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Surface brightness and MPSAS

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#1 CzechAstronomer

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Posted 30 January 2017 - 04:45 AM

Hi there!

 

From many sources, I have determined the background surface brightness of my sky being about 19,5 MPSAS, NELM of about 5 - 5.5. but I repeatedly see and comfortably observe many objects that have a much dimmer surface brightness, like 22 MPSAS, for NGC 2403 that I have seen yesterday, even 23,23 MPSAS. 

 

1. How is that even possible?

 

2. Is my sky MPSAS guesstimate wrong?

 

3. what is the approximate number of objects visible from my place?

 

 


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#2 Jon Isaacs

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Posted 30 January 2017 - 04:54 AM

The surface brightness for an object is given as an average but is actually varies for each point on the surface.  One may see the bright core but not the entire galaxy.. 

 

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#3 happylimpet

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Posted 30 January 2017 - 05:11 AM

And its quite possible to see an object which has a lower surface brightness than the sky - the eye is sensitive to changes in brightness, not just absence/presence of light.


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#4 CzechAstronomer

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Posted 30 January 2017 - 05:17 AM

The surface brightness for an object is given as an average but is actually varies for each point on the surface.  One may see the bright core but not the entire galaxy.. 

 

Jon

well, yes, i have figured this out. But let's say the Owl nebula M97, is round, symetrical, quite big and not the brightest. exept for the eyes, it should be uniformly bright, right? 

 

and it is supposed to have about 22 MPSAS. and it is quite well visible even direct vision low on the northern horizon at small exit pupils.



#5 Redbetter

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Posted 30 January 2017 - 06:41 AM

Convert the MPSAS to flux and compare the ratio.  Our eyes can see a difference in shading/flux (contrast) down to about 5% or so (20:1) if the image scale is a good match (that isn't a precise figure, just what I have observed and gathered from other discussions.)  It varies from person-to-person.  At a greater ratio/lower percent difference in brightness we eventually can't tell them apart.  The 3.5 MPSAS  delta you gave is close to what I can typically see when fully adapted and looking for dim objects from a dark site.  It is ~25:1 ratio.   Remember these aren't absolute values. 

 

This is why a few tenths more sky darkness allows you to see low surface brightness objects better.   If my choice is 21.6 or 21.1 MPSAS skies, I will go to the 21.6 if I can because it is huge in the eyepiece no matter what scope I have.  At 25 MPSAS surface brightness I can target some very low surface brightness objects...just wish I could see a full 1 MPSAS deeper, but that would be like seeing a 1.6% brightness difference, or having a sky that was 1 magnitude darker to start.

 

This is also why we use filters to block unneeded wavelengths.  We are trying to increase the relative amounts of light from the desired source and the background to improve the ratio so that our eyes can distinguish them more easily.  As long as we don't make the overall image too dim, this will result in a relatively brighter impression of the target relative to the background.  If the background is already effectively black to our eye, then further filtering merely reduces the brightness of the target, hurting the impression because contrast is lost.


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#6 GlennLeDrew

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Posted 31 January 2017 - 03:12 AM

Light from an object and the intervening sky always add to make the object brighter than the sky. In the middle of a sunny day any DSO fuzzies up there will be just a tiny smidgen brighter than the adjacent sky.

 

As pointed out, the crucial factor is by how much brighter the [object+sky] resultant is than sky alone. And this limit varies by sky brightness and object contrast against this sky. Under a bright city sky an extended source can have surface brightness (SB) around 3.5 magnitudes fainter than the sky. But under a dark country sky *and* when using a small exit pupil, the object might require to be as bright as the sky, if not brighter, just to be detected. Visual system noise worsens as scene brightness decreases, thus rendering subtler contrasts ever harder to discern.

 

On top of this is the worsening visual resolving power with decreasing scene brightness. Whereas at photopic (daytime) brightness levels we can resolve on the retina down to 1-2 arcminutes, at the lower limit of brightness/contrast, where visual system noise is very much dominating, visual resolution has degraded to as poor as several *degrees*!

 

This therefore requires quite different minimum apparent angular size for detection as sky brightness and object contrast against the sky vary.

 

This can lead to interesting effects. For instance, consider a face-on spiral galaxy possessing a small, bright core in a very extensive, dim disk. Let's suppose the sky darkness would permit the disk system to be seen if sufficiently enlarged. The brighter core, in spite of its much smaller size, can be seen via not-so-large apertures because its higher surface brightness does not so adversely impair visual resolving power. The dimmer disk, however, demands that it subtend a larger minimum apparent size in order to be detected. And so until sufficient aperture/magnification is brought to bear, the small core is seen while, seemingly paradoxically, the much bigger disk remains quite invisible.

 

Obtaining a decent feel for how the matrix of factors sky SB, object SB, object size and varying visual resolving power work in concert to control object visibility can seem daunting. But with some time spent in study these contributing variables can begin to assemble into a coherent picture. The RASC's Observer's Handbook has a section dealing with this. And I've posted on the matter here at CN, producing a couple of charts by which to facilitate understanding. One such chart resides in my Gallery, linked to below, in my signature (if you've not disabled the display of sigs.)

 

My more recent chart appears in the following thread, in the Deep Sky Forum:

 

http://www.cloudynig...sibility-chart/


Edited by GlennLeDrew, 31 January 2017 - 03:17 AM.

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#7 Pcbessa

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Posted 03 March 2019 - 06:02 PM

If the MPSAS for a pristine sky is a max of 22, what happens with DSOs with a MPSAS above 22 (which are the case for many nebulas).

 

Does that mean we will never see them in all their extent or they might simply be invisible even on the darkest skies?

 

Examples include the nebulas Crescent, Pelican, Coccoon, Barnard Loop and IC434-Horsehead, which have MPSAS between 25 and 28. What does that mean? Will they mostly be invisible visually even under the best skies? This may explain why many of these nebulas are either very difficult visually or near impossible. But still they are observed sometimes.

 

Perhaps I assume that these nebulas have MPSAS between 20 and 28, and we happen to see their parts with a MPSAS of 20, when the skies get really dark, but the remaining parts below 22 will never ever be observed visually, and only imaged.

 

One interesting example is M101, the galaxy has a MPSAS of 23.8. Shouldn't most of it be invisible as the sky will always be brighter than the galaxy? However in my Bortle 3 skies, I am able to see its faint spiral arms. 



#8 Araguaia

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Posted 04 March 2019 - 06:26 AM

If I understand correctly, the sky brightness is in front of the DSOs, not in the background.  It is therefore added to the light from the DSO.  It becomes the 22 plus the 25, which would be what, about 5% more light.  So the nebula is a patch of sky 5% brighter than what is around it.

 

If, OTOH, the sky is one magnitude brighter, then the nebula is only about 2% brighter than the foreground, so harder to detect.



#9 Redbetter

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Posted 04 March 2019 - 12:57 PM

I don't know where the nebula brightness values you are quoting are coming from, but I wouldn't put much stock in them since it isn't stated how they are calculated/estimated.  Nebula tend to be irregular, often with large percentages of an enclosed area being empty of visible nebulosity.  What matters is the luminosity of the illuminated area, not the rest.  And there are usually brighter stretches that are of higher surface brightness than the average. These areas might contain the majority of the actual emissions.  It all depends on the nature of the object.  Some things are readily seen, others are rarely (if ever) visible.

 

With emission nebula one can often enhance the view with a filter.  The filter in effect reduces the sky brightness while leaving the primary emissions of the nebula.  Picking a number out of the air a narrowband filter might reduce the background brightness by about 2+ MPSAS while retaining the bulk of the nebula emissions.   A 1 or 2 MPSAS improvement in contrast is quite noticeable.   That is why going from 21 MPSAS to 22 MPSAS sky seems so much darker.

 

As is noted several times in this thread, the actual brightness of a patch of sky containing a nebula/galaxy/etc. is brighter than either alone.  The brightness of the object is added to that of the sky/background.  What one sees visually is the contrast.  That contrast can be very little and barely perceived on a sky that is gray/dark gray/black.  It is also more difficult to detect the contrast when it gradually fades rather than producing a sharper cut off. 

 

We see things brighter than the sky all of the time.  That is the nature of this.  The sky is not black even if pristine (although magnifying dims its apparent surface brightness...as well as that of the target.)  22 MPSAS is actually still rather bright, about 40 times brighter than the eye's "dark light" threshold of ~26 MPSAS; and it is still possible to detect some contrast of a large, evenly illuminated 28-29 MPSAS source against the eye's own 26 MPSAS background noise signal. 

 

There is a sort of rule of thumb that one can detect some objects that are about 3 MPSAS dimmer than the sky background/foreground.  It is not a precise value and it will vary both with the nature of the target (gradient), and the observer.  One might find it to be closer to 1, or 2 MPSAS, or 3.5 and perhaps some can reach 4.  Three just seems to work out as a more reasonable target for detection, and one that still might not be achieved without some practice.



#10 Araguaia

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Posted 04 March 2019 - 01:30 PM

If there is one thing I learned is that surface brightness quotes are deceptive.  As Jaques Cousteau used to say, one must go see.  Objects with low SB often contain bright features that are easy to see.  Objects with high SB often fade slowly towards the edges so there is very little contrast, or consist of a very bright feature (e.g., a galactic nucleus) along with some very dim haze.



#11 dkracht

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Posted 05 February 2022 - 06:07 AM

The surface brightness add up.

Contrast is the crucial thing.

CS Dietrich


Edited by dkracht, 05 February 2022 - 06:31 AM.


#12 spereira

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Posted 08 February 2022 - 02:46 PM

Moving to Deep Sky Observing.

 

smp



#13 EmDrive2821

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Posted 14 February 2022 - 07:48 AM

To CzechAstronomer:  It’s probably a little late, but I noticed this NELM to MPSAS calculator web page.

 

Conversion Calculator - NELM (V) to MPSAS (B) systems.

 

http://unihedron.com...NELM2BCalc.html



#14 Starman1

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Posted 19 February 2022 - 07:02 PM

Though these formulae assume the object is rectangular, and assume a uniform brightness across the object (never the case), you can convert the 

magnitude figure you see in books to mpsas or mpsam with:

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')

Min' and Max' are the sizes in arc minutes

TIM is the Total Integrated Magnitude (the figure you see quoted in books and charts).

 

Note that most deep sky object sizes are quoted to the magnitude 25 mpsas isophote (line of constant brightness).

Of course, most objects are larger than that if the images go deeper.

 

Let's try an example of M33:

TIM = 5.72  It can be seen in a dark sky with the naked eye.

Size is 41.7' x 70.8'

Plugging in the figures, we get surface brightness (SB) = 5.72 + 2.5log (2827.4 x 41.7 x70.8) in mpsas.

SB (averaged across the entire object) = 23.02mpsas

or 14.1mpsam.

Is it any wonder many beginners find it quite faint?

 

Fortunately, 1) small scopes don't see all the fainter parts, mainly the brighter parts, and 2) the galaxy does not have uniform brightness, but is brighter in and near the core than in its outer-most area.

So it is easier to see, even a small scope, than its surface brightness calculation implies.



#15 Starman1

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Posted 19 February 2022 - 07:04 PM

Another thread, as a good example about M33:

https://www.cloudyni...e-7/?p=11558489


Edited by Starman1, 19 February 2022 - 07:04 PM.



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