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Looking for "rules of thumb" for minimum surface brightness for deep sky objects

beginner dso observing
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#1 FredOz

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Posted 28 May 2020 - 11:13 PM

Does anyone have some "rules of thumb" for the minimum surface brightness for seeing deep sky objects?  Obviously, this will depend on local light pollution, the size of the telescope and one's eyes plus other factors.  But a couple of suggested surface brightness levels would be useful to those of us with less experience.

 

In various threads, we are told that magnitude is not meaningful for extended objects like galaxies or nebulae because the light is spread over the surface area in contrast to stars, which have essentially zero size.

 

This request is inspired by a thread at https://www.cloudyni...axies-in-virgo/ that mentions "SB", which I presume refers to surface brightness in mag/arcmin^2.

 

For example, M101, the Pinwheel Galaxy, which I've not been able to see from my Bortle 5 yard but can see (dimly) from a Bortle 4 spot 8 miles away.  Stellarium rates M101's surface brightness as 14.82 mag/arcmin^2 (magnitude 7.86).  So maybe 15 is a limit for my 150 mm Newtonian.  (Note that a bigger number refers to a higher magnitude, which is dimmer.)

 

By contrast, M51 Whirlpool Galaxy rates 12.56 (magnitude 8.1).  I can just see it at Bortle 5 while M81, Bode's Galaxy rates 13.13 (magnitude 6.94) but I can see it (plus its companion M82) better than M51.  And low in the southern sky, I can slightly see Omega Centauri (13.77, magnitude 5.33) with 7x50 binoculars even in the light dome from Phoenix 100 miles to my south plus with tonight's 6-day moon.

 

Is there any correlation between min. surface brightness and sky quality meter (SQM) readings, which are a measure of light pollution and given in the same units of mag/arcmin^2?

 

Finally, if surface brightness is not the key, what is?

 

--- Fred


Edited by FredOz, 28 May 2020 - 11:15 PM.


#2 Taosmath

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Posted 29 May 2020 - 12:30 AM

It's complicated, but try this thread:

 

https://www.cloudyni...sibility-chart/



#3 Astrojensen

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Posted 29 May 2020 - 01:06 AM

Remember that, while the surface brightness is a good indicator to whether a galaxy will be visible or not, their surface brightness is not evenly distributed over their surface. Some, like M81, have a very bright, relatively large core, sitting inside a faint disk, while M51 have a smaller core and a somewhat brighter disk, relative to it. This means that M81 is easier to see, because the bright core punches through the light pollution, even though the disk is completely invisible. 

 

Galaxies can be so difficult to predict, that I find that it's best to just take a glance at the numbers and then go out and see for one self, whether it's visible or not. 

 

 

Clear skies!
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#4 Tony Flanders

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Posted 29 May 2020 - 07:17 AM

In various threads, we are told that magnitude is not meaningful for extended objects like galaxies or nebulae because the light is spread over the surface area in contrast to stars, which have essentially zero size.


That is absolutely untrue. On the contrary, magnitude remains the single best predictor of visibility. However, surface brightness also plays a critical role. To a very crude first approximation, you can think of it like this. In order to see an object, first of all the total brightness (magnitude) needs to be sufficiently bright. And in addition, the surface brightness needs to be sufficiently bright.

Total brightness (magnitude) relates primarily to aperture. Given two objects of identical surface brightness but different total brightness, you will need a bigger telescope to see the object that is fainter in terms of total brightness.
 
Surface brightness relates primarily to sky conditions. Given two objects of identical total brightness but different surface brightness, the one with brighter surface brightness will almost always be easier to see even under dark skies. But that advantage is magnified in the presence of artificial light pollution. And if the sky is sufficiently bright and the surface brightness sufficiently dim, you won't be able to see the object no matter how bright its total brightness is.
 
The classic example is the Milky Way. Its total brightness is quite a bit brighter than Venus. Yet it is invisible from most urban areas because its surface brightness is so much lower than the skyglow from artificial light sources.
 

For example, M101, the Pinwheel Galaxy, which I've not been able to see from my Bortle 5 yard but can see (dimly) from a Bortle 4 spot 8 miles away.  Stellarium rates M101's surface brightness as 14.82 mag/arcmin^2 (magnitude 7.86).  So maybe 15 is a limit for my 150 mm Newtonian.  (Note that a bigger number refers to a higher magnitude, which is dimmer.)


M101 is tremendously bright in terms of total brightness, so aperture plays little role here. It is only a little bit easier to see through my 8-inch Dob than through my 70-mm refractor. A good rule of thumb is that an object that is 3 magnitudes dimmer than the skyglow will be visible.
 
Skyglow is usually expressed in magnitude per square arcsecond, which is equal to magnitude per square arcminute plus 8.9. So M101's nominal surface brightness in magnitude per square arcsecond is 22.9. That means that it should be visible in skies of 19.9 mpss or better. That corresponds to good suburban/bad rural in my part of the world.
 
This is an exceedingly crude guideline; there are numerous exceptions.
 

By contrast, M51 Whirlpool Galaxy rates 12.56 (magnitude 8.1).  I can just see it at Bortle 5 while M81, Bode's Galaxy rates 13.13 (magnitude 6.94) but I can see it (plus its companion M82) better than M51.


There are two different things going on here. First of all, total brightness and surface brightness are not completely independent, as my initial introduction suggested. In this case the very large difference in total brightness outweighs the smaller difference in surface brightness.

 

More to the point, M81's nominally dim surface brightness is misleading. This galaxy's extended disk is indeed quite hard to see even under excellent conditions, but it has a very bright core, and the core is what catches your eye and makes the galaxy easier to see than the numbers suggest.

 

This subject is discussed at length in my Urban/Suburban Messier Guide. Note my suburban sites are somewhere between Bortle 5 and 6, yet I could see all the Messier objects with my 70-mm refractor. That illustrates what might be the most important factor of all: experience. With sufficent practice, I predict that you will have no trouble seeing any of the Messier objects from your yard.


Edited by Tony Flanders, 29 May 2020 - 07:25 AM.

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#5 Jon Isaacs

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Posted 29 May 2020 - 07:21 AM

My thinking:

 

It looks like Tony beat me to it.. A good thing, his answer is much more complete.

 

Many galaxies have a bright core, this makes them much easier to see. I can see M31 , just the core, from my urban backyard with 7x35 binoculars.

 

Sky brightness is normally measured in Magnitudes Per Square Arc Second (mpsas), Stellarium is using Magnitudes Per Square Arc Minute (mpsam) for the average surface brightness, you need to be using consistent units.  To convert, add 8.9 to the mpsam to get mpsas.

 

A Bottle 5 sky might measure 20.0 mpsas, the range is quite broad, 19.1-20.4.

 

So, M101 is magnitude 14.8 mpsam = 23.7 mpsas. Compare this to the 20.0 mpsas sky, its 3.7 magnitudes dimmer. Since it's a logarithmic scale, it means the sky is 30 times brighter than M101. That's very low contrast. 

 

Since the light you see the Galaxy is the sky plus the object, this means the contrast is 31/30-1 = 3%. That's very low. The eye is a very good contrast defector but probably not that good.

 

As a rough rule of thumb, if the surface brightness object more than 2 magnitudes dimmer than the actual sky glow, it will not be easy.

 

Jon


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#6 FredOz

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Posted 29 May 2020 - 12:14 PM

Sky brightness is normally measured in Magnitudes Per Square Arc Second (mpsas), Stellarium is using Magnitudes Per Square Arc Minute (mpsam) for the average surface brightness, you need to be using consistent units.  To convert, add 8.9 to the mpsam to get mpsas.

Thanks to all for the thoughtful replies.  I'd not noticed the difference in units with Stellarium.  That raises Omega Centauri's 13.77 up to 22.67 mag/arcsec^2 and M51's 12.56 up to 21.46, etc.

 

The Wikipedia article on Sky quality meter mentions the units as "magnitudes per square arcsecond" and says the range is 16.00 to 22.00.

 

I also found a good article from Sky & Telescope, https://skyandtelesc...e-your-skyglow/ which is also in units of mag/arcsec^2.

 

--- Fred



#7 Jon Isaacs

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Posted 29 May 2020 - 12:42 PM

Thanks to all for the thoughtful replies.  I'd not noticed the difference in units with Stellarium.  That raises Omega Centauri's 13.77 up to 22.67 mag/arcsec^2 and M51's 12.56 up to 21.46, etc.

 

The Wikipedia article on Sky quality meter mentions the units as "magnitudes per square arcsecond" and says the range is 16.00 to 22.00.

 

I also found a good article from Sky & Telescope, https://skyandtelesc...e-your-skyglow/ which is also in units of mag/arcsec^2.

 

--- Fred

 

Fred:

 

You might notice that the Sky and Telescope webpage was introduced by Tony Flanders.. the  same Tony Flanders who replied to your post just above me.

 

As far as Omega Centauri, it's a globular cluster, much brighter in the center than at the edge. And individual stars can be resolved, so average surface brightness is a poor measure at best. SkySafari 6 Pro has a large enough database of stars to simulate Omega Centauri based on it's database. The number of stars magnitude 13.8 and brighter, brighter than its average surface brightness in mpsam, is definitely significant.

 

Screenshot_20200529-103314.png

 

Jon


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#8 havasman

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Posted 29 May 2020 - 12:52 PM

...it's best to just take a glance at the numbers and then go out and see for one self, whether it's visible or not. 

 

That really is the only sure answer. All the prediction in the world crumbles in the face of an observation.

 

We all sometimes fail to observe an object we intended to see. If we didn't, we'd certainly miss trying observations that turn out to be satisfying accomplishments. The difference may come down to conditions anyway.


Edited by havasman, 29 May 2020 - 12:54 PM.

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#9 Galaxyhunter

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Posted 30 May 2020 - 09:10 PM

-----------

Is there any correlation between min. surface brightness and sky quality meter (SQM) readings, which are a measure of light pollution and given in the same units of mag/arcmin^2?

 

----------

--- Fred

Fred, This is my experience and it only applies to Asteroids.  I used to do a lot of Asteroid hunting (CCD Photography) & I have found that the dimmest Asteroid I could detect was roughly ½ mag less then that my SQM was reading, meaning the SQM was reading 20.45,  I could barely detect an 20.0 mag Asteroid.    I have never tried to tie the two together visually.  One would have to do a lot of experimentation with documentation to build a data base.  But there are a lot of variables that would have to be considered, The weather conditions ( heat, humidity, transparency).  The major variable that has to be factored in is the experience ( and eyeball ) of the observer. It is just a basic fact,  The more eyepiece time one has, the more one will see.

One thing I like to do is point my scope into Virgo. I start panning around looking at all the Galaxies & it is not hard to get lost.  I'll try to match a group of Galaxies to the NGC catalog, and sometimes it is hard.  Yes I could look at the readout on the mount, but that takes the fun out of trying to match what your seeing to the catalog.


Edited by Galaxyhunter, 30 May 2020 - 10:54 PM.


#10 FredOz

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Posted 30 May 2020 - 10:49 PM

RE: 

One thing I like to do is point my scope into Virgo. I start panning around looking at all the Galaxies & it is not hard to get lost.  I'll try to match a grope of Galaxies to the NGC catalog, and sometimes it is hard.  Yes I could look at the readout on the mount, but that takes the fun out of trying to match what your seeing to the catalog.

I've been using (mostly) the star-hopping hints in Turn Left at Orion.  So far, I've tried the Virgo galaxies only once and then I saw 11 of the first 12 described in the book from a Bortle 4 site.  Since then I've looked at other galaxy groups, especially in Leo and at globulars, such as Omega Centauri and in Hercules.  But I intend to go back to Virgo, hopefully in several days when the moon is gone from the early night sky.

 

I have a Dobson mount so no setting circles.


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#11 ShaulaB

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Posted 30 May 2020 - 10:56 PM

It would be a good idea to get your 6 inch scope out to darker skies. A 6 inch (150mm) f8 reflector can show you the entire Messier list, and many NGC objects, under dark skies.
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