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Histogram of Deep Sky Object (DSO) Sizes from Messier Catalog and Starman1's Catalog

Beginner Charts DSO Eyepieces Observing
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#1 Vatsumok

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Posted 28 September 2021 - 02:09 AM

I am new to the hobby, and decided to create a histogram of dimensions for two popular DSO catalogs. 

 

 

How to use these histograms:

 

These histogram will help you figure out how many DSOs will fit nicely with a given eyepiece TFOV:

  1. Calculate the TFOV of your chosen eyepiece in your chosen scope.  There are a number of online calculators that will do it for you.  (For example, a 14mm eyepiece in my 127mm Maksutov scope gives a TFOV of 44 MOA.)
  2. Subtract a 20%-30% framing allowance from the TFOV in previous step 1.  This allowance is to make sure a DSO can fit comfortably in the middle of the eyepiece with some space to spare.  (30% allowance makes my eyepiece 26 "usable" MOA).
  3. Use the histograms below to find a corresponding bucket and see precisely how many objects would fit perfectly in your eyepiece! BOOM!  (Continuing our example, 20-25 MOA bucket and 15-20 MOA bucket would fit nicely in my 26-usable-MOA eyepiece.  These two buckets contain 18 DSO objects from the Messier catalog.)
  4. All smaller histogram buckets will obviously fit in the eyepiece, but they might look too small.  Consider an eyepiece with a higher magnification and smaller TFOV, which will be better suited to see something two or three buckets to the left.   Anything right of the bucket will need a lower magnification wider TFOV eyepiece.

 

 

Fine-print about the data:

  • All dimensions are in MOA (Minutes of Angle / arc-minute).  For fellow beginners, 1 degree angle = 60 arc-minutes = 3,600 arc-seconds. 
  • Each DSO has a height and a width: I selected MAX( height, width ) and then rounded-up to the nearest MOA prior to plotting the histogram.

 

 

Future improvements:

  • I should figure out a way to incorporate Magnitude brightness in all this.

 

 

 

1) Messier catalog histogram of sizes:

 

Total objects = 110

 

Full Histogram chart:

cGokXi2.png

 

 

Zoomed-in Histogram, leaving out the outliers:

Omitted outliers: 80, 90, 90, 190, 71, 65, 95, 110, 54

 

9wSXESg.png

 

 

 

Histogram in table format:

vflaLPQ.png

 

Mean:  20.24545
Standard Deviation:  25.84292
Skewness:  3.83046
Kurtosis:  18.66079

 

 

 

2) @Starman1's catalog of 500 best North American objects histogram:

 

Total Objects: 502

 

Full Histogram chart:

 

Hov1IEP.png

 

Zoomed-in Histogram, leaving out the outliers:

 

ZEaNyDd.png

 

Histogram in table format:

 

hkZEeim.png

 

 

Mean  10.42724
Standard Deviation (s)  13.4791
Skewness  7.72916
Kurtosis  99.32673

 

 

 

Hope this helps someone!

 

waytogo.gif


Edited by Vatsumok, 28 September 2021 - 11:14 PM.


#2 Redbetter

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Posted 28 September 2021 - 04:19 AM

While it is a good technique (particularly including framing), keep in mind that the lists chosen represent a subset of bright objects in telescopes.   As you note, incorporating overall magnitude can be an important factor in determining what is most worth observing.

 

There is self-selection in terms of size for the observer's preserved instrument in the 500 list.  (That is even more true for Herschel's observations which formed much of the basis of the NGC.) There are a number of large and bright objects not on the lists, and there are some for which the max size is unintentionally misleading.  In some cases, components of larger objects are listed, resulting in max sizes that represent only part of the object--see the Veil or the Lagoon.  The Double Cluster is best seen as the pair, rather than individually.  

 

In some threads I have gone through lists of larger objects that are frequently omitted.  Many are nebulae, some are open clusters.  One of these days I need to make a "large objects for visual" list.  Then I could just point to it whenever it was relevant to the topic.

 

Among the largest objects out there is Barnard's Loop--challenging but rewarding when finally seen.  Similarly large and challenging to see in its extent is the Lambda Orionis/Angelfish Nebula.  But those understandably might not make most people's highlight reel.  The North America Nebula should however, as should the California Nebula and the cluster and nebula IC 1396.  The Soul and Seagull nebulae might also make some lists.  There are a number of quite large, but lower surface brightness nebulae.   

 

And while most individual galaxies are relatively small (even many bright ones), there are a number of fine groups of them, most impressive when seen together.  


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#3 TOMDEY

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Posted 28 September 2021 - 06:10 AM

That certainly addresses the angular size of the image relative to the telescope + eyepieces' field of view. Another consideration (especially for Deep Sky Objects) is the resulting exit pupil size, which will determine the image's ~Surface Brightness~ relative to what it would be if you were floating in outer space that much closer to the target. If the exit pupil is smaller than your eye's, the surface brightness diminishes by the square of that pupil size ratio... no free lunch. This favors larger and larger telescopes as the size of the target gets smaller and smaller. I made this graphic of Rich Field Telescopes to illustrate the dilemma. The boundary condition is that exit pupil is maintained for invariant (bright) image. It's sobering how big the scope has to get so that image surface brightness is not compromised on smaller targets. Tom

 

~click on the image~ >>>

Attached Thumbnails

  • 73 The Aperture Advantage Rich Field Invariant Luminance.jpg

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#4 Vatsumok

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Posted 28 September 2021 - 01:14 PM

There are a number of large and bright objects not on the lists, and there are some for which the max size is unintentionally misleading.  In some cases, components of larger objects are listed, resulting in max sizes that represent only part of the object--see the Veil or the Lagoon.  The Double Cluster is best seen as the pair, rather than individually.  

 

In some threads I have gone through lists of larger objects that are frequently omitted.  Many are nebulae, some are open clusters.  One of these days I need to make a "large objects for visual" list.  Then I could just point to it whenever it was relevant to the topic.

 

These are great pointers @Redbetter!  Popular catalogs skew heavily towards smaller objects, so I was wondering about the large objects are out there.  If you compile a list on Google Sheets or something it will be very helpful to beginners like me. 



#5 AstroVPK

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Posted 30 September 2021 - 07:22 AM

I like this thread!!! Can we please redo the histograms but with angular size replaced by surface brightness estimated as integrated magnitude divided by the square of the angular size?

Edited by AstroVPK, 30 September 2021 - 07:22 AM.

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#6 Starman1

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Posted 30 September 2021 - 03:09 PM

These formulae allow you to calculate the approximate Surface Brightness of any object whose size is known:

surface brightness in sq.arc-sec=TIM+2.5log (2827.4 x max' x min')
surface brightness in sq.arc-min=TIM +2.5log (.7854 x max' x min')

TIM = total integrated magnitude (the magnitude often listed).

Min' and Max' are the sizes in minutes of arc.

 

It is very true my list of 500 deep sky objects is biased against objects larger than about a degree.

It is also biased against objects below -40°.

It would have included more objects if expanded in either case.

 

I aimed the list at a northern hemisphere observer, and also aimed it at all scope, including the myriad of scopes out there with maximum fields of

maybe 1-1/4° like many SCTs, MCTs, and many reflectors used with 1.25" eyepieces (what is usually provided).

I also aimed it at small scopes, i.e. 4.5" and smaller, where the light grasp may preclude the visibility of large faint nebulae.

And I aimed it at beginners, not really experienced observers.

 

I have no objection at all to expanding the list to larger objects or objects south of -40°.  Please feel free to do so.

And if you have some favorite large objects that you can see in your skies, add them.

The list is supposed to be a "starting point" for DSO observing, not an end point.

My own personal "Favorites" list is 5x as large, and I'm certain I left out hundreds of star clusters that look nice in small scopes.

 

What the histograms show me is that my own impression of DSOs in the sky is not far off--there are basically 5 sizes--XL, L, M, S, and XS

(for this writing, I'll ignore the thousands of planetaries less than 30" across--I left them off my beginner's list on purpose).  It's a good reason why I, and many other observers, think that 4-6 eyepieces pretty much

cover the bases where observing is concerned.  I have a lot more eyepieces, but I'm certain I could get by without a few of them and be just as happy.

I'm not trying to outfit a dozen scopes like some people here on CN.

 

Redbetter is right about unintentionally making objects appear smaller than they are.

M8, for instance, in a dark sky, with a large aperture, and using a nebula filter, can be traced out to 2° wide.

But is that the size you would list for a small scope?

It's a dilemma.  So I went with an online listing for size, which might not be the best choice.

 

At least my list contains Surface Brightness.  Just know that for large objects, like some nebulae, it won't be very indicative of what you see.


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#7 Vatsumok

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Posted 30 September 2021 - 07:56 PM

I like this thread!!! Can we please redo the histograms but with angular size replaced by surface brightness estimated as integrated magnitude divided by the square of the angular size?

Thanks Viking - I am also curious what the brightness distribution would look like.  I will create brightness histograms using @Starman1 's formula:

surface brightness in sq.arc-min=TIM +2.5log (.7854 x max' x min')

 



#8 Vatsumok

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Posted 01 October 2021 - 12:57 AM

Surface Brightness Histogram of Messier and @Starman1's catalogs

 

 

Why Surface Brightness Matters:

The magnitude scale dates back to the time of Hipparchus (about 130 BCE).  For stars, their light is always concentrated into a point so that there is a clear relationship between a star’s brightness and the stellar magnitude scale.

 

But “Integrated magnitude” for DSOs is estimated as if its light were concentrated to a stellar point.  This results in weird problems where DSOs with "bright" integrated magnitude are less visible than other DSOs with a "dim" integrated magnitude.

 

For example the galaxy NGC 55 at magnitude 8 spreads its light over about 180 square arc minutes. This gives it a surface brightness of about magnitude 13 per square arc minute. The planetary nebula NGC 3132 in Vela is also at magnitude 8 but has its light concentrated into a much smaller area about 1.3 square arc minutes. This results in a surface brightness just under magnitude 8 per square arc minute — almost the same as its “stellar” magnitude. This makes NGC 3132 much easier visually detect than NGC 55.

 

Surface brightness concept by Martin Lewicki solves this dilemma.  For more on this concept, read Martin's blog here: https://martins-arti...brightness.html

 

I used Martin's ellipse formula to calculate surface brightness for two popular catalogs and histogram them.   A tip of the hat to @starman1 for the pointer.

 

 

 

Surface Brightness Histogram of 110 Messier Objects:

 

9fHgifJ.png

l1DJcCj.png

 

7LQPBzj.png

 

 

Surface Brightness Histogram of 500 Objects from @Starman1 Catalog:

 

nvG47wr.png

5AheVcc.png

2sYFV3d.png

 

 

 

Enjoy!

 

waytogo.gif



#9 Starman1

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Posted 01 October 2021 - 01:09 AM

1) you can't calculate surface brightness in open clusters--it just doesn't work.

2) We don't only see the surface brightness.  We also see size and overall light from the object.

M33 is easy in very small scopes, yet it has an average surface brightness of mag.14.1

It's often visible to the naked eye at mag.5.7 but it can be faint with a little light pollution.

Surface brightness doesn't form a "visibility index" any more than total integrated magnitude.

You really have to look at both.

My next post will be something I posted back in 2006.


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#10 Starman1

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Posted 01 October 2021 - 01:10 AM

A VISIBILITY INDEX FOR GALAXIES IN PORTABLE TELESCOPES

The issues, and the problem:

Recent conversations on several web sites, and most notably on Cloudy Nights.com, 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.ngcicproject.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.com 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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#11 spereira

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Posted 01 October 2021 - 07:56 AM

Moving to General Observing.

 

smp


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#12 Vatsumok

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Posted 01 October 2021 - 02:51 PM

1) you can't calculate surface brightness in open clusters--it just doesn't work.

Ah you are right - I forgot to filter out the star clusters before making the histogram.  I will correct it in a day or so.  Should I exclude globular clusters as well?


Edited by Vatsumok, 01 October 2021 - 02:53 PM.


#13 Starman1

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Posted 01 October 2021 - 03:51 PM

Ah you are right - I forgot to filter out the star clusters before making the histogram.  I will correct it in a day or so.  Should I exclude globular clusters as well?

If the globular class is I-VI, a SB figure may work.

With lower densities, you end up with the same problem as open clusters.



#14 Redbetter

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Posted 01 October 2021 - 04:00 PM

FWIW, while historical for extended objects in some instances, the units of surface brightness, magnitude per square arc minute (MPSAM), are less useful than they should be.  MPSAS (magnitude per square arc seconds) are more intuitive/useful to what we are doing, even though they are simply a scale shift from adding 8.89 to the MPSAM value.  While many catalogs and other resources use MPSAM values, including Uranometria's Deep Sky Field Guide, studies of specific galaxy and globular surface brightness profiles often report in MPSAS. 

 

The reason I prefer MPSAS is that this is the most commonly reported sky brightness measure.  If you want to understand visibility of an object in different sky, the magnitude difference between the object and the sky is the key.   This is what sets the contrast (along with the specific surface brightness profile of the object.)  In general objects that are not too close to the TIM threshold for the aperture/conditions/observer can be detected as long as their surface brightnesses are no worse than 3 magnitude dimmer than the sky.  If the delta is 2 or 1 then things are more readily seen.  At 0 they are easy to see, and at -1 or so they appear rather bright.

 

The other problem with MPSAM is that too many folks confuse surface brightness for actual magnitude (TIM) or mistakenly infer that they are interchangeable.  That is because the raw values are very similar for many objects.  It is easy to swap the two values and not realize the mistake...  MPSAS simply avoids that pitfall because the scale is shifted.  I don't need to see the units to know that someone is talking about TIM or MPSAS.

 

An issue that applies to any of the published and personal lists with respect to surface brightness is the instrumental self-selection bias I mentioned earlier.  While it is obvious why Messier's objects would be among the highest surface brightness, and most "best of" lists will feature medium to high surface brightness objects, the problem extends far more deeply than that.  NGC objects were mostly visual discoveries, which means that even as observers went to higher magnitudes with larger instruments, the objects seen most readily had better surface brightness.  Nearby objects of similar total magnitude but low surface brightness were often missed by observers during the same sweeps.  

 

The IC catalog included objects that were detected with early photography.  These cover a wider range of surface brightness and can be far more challenging visually.  Some are simply dim and small, but with good surface brightness, some small and average surface brightness, while others are larger and more diffuse with low surface brightness.  As imaging surveys became more comprehensive, lower surface brightness objects were cataloged.   The many diffuse emission and reflection nebulae discovered this way remain largely absent from visual observing lists--although many of these can be detected visually.

 

Galaxies are more plentiful by many orders of magnitude.  However, traditional measure of size and surface brightness was to the 25 MPSAS B isophote--meaning that the average surface brightness of any given galaxy would normally be substantially brighter than that.  The UGC galaxy catalog extended to roughly this level to include more of the lower surface brightness galaxies that had been missed in earlier compendiums--some of these are relatively large such as UGC 2885.  CGCG and MCG had their own criteria, with a mix of sizes and surface brightness.  Our Local Group of galaxies and those not far beyond include many objects largely missing from traditional catalogs and of very low surface brightness:   Fornax and Sculptor Dwarf galaxies, Maffei 1 and 2,  Dwingeloo 1 and 2, etc--some of these are heavily obscured by Milky Way dust.  And then there are the ultra low surface brightness galaxies with large apparent size, but extremely difficult to detect visually, things like the Draco and Ursa Minor dwarf galaxies.

 

Globular cluster discoveries have followed a similar progression to galaxies, but somewhat delayed.  Messiers, NGC's, IC's, then various surveys and discoveries such as Palomars, Terzans, and a smattering of lists of 2 or 3 objects in various searches.  Recent discoveries have been primarily ultra low surface brightness, heavily obscured by Milky Way dust, and/or dim enough and isolated enough to have been missed in prior sweeps.  


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