galaxy magnitudes has never been adequately 
addressed in the popular literature or in discussion groups. The most useful 
standard for amateurs is the true "visual" magnitude - but no one seems to 
agree on just what that means. My point in writing to you originally was 
really to express my belief that most long held values are probably too 
optimistic; that those now being adopted in the "new" NGC probably do a 
better job of predicting what the average observer will see under most decent 
conditions.

To be sure, Bortle class 2 skies are rare - to the point that few of us will 
ever experience them. And sky darkness obviously is a BIG determining factor 
in how much of any faint fuzzy you can see on a given night. However, 
regardless of sky conditions, all galaxies emit a quantifiable amount of 
light in the visible wavelengths of the spectrum - and so it remains a 
mystery to me why the pros haven't been able (at least up till now) to arrive 
at truly realistic - and broadly accepted - values for visual observers. 
Values that are actually useful in predicting an object's visibility in your 
eyepiece. Some may feel that the new higher (dimmer) numbers in the NGC for 
virtually all galaxies are too conservative. But so far, based on my long 
years in the field, I am finding them to be better indicators than their 
predecessors.

Looking forward to any and all responses!

Best regards,
Doug Stewart
=====
Post to the entire list by writing to snegazers@brainiac.com.
Unsubscribe by mailing to majordomo@brainiac.com with the message body
"unsubscribe snegazers"