A VISIBILITY INDEX FOR GALAXIES IN PORTABLE TELESCOPES
The issues, and the problem:
Recent conversations on several web sites, and most notably here on Cloudy Nights,
about the visibility of galaxies in small scopes (I use the word portable) has led me to
research this issue further. Let me explain the issues:
Every observing guide seems to list a figure, Magnitude, for the deep-sky objects. This
magnitude figure is generally the “total integrated magnitude” of the object. Picture
reducing the object to the size of a star, or blowing up the image of a star to the size of
the galaxy, and you have a rough approximation of the Total Integrated Magnitude
(henceforth called TIM) of that extended object.
Many, but not all, observing guides also list Surface Brightness Magnitude (henceforth
called SB) as an additional magnitude of the deep-sky objects in the lists. This can be
calculated more than one way, but is usually expressed as the average brightness of each
square arc-minute (1/3600 of a square degree) of the extended object. This figure is
usually lower (though not always) than the TIM figure.
The issue for both of these magnitudes is where to cut off the “edge” of the deep-sky
object. Longer time-exposures of most deep-sky objects just keep showing everincreasing sizes for those objects. So an arbitrary cut-off point, the faintness “isophote”
(contour line of equal brightness) is often set at magnitude 25.0 per square arc-second
(very approximately mag.16.33 in sq. arc-min parlance). This is about the brightness
“edge cutoff” easily seen on most of the photographic plates taken for magnitude studies
at the professional observatories.
The relevance of these two measurements for we amateur observers is that the chosen
cutoff is usually fainter than we can see, so we don’t actually see all the surface area used
for the calculation of the overall brightness of the galaxies in question. Accordingly, the
TIM calculation OVERSTATES the brightness we see because it includes a lot of area
we can’t see, and the SB calculation UNDERSTATES the brightness of the object
because it calculates the average brightness using a lot of faint area we can’t see.
You can understand all that, and still be confused about which figure has the most
relevance for the amateur observer.
Take M33, for example. At magnitude 5.7 (TIM), it’s visible to the naked eye in a dark
site. Yet, it’s not easy in most scopes. A lot of beginners have trouble finding it. The SB
figure tells why—it’s magnitude 14.1 per square arc-minute. That’s not bright, and
though it should be within reach of most small scopes, it certainly won’t stand out as
spectacularly as, say, M32, the companion of M31 (which, although small, is
outstandingly bright).
So, is TIM or SB more indicative of a visibility index figure we can use to discern the
ease of seeing a particular galaxy? Or neither?
Let’s explore further.
The comparisons:
Let’s take some popular galaxies that everyone eventually views and rank them by TIM
(figures from the Deep Sky Field Guide of Uranometria 2000.0, 2nd Edition):
1. M31 mag.3.4
2. M81 mag.6.9
3. NGC 253 mag.7.2
4. M104 mag.8.0
5. M32 mag.8.1
6. M110 mag.8.1
7. M82 mag 8.4
8. M74 mag.9.4
9. NGC 4565 mag.9.6
10. NGC 891 mag.9.9
This seems like a fairly good ranking (M31 is on the top). Let’s see how surface
brightness (SB) magnitude stands up:
1. M104 mag.11.6
2. M32 mag.12.5
3. M82 mag.12.5
4. NGC 253 mag.12.8
5. NGC 891 mag.13.0
6. NGC 4565 mag.13.2
7. M81 mag.13.2
8. M31 mag.13.5
9. M110 mag.14.0
10. M74 mag.14.2
And now, to compare those lists with my personal, subjective, ranking of ease of
visibility:
1. M31
2. M32
3. M104
4. M81
5. M82
6. NGC 253
7. NGC 4565
8. M110
9. M74
10. NGC 891
So, which magnitude list did better?
TIM—2 identical, 2 only 1 place off, and 6 weren’t close.
SB---1 identical, 3 only 1 place off, and 6 weren’t close.
Total Integrated magnitude is the winner. Well, sort of… You see, M101 and M33
weren’t even in my top ten (I purposely left them off), yet M33 would have been #2 on
the TIM list (had I listed it), and M101 would have been 4th. These simply wouldn’t have
fit on the chart of my personal visibility evaluation (I would judge them less visible than
all except, perhaps, NGC 891). SB figures wouldn’t have listed them in the top ten. Is
SB closer to what we see? The comparison of the lists says no. Is there a compromise
that works better? Because neither magnitude type is particularly useful for these
galaxies.
Enter the Third Solution:
There is a list of galaxies used by a lot of professionals. It is the Reference Catalogue of
Bright Galaxies, originally developed by Gerard de Vaucouleurs in the 1950s, and
revised in 1991 to form the 3rd Reference Catalogue of Bright Galaxies—informally
called the “RC3”. Among other features of usefulness is the field in this large galaxy list
called the m’_e, which is a field that is only calculated for a small number (about 3000)
of the brightest galaxies in the list.
The field, m’_e is a magnitude field, and is calculated by taking the average surface
brightness of only the brightest half of the galaxy (from the mean brightness to the peak).
This figure comes much closer to calculating surface brightness from the parts of galaxies
seen in typical amateur’s telescopes, so also might come closer to corresponding to a
Visibility Index for galaxies typically viewed by most of us.
Let’s see how well it does (using the RC3 figure for our sample galaxies):
1. M32 mag.10.1
2. M104 mag.11.6
3. M82 mag.12.0
4. M81 mag.12.4
5. NGC 253 mag.12.8
6. M31 mag.12.9
7. NGC 4565 mag.13.1
8. M110 mag.13.2
9. M74 mag.13.7
10. NGC 891 mag.14.6
How well did we do?
5 identical matches, 3 only one place off, and only 2 weren’t close.
Which ones weren’t close? M31 and M82. Hmmm. Could it be my subjective
evaluation is wrong because I’m using a large scope at a dark site? Is M82 really easier
to see than M81? Is M31 really harder than the others?
I got out my 5” Maksutov and went to look at all of these in a fairly light-polluted
place—my home in LA. I had to wait a while, but I finally got a night in which
magnitude 4 stars were faintly visible, and the Moon was going to set early. I observed
all night.
My neighborhood has no street lights, and house lighting is architecturally controlled, so
lighting in the immediate vicinity of my scope was non-existent. But, this IS LA, where,
in the direction of downtown, the sky can be yellow-orange on most nights, and plain
blue if there’s a little water vapor in the air.
I tried to view as many of these galaxies as possible. Several weren’t visible at all: NGC
4565, M110, M74, NGC 891, and NGC 253 (because it wasn’t convenient to view). But
I did verify, to my satisfaction, that M82 is, indeed, easier to view than M81 in the
circumstances of use. The one galaxy that was definitely easier to view than the RC3
would indicate was M31.
But I only could see the core of the galaxy. All the visible parts fit within a 30’ field of
view. Instead of seeing the brightest 50%, I was probably only seeing the brightest 10%,
which was quite noticeable in the scope.
Well, I couldn’t reconcile the RC3 to my subjective visibility list completely—only 9 of
the 10 samples matched up.
So far, so good. How did the RC3 stack up when a larger selection was made? I went
back to the RC3, imported the entire catalogue into Excel, sorted the list, and compared
about a hundred galaxies with my observing notes. Though my observing notes aren’t
ranked, did I comment in them that the top hundred galaxies (in the RC3) were all bright
and detailed?
Unfortunately not. The 100 brightest galaxies, using the m’_e figure from the RC3 only
put 4 Messier galaxies in the 100 brightest galaxies: M94, M32, M77, and M105. The
rest of the hundred brightest galaxies included some faint Index Catalog (IC) galaxies,
and a host of very small galaxies with bright cores. Perhaps the ranking really IS
indicative of the brightness of the brighter half of those galaxies, but too many of them
are tiny faint galaxies with very bright nuclei. Though this column in the RC3 may
represent the ease of detection for the galaxies’ cores, it does not truly describe the ease
of seeing the galaxy itself. I don’t view as successful any list that makes M94 the
brightest Messier galaxy.
In conclusion, the RC3 has potential to describe successfully the average brightness of a
lot of galaxies as seen in large scopes; but, as a Visibility Index, it does not work well.
Despite its success at dealing with a select group of bright galaxies, it fails at predicting
the visibility of galaxies—especially the bright ones.
Creating an Index to Visibility for all galaxies for the amateur astronomer is the goal.
Until someone comes up with a viable formula for that, I have some recommendations:
1. Look at the Total Integrated Magnitude for the object. If the galaxy is small, you
needn’t look any further. But if the galaxy is large, say 5’ or larger, then…..
2. Look at the Surface Brightness of the galaxy (or calculate it). Use that figure to
decide whether the galaxy’s fainter regions will be visible in your scope. It will
help to know what the limit of your scope is—you can gain that with experience
of looking for galaxies and either finding or not finding them. Just be aware the
positions of many galaxies in the literature are wrong—especially for the fainter
NGC ones—so don’t trust what you read if the galaxy is not found. I recommend
going to the data files on www.ngcic.org and looking up (or downloading) the
corrected position to see if your not having found an object means either that you
couldn’t see it or were looking in the wrong place.
3. Use the m’_e field of the RC3 to compare to the SB figure to see if there’s a large
discrepancy between the normal SB figure and the brightest 50%.
4. Look at a lot of galaxies. My “Faint” notation from the early ‘90s might rate a
“Bright” designation today. Experience makes faint stuff more visible.
5. Take notes. Keep a note card handy with the following questions and answer
them all in your notes. Eventually, you’ll not need the card. Until then:
 What’s the shape? (Round/Oval/Lenticular/VeryExtended/odd)
 What’s the brightness? (Very Bright/Bright/moderate/Faint/VeryFaint/UltraFaint)
 What’s the size? (V.Lrg/Lrg/Average/Sml/V.Sml/VV.Sml)
 What’s the orientation? (Face-on/Edge-On/Some degree in between)
 What shape is the core?
 Along what axis does the core lie?
 What shape is the nucleus?
 Are there superimposed stars?
 How is the field? Rich/average/sparse
 Are there nearby companions?
 How is the edge definition? Diffuse/Sharp on one side—describe
 Is the core a lot brighter than the outlying areas?
 Are there any stand-out features or details? Describe.
It’s been my experience that even rank novices can often see quite a few details if
answering those questions.
In the long run, you won’t need a visibility index. In the meantime, feel free to work on
your own scale created from your own experiences. Let all the members of Cloudy
Nights know what you’ve found. And know that some of us are still working on it. But
the RC3 isn’t it. Alas.
Don Pensack, Los Angeles, May 2006.