A visibility index for galaxies in portable telescopes

Дата канвертавання24.04.2016
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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 ever-increasing 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.

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