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Relationship of Surface Brightness & Light Pollution

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#1 astronomus1930

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Posted 27 February 2021 - 01:23 PM

Greetings Friends!
I'm hoping to understand the impact of light pollution on surface brightness. Also, does surface brightness of OC somehow differ from that of a PN or GC?  That LP brightens the sky and must limit views of DSOs from an urban setting is clear but is there a formula to estimate the limit of what I can see? 

 

Specifically: Using 15x70 Oberwerk DeLuxe, supported, I am in Chicago but observe to the east over the lake so my guess is Bortle 8. I only go after DSOs on nights when the transparency is very good. I have seen M37, an OC with a surface brightness of 12.8 but have not been able to see neither M27, a PN with a surface brightness of 12.3 nor M78, a  reflection nebula with a surface brightness of 12.6- despite multiple attempts.

 

Perhaps it's just a matter of time, but any insight would be appreciated.

 

Clear skies


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#2 lee14

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Posted 27 February 2021 - 01:35 PM

It's about contrast. Objects such as reflection or even planetary nebulae (and galaxies) will suffer the most. It's not the overall magnitude that determines visibility in challenging conditions, it's surface brightness. Even though the stated magnitudes are similar, open clusters like M37 are less affected by light pollution than extended objects like nebulae because the light is concentrated into pinpoint stars, offering greater contrast. You should be able to get a glimpse of many planetary nebulae by stepping up the magnification, which has the effect of improving the contrast by darkening the background sky. 

 

Lee


Edited by lee14, 27 February 2021 - 01:43 PM.

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#3 Tony Flanders

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Posted 28 February 2021 - 07:12 AM

Greetings Friends!
I'm hoping to understand the impact of light pollution on surface brightness. Also, does surface brightness of OC somehow differ from that of a PN or GC?  That LP brightens the sky and must limit views of DSOs from an urban setting is clear but is there a formula to estimate the limit of what I can see? 
 
Specifically: Using 15x70 Oberwerk DeLuxe, supported, I am in Chicago but observe to the east over the lake so my guess is Bortle 8. I only go after DSOs on nights when the transparency is very good. I have seen M37, an OC with a surface brightness of 12.8 but have not been able to see neither M27, a PN with a surface brightness of 12.3 nor M78, a  reflection nebula with a surface brightness of 12.6- despite multiple attempts.


Here are some thoughts. First of all, take all astronomical data with many grains of salt. In particular, it is hard both to define and to measure the brightness of nebulae. That's due to many factors, the two main ones being they typically have uncertain borders and the fact that they typically shine in only a few specific wavelengths rather than being broad-spectrum sources like stars.

 

Open clusters also have two different aspects -- they may be detectable either as diffuse glows or as individual stars. My guess is that using 15x70 binoculars in Chicago, you detected M37 only as a diffuse glow; is that right? In that case, the surface-brightness figure is at least approximately relevant. When you're detecting a cluster as a collection of individual stars, other criteria apply.

 

For the record, M27 is generally fairly easy to detect with small instruments under bright skies, whereas M78 is quite hard. That is due not only to surface brightness but also to size; given two objects with identical surface-brightness profiles, the bigger one will always be easier to detect. Often a lot easier. To phrase exactly the same thing in different words, given two objects with identical surface-brightness profile, the one with the greater total brightness (lower apparent magnitude) is generally much easier to detect.

 

Another factor is that you're observing to the east, when objects are far from their highest in the sky. That doesn't matter much with M37, a far-northern object that's already quite high even when it's due east. It matters a lot more with M27, which is just 20 degrees north of the celestial equator. That means that it's quite high when it's due south, but still down low in the light-pollution haze when it's rising in the eastern part of the sky. Yes, that light-pollution haze can be quite strong even directly opposite the major light sources.

 

There absolutely is and can be no formula that can predict what you will see, but you'll get a good sense of it over time. For the Messier objects in particular, you might want to look at the ratings in my Urban/Suburban Messier Guide.


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#4 Second Time Around

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Posted 28 February 2021 - 08:30 AM

For the Messier objects in particular, you might want to look at the ratings in my Urban/Suburban Messier Guide.

I can thoroughly recommend this excellent resource.  Thanks once again for all the hard work, Tony.


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#5 Jon Isaacs

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Posted 28 February 2021 - 10:14 AM

For the record, M27 is generally fairly easy to detect with small instruments under bright skies, whereas M78 is quite hard. That is due not only to surface brightness but also to size; given two objects with identical surface-brightness profiles, the bigger one will always be easier to detect.

 

 

 

My thinking:

 

Looking at the resources I have for M27 and M78, M78 is about the same size as M27 but it's visual magnitude is about 1.2 magnitudes dimmer. 

 

M78 is listed as being 8'x6' with visual brightness of 8.3 while M27 is also about 8'x6" with a visual brightness of 7.1.  I verified the sizes using images on the sky safari screen and measuring them with calipers. 

 

I calculate the surface brightness of M27 at 19.9mpsas and the surface brightness of M78 at 21.1mpsas. 

 

For an urban observer, whose skies might be 18.0-19.0 mpsas, this difference in surface brightness (and therefore contrast) is very significant.  In terms of surface brightness, it says M27 is about 1.4 magnitudes dimmer than a magnitude 18.5 sky, M78 is about 2.6 magnitudes dimmer, much more difficult.  

 

The numbers seem consistent with what I see from my urban backyard.  M27 is easily seen in binoculars and finder scopes, M78 is considerably more difficult.

 

Jon


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#6 Tony Flanders

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Posted 28 February 2021 - 05:08 PM

Looking at the resources I have for M27 and M78, M78 is about the same size as M27 but it's visual magnitude is about 1.2 magnitudes dimmer.


Thanks for the correction, Jon. Regardless, M27 is certainly a much easier object to see -- and also a much more impressive object even when both are easy to see. M78 has that cool embedded star pair to juice it up -- often described as two headlights in the fog. But the nebulosity itself is a bit drab. M27, by contrast, has lots of structure.
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#7 radiofm74

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Posted 01 March 2021 - 07:47 AM

For the Messier objects in particular, you might want to look at the ratings in my Urban/Suburban Messier Guide.

I discover it now, and promptly jump to read it. Thanks!


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#8 Jon Isaacs

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Posted 01 March 2021 - 08:17 AM

Thanks for the correction, Jon. Regardless, M27 is certainly a much easier object to see -- and also a much more impressive object even when both are easy to see. M78 has that cool embedded star pair to juice it up -- often described as two headlights in the fog. But the nebulosity itself is a bit drab. M27, by contrast, has lots of structure.

 

Two headlights in the fog is an excellent description.  I had my 13.1 inch out last night in my San Diego backyard.  Because of this thread, I thought I'd give M78 a look even though the moon was already up.  The two headlights allowed be to identify the region and the see the faint nebulosity.. 

 

Jon



#9 MeteorBoy

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Posted 03 March 2021 - 10:09 AM

Lee is correct.  You need more magnification.

 

I'm in the center of Toronto, population 5 million.  Here's my personal light-pollution experience...

  • Increasing magnification is one of two ways to see deeper (because it darkens the sky background thereby increasing contrast).  But you'll need 100x or more.
  • Your 15x70 binoculars should give you a visual LM of about 9.3 in the city.  Increasing this binocular's magnification to 100x (not practical with binoculars of course) would yield a LM of 12..., just because of the increased magnification.
  • For city observing my 20cm SCT at 50x yields a visual LM of 11.8 but at 300x the LM is now 14.4.
  • The other way to see deeper is to use imaging instead of visual astronomy.  Imaging (at 100x equivalent) on my 20cm SCT in the city now pushes the LM to 16 with a 5-second exposure and to 19 with a 15-minute exposure,

I find low power visual observing in a badly light-polluted environment not very satisfying.

 

You can find out a lot more about this by checking out my city light-pollution observing link below called, The White Zone.


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