•

# How to figure out the max SB LP will allow you to see ?

17 replies to this topic

### #1 Diomedes

Diomedes

Messenger

• topic starter
• Posts: 462
• Joined: 04 Feb 2020
• Loc: New York City

Posted 07 July 2020 - 07:39 AM

Hey, an interesting thought popped into my head this morning. I was reading about NGC 6503 in Draco which has a surface brightness of 21.  I know I won't be able to see it from my skies, since I struggled to make out a hint of the galaxies in the Leo triplet and those are much much brighter.   Is there a point in which a galaxy is rendered invisible due to light pollution regardless of the scope you have?  what would that calculation look like ?

### #2 stevenrjanssens

stevenrjanssens

Vostok 1

• Posts: 195
• Joined: 24 May 2018
• Loc: Vancouver, BC

Posted 08 July 2020 - 12:42 AM

Check out Section 3.3 and Figures 15 on.

• Redbetter and Diomedes like this

### #3 Feidb

Feidb

Mercury-Atlas

• Posts: 2,939
• Joined: 09 Oct 2009

Posted 08 July 2020 - 08:23 PM

Sounds like a math question and since I'm allergic to math, all I can say is that I just go for it.

Period.

I know the ballpark my scope can do and I push the limit.

That's it.

If I'm successful, fine. If not, I move on to the next object. I don't sit around doing math formulas, fretting over whether I was supposed to see something or not.

Then again, you have to take into consideration the highly unpredictable sky conditions, which throw any math calculations right out the window anyway.

So, in conclusion, just know your basic limits and from there, go for it and see what happens. You don't need math formulas for basic limits. They usually come with the scope specs.

The SBs are notoriously inaccurate to vague, depending on which catalog you're using, so go with the general brightness, try it and if you fail, move on to the next object and don't scratch that failed one off the list just yet. Save it for another day. You never know when you'll have that magic moment.

As a general rule, given the mag limit of my scope, I start two mags brighter as my main targets. Then keep some at the limit on the list. I also throw in a few over the limit just for giggles. Sometimes I'm surprised.

Saves a lot of formula figuring.

Well, that's my take. Sorry to disappoint you with no formulas. Maybe some of the PHDs can chime in here.

• Mark SW and Diomedes like this

### #4 Tony Flanders

Tony Flanders

Hubble

• Posts: 18,269
• Joined: 18 May 2006
• Loc: Cambridge, MA, USA

Posted 08 July 2020 - 09:00 PM

Is there a point in which a galaxy is rendered invisible due to light pollution regardless of the scope you have?  what would that calculation look like ?

Yes, there are some galaxies too faint for the human eye to see. None of the NGC galaxies qualify, since all the NGC galaxies were discovered by visual observers rather than by photography.

To a very crude first approximation, it's only possible to see objects whose surface brightness is within 3 magnitudes of the background skyglow. Natural skyglow is around mag 22 per square arcsecond, so the faintest galaxy visible to the human eye would have surface brightness of mag 25 per square arcsecond or fainter. However, most galaxy's cores are much brighter than the galaxy as a whole. So technically, the core would have to be 25 mpss or fainter, making the galaxy as a whole likely fainter than 26 mpss.

Very few such galaxies are known, not because they're rare -- they are not! -- but because they're incredibly hard to detect even with the best modern instruments.

• Jon Isaacs, Keith Rivich, Redbetter and 3 others like this

### #5 Redbetter

Redbetter

Cosmos

• Posts: 9,215
• Joined: 16 Feb 2016
• Loc: Central Valley, CA

Posted 09 July 2020 - 05:54 AM

You asked about what is visible in the way of surface brightness of the object relative to the light pollution level.  As a first order approximation, an object 3 MPSAS of delta is required, and that will be very subtle/faint.  3.5, possibly even 4 may be detectable depending on the observer's eye and experience with these type of challenge objects. 3 MPSAS dimmer ~16x fainter than the background glow, 3.5 is ~ 25x dimmer than the background, and 4 is ~ 40x dimmer than the background.  The apparent size of the object typically has to be quite large for values greater than 3 MPSAS dimmer.

A 21 MPSAS object is of reasonably good surface brightness...in dark sky, but for a 3 MPSAS delta you still need about 18 MPSAS skies for detection.  Good luck with that in New York City!  The sky brightness there is likely to be closer to 17 than 18.  An experienced observer might detect some of the brighter central region (as Tony explained above), but that would seem a tough object for the average observer in such sky.

In very dark sky the two dimmest I believe I have seen are the Draco Dwarf and Ursa Minor Dwarf galaxies.  These happen to be in exceedingly dark portions of the sky, improving contrast and giving the observer a fighting chance.  Both are barely noticeable diffuse over-brightenings of the background sky on photographic plates.  Fortunately, they also happen to be quite large in apparent size.  Some of the published values suggest they are almost 27 MPSAS...but my visual impression with the 20" is that they--or at least their somewhat more apparent major axes are closer to 26 MPSAS or even a little brighter.  There is just enough suspected to tease the eye while panning about.  I could be seeing some other subtle background effects or even diffuse glare, but I have seen the same things consistently, several times over several hours with different scope orientation. (A large enough aperture could actually resolve enough individual stars within as way of confirmation/observation...cheating because one would see tiny parts rather than the whole.)

• Dave Mitsky and Diomedes like this

### #6 Starman1

Starman1

Vendor (EyepiecesEtc.com)

• Posts: 46,611
• Joined: 23 Jun 2003
• Loc: Los Angeles

Posted 13 July 2020 - 07:58 PM

Hey, an interesting thought popped into my head this morning. I was reading about NGC 6503 in Draco which has a surface brightness of 21.  I know I won't be able to see it from my skies, since I struggled to make out a hint of the galaxies in the Leo triplet and those are much much brighter.   Is there a point in which a galaxy is rendered invisible due to light pollution regardless of the scope you have?  what would that calculation look like ?

The answer is "of course" and Redbetter explained it well, as did Tony but it is more complicated than that:

https://www.cloudyni...x-for-galaxies/

As an aside, NGC6503 is very bright.  If your LP is so bad you cannot see it, even bumping the power up a bit,

then confine your observing to star clusters, Moon, planets, asteroids, carbon stars, bright planetary nebulae and very bright stuff, and leave the galaxies and nebulae for when you can get out to a darker site.

I totally agree with Feidb--just look for it anyway.  I've seen a lot of stuff supposedly needing larger scopes that way.  You only know you can't see it if you can't see it.

I always have a lot of NFs in my notes (not found), but sometimes I see it the second time I look, so I won't give up on anything now.  Look for it first, then look up how faint it was.

Pretty soon you'll have a good idea what your limit may be.

• Dave Mitsky and Feidb like this

### #7 Thomas Pfleger

Thomas Pfleger

Vendor - Eye&Telescope Software

• Posts: 374
• Joined: 25 Apr 2006
• Loc: Germany

Posted 30 July 2020 - 05:33 AM

The perceptability of an object does not only depend on it's size and magnitude, yielding the surface brightness. It also depends on the aperture of the telescope used. Comparing surface brightnesses of DSO and sky is not enough to assess the chances for successful detection. The apparent angular diameter also comes into play.

As you probably know, all this has been treated in detail by Roger Clark in his groundbreaking book "Visual Astronomy of the Deep Sky". Highly recommended! A lot of programs or web sites dealing with DSO visibility use knowledge and data given in this book.

In the case of NGC 6305, let's look at what comes out of Clark's formulae, with a little bit adjusted data based on my own observations. Let Eye&Telescope be our vehicle, because it is all built around the core question of deep sky observing: given object, scope and sky quality, how are my prospects?

You did not give details on your scope, so let's look at three different apertures: 8", 14.5" and 24". This is, of course, just a possible choice. Using the software, you are not restricted to "canned" apertures. Just enter data for your own stuff.

The quantitative property defining the "how good" is the contrast above threshold, aka CAT. The CAT is given for scope and sky quality. Depending on the value of CAT, a category like "Questionable" or "Difficult" is presented. From my experience, the transition of "Questionable" to "Difficult" is the point where you have a fair chance.

In Eye&Telescope, we can set and modify the sky quality (simply by turning the mouse wheel) and (having the DSO and the scope selected) look for "questionable becomes difficult".

The green box (highlighted in the screenshot) is where we give the Sky quality. This can be an SQM measurement value or a personal, subjective estimation of the faintest star.

For a sky quality of 20.16 mags/"^2, NGC 6305 is "questionable". A bit darker, and it becomes difficult:

So, for 8 inches, a sky quality of ~ 20.3 is required for a fair chance to see the object.

To illustrate how aperture helps, we have two other situations where I already dialed in the sky quality to reach the questionable becomes difficult point (where CAT is ~0.1).

14.5" (my most used aperture):

Now we need only 19.6 mags/"^2, half a magnitude brighter than with 8".

And finally the big 24":

Yet another half magnitude.

We find that larger aperture can a bit compensate for brighter skies. But this is not to claim that going for the darkest spot you can reach is a must for the most pleasant experience.

But how comes this? We have to take into account the apparent diameter of the object in the eyepiece. It has to be large enough for a good detection probability. This means that there is an "optimum magnified visual angle", to use Clark's wording. This in turn means that one of your eyepieces best matches this fine magnification. And if we need, say magnification V=100*, a larger scope will simply give a brighter image. For details, please see Clark. Once again, highly recommended.

Using the CAT in a rendering of the eyepiece impression (seen in the circle right to the numbers) shows differently difficult (or easy) objects. The next screenshot shows a view of the M84/M86 area, right at the center of the Virgo cluster of galaxies.

Clear skies

Tom

### #8 Redbetter

Redbetter

Cosmos

• Posts: 9,215
• Joined: 16 Feb 2016
• Loc: Central Valley, CA

Posted 02 August 2020 - 04:18 AM

Tom,

There are some problems with Roger Clark's way of determining DSO visibility.  In general it seems to be too pessimistic (I have found the 3 MPSAS delta guide to be a better all around starting point with 4 MPSAS as close to an actual limit.)  Some of it is inherent to the datasets he used.  That is an issue that arises with published values even now since the data isn't really updated for visual observation. Both sizes and magnitude are often of mixed bases.

Relying on published surface brightness is not advisable, particularly for low surface brightness galaxies.  If I really want to estimate a surface brightness, I look through published visual magnitudes, then I examine Wikisky to determine likely visual limits of the object and obtain major and minor axes.  I use that to calculate a new surface brightness.  Obviously this is still only a mean, and the central surface brightness is typically higher (while the periphery is lower.)

I have gone though the above exercise on many low surface brightness objects in Roger Clark's DSO table and arrived at much brighter surface values on average.  He notes the Sculptor dwarf as being visually "the only object in the table that is not observable."  His 26.3 MPSAS figure is way off for actual visual, I arrive at 25.0.  Even at the time the book was published folks were beginning to report observing it.  Despite its very low declination here, I have seen the Sculptor Dwarf in Bortle 2 sky with apertures as small as 80mm and I don't consider it particularly difficult for large aperture as long as the skies are very dark and transparent--although it remains subtle.  The Fornax system data and visual impression are similar.  Some of the discrepancy is likely due to differences in a broad central surface brightness, vs. a larger area that is very diffuse and much poorer surface brightness than typical cut offs in determining size.

The way Clark determines contrast is interesting as he uses a 24.25 MPSAS background value.  While I and others have been skeptical of that (using an actual sky value of ~22 MPSAS as pristine) it probably helps his method to some degree, even if the overall handling proves to be problematic.  The actual value he uses could even be close to correct if one is using a mean "background between stars" in pristine sky.  (This is something I intend to follow up on eventually.)   Certainly the background is darker than what is seen in the eyepiece with the influence of many stars, so there are some competing things happening visually that aren't so easy to disentangle.

• Dave Mitsky likes this

### #9 Starman1

Starman1

Vendor (EyepiecesEtc.com)

• Posts: 46,611
• Joined: 23 Jun 2003
• Loc: Los Angeles

Posted 02 August 2020 - 10:50 AM

You aren't the only one who had issues with Clark's calculations.  They just didn't seem to correspond to what I saw in a 6", 8" or 12.5" aperture.

It does point out the difficulty with trying to pre-determine the visibility of an object in the sky, however.

Going back over a decade, we've gone around and around on the issue of visibility of an object in a particular scope size, and the answer has always been

that this cannot be determined due to there being too many variables.

When I discovered that Clark's calculations and object lists simply didn't correspond to my reality,

I stopped recommending his writings.

His book, however, is just about the only one I've seen with star clusters with star magnitudes listed, and it is a powerful reference for

that purpose--to find your personal limit.  Unfortunately, the charts are only good up to about an 8" aperture--larger apertures will see deeper than any of his magnitude limit charts.

• Dave Mitsky likes this

### #10 Tony Flanders

Tony Flanders

Hubble

• Posts: 18,269
• Joined: 18 May 2006
• Loc: Cambridge, MA, USA

Posted 02 August 2020 - 12:11 PM

The way Clark determines contrast is interesting as he uses a 24.25 MPSAS background value.  While I and others have been skeptical of that (using an actual sky value of ~22 MPSAS as pristine) it probably helps his method to some degree, even if the overall handling proves to be problematic.  The actual value he uses could even be close to correct if one is using a mean "background between stars" in pristine sky.

That is indeed what he means; he says so explicitly. But even so, the 24.25 figure seems too dark.

All I can say is ... whatever.

Roger Clark's book was immensely important historically, encouraging deep-sky observers to use much higher magnifications than most had previously. And it's also a good read. But of course it's not definitive -- nor will anything ever be.

• Redbetter and Diomedes like this

### #11 Redbetter

Redbetter

Cosmos

• Posts: 9,215
• Joined: 16 Feb 2016
• Loc: Central Valley, CA

Posted 02 August 2020 - 07:21 PM

That is indeed what he means; he says so explicitly. But even so, the 24.25 figure seems too dark.

All I can say is ... whatever.

Roger Clark's book was immensely important historically, encouraging deep-sky observers to use much higher magnifications than most had previously. And it's also a good read. But of course it's not definitive -- nor will anything ever be.

Thanks for confirming that.  Any single value is problematic, because the background sky between stars is not uniform either.  I see three primary extreme cases:  1.  regions with few stars, away from the Milky Way and toward the galactic poles  2.  Many bright sections in the Milky Way with a dense unresolved starry background (as with the NAN discussion.)  3.  Less common areas of the Milky Way that happen to be windows through or interposed on top of dark nebulae and little visible background glow.  (While the latter is rare, it can make things stand out dramatically, I vaguely recall a Terzan that was surrounded by blackness that made it much easier than anticipated.)

When I was trying to determine what my visual limit for full on black was in dark sky (threshold where I was unable to see the field stop) I selected regions with few background stars, in dark sections of sky away from the galactic plane.  Part of the trick there was leaving one dim field star somewhat near the middle of the field that I could use as a focal point and reference for my eye.  Without some such dim star, I could not find the exit pupil to position my eye, much less scan fully around the field.

Any faint stars near the field stop could illuminate that portion of field slightly, so what i was looking for was whether or not I could still detect sections of the arc away from such areas.  I was using a hood and cupping around my eye, but it was quite difficult to hold the eye in position with only a flicker or two of reference.  I knew I was getting close to the limit by how difficult this became, even with a driven scope..  It is similar to when one inserts a deep red filter into the eyepiece for planetary magnification, the background sky largely disappears and it becomes difficult to determine where the field stop is for centering a planet.

So where is all of this leading?  For about 21.6 MPSAS conditions I used exit pupil to calculate a little over 28 MPSAS at the threshold for the tests I made on different nights with three different scopes.  If I used 24.25 MPSAS instead and tried to adjust this by adding a light pollution component (22.0-21.6 MPSAS surface brightness delta) I would arrive at a considerably darker background sky that I was detecting.  If my math is right the effective background would be 22.6 in such a case, rather than a pristine 24.25 between stars.  That would yield 29+ MPSAS, which from what I back calculated from some old papers, is what some of the best young subjects measured during WWII.  It is also, probably not coincidentally, 3 MPSAS dimmer than the "dark current" background of the dark adapted eye.

Lots of numbers there, so not the easiest to digest.  What I have been trying to wrap my head around and quantify, is the real effective contrast at dark site levels of light pollution, and with varying levels of transparency.  Transparency is more difficult to determine than overall site darkness (which can be calculated.)  The combination has a huge impact on observed contrast because it whittles away at the "24.25" very quickly.  21.6 MPSAS sky is relatively dark, but that level of light pollution will knock 24.25 regions of the sky down to 22.6.  21.8 MPSAS sky would become 23.2 between stars, and 21.9 would be 23.6.  Those are huge ranges of effective contrast even though the sky itself has gone from only 21.6 to 21.9.

### #12 Starman1

Starman1

Vendor (EyepiecesEtc.com)

• Posts: 46,611
• Joined: 23 Jun 2003
• Loc: Los Angeles

Posted 02 August 2020 - 09:21 PM

Lots of numbers there, so not the easiest to digest.  What I have been trying to wrap my head around and quantify, is the real effective contrast at dark site levels of light pollution, and with varying levels of transparency.  Transparency is more difficult to determine than overall site darkness (which can be calculated.)  The combination has a huge impact on observed contrast because it whittles away at the "24.25" very quickly.  21.6 MPSAS sky is relatively dark, but that level of light pollution will knock 24.25 regions of the sky down to 22.6.  21.8 MPSAS sky would become 23.2 between stars, and 21.9 would be 23.6.  Those are huge ranges of effective contrast even though the sky itself has gone from only 21.6 to 21.9.

Which explains why the views through scopes differ when going to a site of 21.6/7/8/9.

I could detect at least 5 discrete different skies and views between 21.0 and 22.0, which is why I always thought

the color zones for light pollution and the Bortle Scale were all way too coarse at the darker end.

### #13 Redbetter

Redbetter

Cosmos

• Posts: 9,215
• Joined: 16 Feb 2016
• Loc: Central Valley, CA

Posted Yesterday, 01:19 AM

Don,

Yes, that is where I was coming out on this as well.  The difficulty is in actually identifying/measuring differences in overall sky brightness this small, since the measurements cover substantial swath of sky and the values include the contribution of so many stars, plus the Milky Way glow.  On top of that is the variable air glow that will show up as mild levels of light pollution, as well as any transparency degradation from dust/aerosols/mist.

Those of us who use meters frequently understand how much the measured brightness of the sky varies at our sites throughout the night and time of year.  So we can tell when peak and minimum values should be anticipated as well as have indications of when a night is abnormally good or bad.  The differences in the measured numbers for good/average/poor conditions are not all that great as long as the light pollution level is not large.  But it is challenging to evaluate the transparency as more than a subjective impression.

The difficulty as I see it is one of scale.  We don't have a very good way of measuring the darkness of the region between stars directly on a given night, because our view/impression is impacted by the whole sky--which is better reflected by the SQM/SQML type reading.  NELM doesn't really do it, it is again on too wide of a scale.  So we use other things, like how visible dark bands are in the Milky Way, zodiacal band/gegenschein, etc.  We look for signs that lower surface brightness DSO's have more contrast than normal.

As a practical matter, for the same segment of sky overhead, there is considerable difference in those last few tenths at small scales that we use to look at galaxies and such.  It won't be linear in the overall 21.5-22.0 range.  If one were to propose ranges assuming 24.25 MPSAS background with 22 MPSAS overall being pristine, then some natural break points appear to be in roughly 0.6 MPSAS delta net background contrast.  My calcs suggest:

• 22.0 overall (pristine) with 24.25 net background
• 21.9, 23.63 net background, 0.62 delta from above
• 21.75, 23.04, 0.59 delta from above
• 21.5, 22.37, 0.67
• 21.2, 21.79, 0.58
• 20.8, 21.17, 0.62

From there on out the overall sky brightness and background levels are beginning to merge, so a 0.5 MPSAS delta becomes convenient again.

One might lump ranges of 21.9 to 22 as pristine/near pristine, 21.75 to 21.9 as superb, and 21.5 to 21.75 as very good dark sky, while 21.2 to 21.5 as becoming merely decent rural dark, and 20.8 to 21.2 is rural transition.

Of course, the ranges and break points depend on the effective value of a pristine background.  A value of 23.5 gives somewhat wider/different break points, but perhaps a more intuitive 0.5 mag net background delta retains the step count (21.85, 21.65. 21.4, 21.1, 20.7).

### #14 Thomas Pfleger

Thomas Pfleger

Vendor - Eye&Telescope Software

• Posts: 374
• Joined: 25 Apr 2006
• Loc: Germany

Posted Yesterday, 02:29 AM

IMHO, the discussion shows that there are some points where it becomes hard to give "correct" numbers.

The 24.25 MPSAS was the first I stumbled upon. Could this be true? In an era before the SQM devices, we were restricted to faintest star estimations insdtead of measurements. Attending star parties, you can hear very different fst numbers for the same sky. Today I know that visual acuity is the strongest driver for different estimations. At night, people tend to become a bit shortsighted. Glasses might not yield a perfectly sharp image of the sky. And people not wearing glasses might also be a bit off. At least this differences mostly vanish when we use telescopes, as every observer works with a properly focused scope.

So with and SQM or without, getting the "correct" sky brightness is no easy matter.

Transparency also can make a considerable effect and is not measured by an SQM.

The next problem is that a perception model reduces a deep sky object to a uniformly glowing ellipsis. This oversimplyfication nevertheless can yield useful results, but as soon as it is just plain wrong, results willl also become wrong. For galaxies, a brightness profile is not taken into account. The small and large axis depend on the isophote level you take. Apart from this, the core is brighter and what is uncertain in a model is how far out the "diameters" reach that we might glimpse. Bright nebulae have a SB that hardly ever is (nearly) constant. The distribution does not depend on the distance from the center. Other parts or knots can stand out. And so on... We all know a lot of objects that just do not fit the oversimplification of a uniformly lit ellipsis.

The next problem is the visual magnitude given in data sets. It hardly was determined from observers' estimates but calculated from intensities in certain spectral ranges. This is a source of unsharpness of some tenth of a magnitude. Even if we could agree on a model, there would still be doubt about the "correct" data.

I had to adapt Clark's model here and there and did some tuning to lessen the differences between the numbers and my observations. While this can never be completely perfect, I found it to work good enough to derive lists of objects for a session or to plan for a trip. Albeit all the uncertain points mentioned, the tool is useful.

### #15 Redbetter

Redbetter

Cosmos

• Posts: 9,215
• Joined: 16 Feb 2016
• Loc: Central Valley, CA

Posted Yesterday, 04:32 AM

Tom,

The ~24 MPSAS value for the area between stars (excluding stars down to some dim magnitude I suppose) seems a reasonable one.  This is something I have been wrestling with for a time as I have seen similar descriptions, but didn't realize that was Clark's basis--I thought he was using it for the overall night sky with stars.  Clark's familiarity with background intensities for imaging suggests he would have had some feel and data for this basis.

It makes some sense that it would be somewhere in this range.  If 22 MPSAS is pristine sky with stars, then the area between stars will be somewhat dimmer.  I accept that with SQM or other types of photometric measurement.  2 MPSAS delta for background versus with stars seems reasonable based on how things look in the eyepiece in very dark sky as well as contrast in survey images.  Maybe it is actually only 23 MPSAS or perhaps it is as much as 25.  That is something I have wanted to get a handle on for regions that are not star rich.  I have not tried doing some magnitude census evaluations for brightness contributions in specific areas, but that is something I have been contemplating.

This is a topic I intend to broach with Brian Skiff eventually, because his visual limits appear to have been similar to mine.  I expect he will have some relevant information to add along with photometry and familiarity with methods and units that will make sense of things that I don't yet understand.  I have been wanting to go through Crumey's paper and calcs again, and this is part of it.  Crumey's work is very useful, with a problem creeping in with an incredibly conservative 25 MPSAS cut off for visual.  Remove that limit and replace it with something like 27 or 28 MPSAS...and remove "nerfing" for observer factors and the like, and I can duplicate my own results.  But I need to go back through all of this again to check my work.

Glenn LeDrew likely has some valuable insight on this as well.  He had some helpful information about the gegenshein when I noticed how much it washed out the area around low surface brightness galaxies like IC 1613, IIRC.  (Clark's table gives IC 1613 it as 25.6 MPSAS, but I calculate it as 24.4.  It has irregular knots that I have mapped it out with the 20" in Bortle 2 sky.)

Another factor is the way our eye handles glare.  Some surprisingly dim stars show up as "glare" when trying to see faint galaxies of low surface brightness in the same field of view.  It is difficult to mathmatically account for this close to the star, as well as far from it where the impact is less. It has some impact on telescopic limiting magnitude estimates in rich open clusters--some stars hide in plain sight because they are just near enough/oriented just right around a slightly brighter star or stars.  The effective "surface brightness" of the sky is somewhat elevated (as examining the field stop at exceptionally small exit pupils will reveal.)  And without some key dim stars as markers, it can be difficult to pin others down.

Of course, the variability in the quality/applicability of the data means that we should not take anything for granted as visible or invisible.  The way we were learn the limits of our eye/instrument/sky is to observe.  One of the tricks I have learned is that very narrow edge on galaxies close to my DSO magnitude limits often prove exceptionally difficult compared to ellipses of similar magnitude.  They have reasonable surface brightness, but are too thin to register to the wider scale of the dark adapted eye at the threshold.  They require more attention to detect and pin down in extreme averted vision.

### #16 Tony Flanders

Tony Flanders

Hubble

• Posts: 18,269
• Joined: 18 May 2006
• Loc: Cambridge, MA, USA

Posted Yesterday, 05:09 AM

Thanks for confirming that.  Any single value is problematic, because the background sky between stars is not uniform either.

Indeed! The main natural sources of diffuse skyglow are airglow and the zodiacal light, both of which are highly direction-dependent. Theoretically, the Milky Way doesn't count, because a sufficiently large telescope resolves it completely into individual stars.

Contrary to popular belief, the zodiacal light extends over the entire sky; it is quite significant even at the ecliptic poles.

It's an interesting question just how deep your telescope needs to go before the unresolved  stars become vanishingly faint with respect to the resolved stars. That no doubt depends what part of the Milky Way you're looking at. Not very deep at all in the nearby Cygnus Star Cloud, I would think, but very deep indeed in the distant Great Sagittarius Star Cloud, a.k.a. galactic bulge.

### #17 Redbetter

Redbetter

Cosmos

• Posts: 9,215
• Joined: 16 Feb 2016
• Loc: Central Valley, CA

Posted Yesterday, 06:40 AM

The zodical light is less of an issue than the band and gegenschein.  The light is more of an issue near astronomical twilight.  While this glow light/band/gegenschein extends across the sky at all ecliptic latitudes, it will form part of the 22 MPSAS average and any background level.  The strongest portions are in a narrower band because of the nature of the back scatter, serving as light pollution.  From Mauna Kea these are obvious when tracing the ecliptic...an eye opener in my early days.  On good nights at appropriate times of the year they show up in Bortle 3 conditions, while in Bortle 2 (and somewhat better 1 to 2 transition) they become easier to see and track from horizon (or Milky Way) to horizon.

There was an interesting paper from Hawaii about mapping the photometric intensity of the gegenschein (football or eye of Sauron) and nearby bands.  I did some conversions of these to visual surface brightness units, but will have to revisit that.

I actually consider the Cygnus star clouds relatively bright.  Visually, on a large scale and with a narrow meter the impact is substantial.   True, they aren't as bad as Sagittarius, but the large number of stars in Cygnus makes it more difficult to see/distinguish what might otherwise be more obvious nebulosity when panning about.  I spent some time last Moon cycle tracing what I could see with nebula filters north of the Propeller nebula.  I eventually traced out quite a few hydrogen alpha regions down to the low levels as surveyed as DWB objects (no Sharpless in this area.)  I went somewhat deeper than what I had considered possible, so I didn't mark deep enough with my target list based on the paper's surface brightness estimates.  What I came up with corresponded with actual emission, but also a lot of field stars, which complicated things since their glow spills out similar to glare near reflection nebulae.  It turns out that these stars and nebulosity tended to coincide in images too, although visually there were a few bright spots that lacked significant stars and there were dimmer areas extending away from them, confirmed via images.  I assume I was seeing primarily the 1/3 or less H-beta portion of the recombination emissions, although a DGM NPB was used as well to augment any H-alpha if it was strong enough to give any visual impression whatsoever.

### #18 Starman1

Starman1

Vendor (EyepiecesEtc.com)

• Posts: 46,611
• Joined: 23 Jun 2003
• Loc: Los Angeles

Posted Yesterday, 12:28 PM

The Gegenschein is easy to see in skies of 21.3 (anyone seen it in brighter skies?) and darker as long as it is not in the Milky Way.

https://amazingsky.n...of-gegenschein/

I've never regarded it as particularly hard when seen against constellations like Aquarius or Pisces or Virgo, for example.

There are galaxies I've seen that were a LOT harder to see and took a lot more time.

Edited by Starman1, Yesterday, 12:29 PM.

## Recent Topics

 Cloudy Nights LLC Cloudy Nights Sponsor: Astronomics