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Magnitude vs. Surface Brightness or why I can't see stuff?

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#1 aeajr

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Posted 20 August 2018 - 10:47 AM

When I first started in astronomy, three years ago, I wanted to see galaxies and nebula because the pictures in books and magazines were all so beautiful and colorful.   How disappointed I was to learn that those colors were often false, added for contrast.  I would never see anything like that in the eyepiece.  

 

Not only did I not see colors,  I could not see them at all, even in my 8" scope.   Looking at lists I would see magnitude ratings for galaxies and nebula that seemed to suggest I should be able to see them.  I could see stars of that magnitude and start clusters of that magnitude.  Why couldn't I see galaxies and nebula of that magnitude?

 

That was when I learned about surface brightness and the reason I would like to discuss this topic.  I am not really sure I fully understand this topic and would like the more knowledgeable on CN to help explain this magnitude vs. surface brightness topic and how it relates to what can be seen in an eyepiece. 

 

What I learned was to give up on these targets from my very light polluted home location.   On the www.darksitefinder.com map I am in a dark white area, the second worst.  I don't even try for most nebula and even M31 is barely a gray smudge.   OIII and Nebula filters don't seem to be much help either.

 

Any input on the subject would be welcomed by me and I am sure it would be helpful to those entering the hobby who quickly become frustrated because they can't see things that they feel they should be able to see.


Edited by aeajr, 20 August 2018 - 11:46 AM.

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#2 TOMDEY

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Posted 20 August 2018 - 11:24 AM

Hi, aeajr! I'm anticipating that you will get a flood of explanations and references.

 

So, I'll only give a minimalist summary here:

 

>Your scope's textbook Visual Magnitude Limit assumes that you are looking at a well-focused white star under decent conditions, using averted vision.

>"Little Fuzzies" are catalogued for Total Integrated Magnitude, same amount as a star that delivers as much light to your eye thru your scope... but spread out over the size of the object, at that magnification. That makes it harder to see.

>Indeed, intentionally defocus an equivalent star to that size, and you can, literally... watch it disappear!

>The brightness (luminance, actually) of the (light-polluted) sky exacerbates things, by adding a uniform wash over everything.

>Higher magnifications dilute both sky and little fuzzies proportionally. So upping the magnification does little to assist visibility of the (already difficult!) little fuzzies.

>Stars remain dots up to pretty high magnification; so they DO benefit.

>Resolved clusters, made of discretely discernable stars, therefore also benefit.

>Nebular filters help on many emission nebulae, because they mute the sky but let thru the nebl light.

>But don't do much for stars or galaxies aka unresolved stars.

 

>Best solution is to move to darker skies.

 

All this stuff can be quantified in terms of mag/arc-sec squared, size of Airy Disc (image of a star) in arc-sec, contrast discrimination of the human eye, as a function of luminance... etc.  But the fundamental mechanism of what is going on is consistent.  Tom


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#3 aeajr

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Posted 20 August 2018 - 11:38 AM

Thanks Tom.  I hope so. 



#4 Kendahl

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Posted 20 August 2018 - 12:24 PM

Your best bet will be to investigate the state parks at the far end of Long Island. According to this light pollution atlas (https://djlorenz.git...erlay/dark.html), they are light green which is far better than white. Observe objects in the eastern sky where there is nothing beneath to generate light pollution.


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#5 aeajr

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Posted 20 August 2018 - 12:33 PM

Kendahl,

 

Thanks for the suggestions on how to get away from the light pollution.   However that isn't the purpose of this discussion.   

 

The goal of the discussion is to delve deeper into and gain a better understanding of the difference between magnitude and surface brightness and how it impacts what we see through the eyepiece. 

 

Also, if there are ways to address the issue, without running from it. 

  •  filters
  • aperture
  • Other non-AP, non-electronic methods that could be used by the typical visual observer.

The goal is understanding and knowledge.


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#6 Tempus

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Posted 20 August 2018 - 12:36 PM

Ed, our very own Tony Flanders has an excellent write up in this very topic on his blog entry titled Surface Brightness


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#7 NYJohn S

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Posted 20 August 2018 - 01:58 PM

Interesting topic Ed. It seems to come up often in the deep sky observing forum. It's usually when someone is wondering why they can't see a certain galaxy or nebula even though it's listed as magnitude X or Y.

I read Tony's write up a while back and it did help me to understand the role surface brightness plays. I'm not even sure where the surface brightness is listed for each object but I find trial and error the only way for me to determine if I'll be able to see something in my home skies. I do find filters help with certain nebula. I really struggled to see NGC 2438 in M46 and a UHC filter help me finally succeed. I recently viewed the Helix nebula, something I never thought I'd see from home with an LPR filter. The Owl Nebula is another where a filter helped. All 3 could be seen without a filter but the filter made them more obvious. For galaxies it's just been aperture, waiting for them get into good position in the ski and nights of good transparency. Filters don't seem to help. Some may have low surface brightness but if the core is bright enough then I can detect it. Then there's some I just can't see from here so I save them for dark skies.

 

It will be interesting to see what others can add. 


Edited by NYJohn S, 20 August 2018 - 03:37 PM.

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#8 Jon Isaacs

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Posted 20 August 2018 - 02:28 PM

Cartes du Ciel lists the average surface brightness for many objects,  the come from SAC database . Note that Cartes du Ciel uses magnitudes per square arc minute.  (mpsam )

 

One can compute the average surface using the visual brightness or total integrated brightness and the area of the object .

 

SB = VB + 2.5 x log (area )

 

The units of surface brightness are the units of the area . I like to use square arc-seconds because sky brightness is typically measured in mpsas or magnitudes per square arc second. 

 

To convert form mpsam (magnitudes per square arc minute) to mpsas, simply add 8.89.

 

This is average surface brightness.  In reality,  each point on an object has a surface brightness. The core of M31 can be seen from urban skies because the core has a much higher surface brightness than the rest of the galaxy. 

 

Assigning a surface brightness to a cluster is questionable.  If the stars can be resolved,  then they act as individual stars and the area of the cluster is irrelevant.  If the cluster cannot be resolved,  then the surface brightness has meaning. 

 

Jon


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#9 Dave Mitsky

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Posted 20 August 2018 - 03:08 PM

How disappointed I was to learn that those colors were often false, added for contrast.

I don't think that's the right way of looking at the matter.  The plain fact is that human night or scotopic vision is not sensitive to color.  It also has poor resolution.

 

https://isle.hanover...otopic_evt.html

The wavelengths that a camera can detect are certainly there but our eyes can't perceive them when operating scotopically, except in a limited number of DSOs such as M42, the Homunculus Nebula, and various high surface brightness planetary nebulae like NGC 6543, NGC 6572, NGC 7009, and NGC 7662. People who have the opportunity to observe through very large apertures often detect abundant color in some objects.

There's also the fact that color perception varies quite a bit in individuals.

 

Imagers sometimes use various filters to produce color palettes that display various elements, sulfur for example, not normally present in typical RGB astrophotographs. 

 

http://www.astronomy...ble_palette.htm

 

There's more on surface brightness at http://www.astrobuys...com/paul/sb.htm and https://dept.astro.l...ness/index.html

 

Dave Mitsky


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#10 Keith Rivich

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Posted 20 August 2018 - 03:17 PM

I suspect this thread will progress like similar topics on SB have so I will leave it to the technical writers to explain the nuts and bolts.

 

As an observer I don't dwell to much on SB. I find the numbers very misleading and they have more then once led me away from observing an object that was fairly easy to see. 

 

I usually look at the SB only if I cannot see the object.

 

As far as magnification goes I find that at the right magnification DSO's tend to pop. Not very visible on either side of the magic x but jumps out at that magic x. I suspect it has to do more with my eye/brain then it does with the actual properties of the DSO.

 

On larger apertures filters work quite well on their intended DSO's. 


Edited by Keith Rivich, 20 August 2018 - 03:33 PM.

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#11 CSG

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Posted 20 August 2018 - 04:36 PM

You can't do much with polluted skies but part of the reason I stick, primarily, to small refractors is that I live under dark rural skies (Bortle 3, maybe pushing towards 4 over the next decade).  Anything I look at in DSOs are mostly gray smudges.  HOWEVER, I know what they look like based on all the excellent imaging out there and I find my mind's eye helps me to see what they look like when well photographed.  I also have a few apps on my phone that can show me images if it's something I don't know or had forgotten.  It's seeing the thing with my own eyes that keeps me excited even if it doesn't look like it might with a large scope under dark skies or what the published image looks like.


Edited by Chris Greene, 20 August 2018 - 11:27 PM.

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#12 Tony Flanders

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Posted 20 August 2018 - 07:05 PM

As an observer I don't dwell to much on SB. I find the numbers very misleading and they have more then once led me away from observing an object that was fairly easy to see.

Surface brightness is overwhelmingly important, and it's extremely helpful to understand it. But I agree that I rarely bother looking at the published surface-brightness values. There are just too many different aspects to an object's surface-brightness profile to capture in a single number. Besides, the published numbers sometimes seem to be flat-out wrong, no matter how you measure surface brightness.

In general, surface brightness is most important to people who observe under heavy light pollution and to people who observe exotic objects such as aging planetary nebulae and dwarf galaxies.

All the objects in the NGC have reasonably high surface brightness as far as dark-sky observers are concerned. Otherwise, they wouldn't be in the NGC, which was compiled based entirely on visual observations.


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#13 Keith Rivich

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Posted 20 August 2018 - 09:02 PM

Surface brightness is overwhelmingly important, and it's extremely helpful to understand it. But I agree that I rarely bother looking at the published surface-brightness values. There are just too many different aspects to an object's surface-brightness profile to capture in a single number. Besides, the published numbers sometimes seem to be flat-out wrong, no matter how you measure surface brightness.

In general, surface brightness is most important to people who observe under heavy light pollution and to people who observe exotic objects such as aging planetary nebulae and dwarf galaxies.

All the objects in the NGC have reasonably high surface brightness as far as dark-sky observers are concerned. Otherwise, they wouldn't be in the NGC, which was compiled based entirely on visual observations.

Yes it is important from a physical standpoint. Visually it is less so unless one puts it together with all of the other attributes of the DSO. If its a galaxy or a glob does it have a condensed core or is it smooth. For a PN is the central star visible and if so its estimated magnitude. 

 

Here is my order of importance for most DSO's:

 

Type (galaxy, globular...)

Size

V. Magnitude (or if P. magnitude only is given add 1 mag of brightness) 

SB


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#14 Migwan

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Posted 20 August 2018 - 10:16 PM

Ouch on the white.  I'm 4 colors better and I was whining.  I am ashamed of myself.  Can you see M81 in the spring when it's higher?   

 

Looks like much of the beach is accessible out by the light house.   This bit is off N.Road and them's campers, cars and a circle of people.  Would put you in the green.

 

jd

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#15 penguinx64

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Posted 20 August 2018 - 10:54 PM

For me, light pollution is brighter than the surface brightness of most DSOs.  It's very difficult for me to see anything with a surface brightness dimmer than magnitude 5.  My solution is to retire in a couple of years and move someplace with darker skies!


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#16 GlennLeDrew

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Posted 21 August 2018 - 02:39 AM

The sky itself provides some interesting perspective.

 

A pristine dark sky at the zenith has a surface brightness (SB) of 22 MPSAS. This corresponds to a 1 square degree patch having an integrated magnitude of 4.2, and a 180 degree hemisphere (assuming equal SB down to the horizon) having an integrated magnitude of about -7.

 

If you have a nebula of 1 square degree in area listed as being 4.2m integrated, or having SB = 22 MPSAS (let's assume essentially uniform SB over its full extent), as seen under a pristine sky its light will add to the equally bright sky glow and thus present as 2X brighter than the surrounding sky. (This is why we can see objects actually fainter than the sky; object and sky always add.)

 

Such an object having SB equal to sky glow SB and thus appearing 2X brighter than sky alone is actually considered to have pretty good contrast. In the main, one can see objects of SB some 3.5 magnitudes fainter than sky glow.

 

Let's take a well known example--the North America nebula. It has a characteristic diameter of about 2.3 degrees and a SB of around 24 MPSAS (and of not particularly large variation). The integrated magnitude is about 5 to 5.5, and indeed under a decent sky is dimly seen with the unaided eye. If we use the aforementioned delta of 3.5 magnitudes, a sky 3.5m brighter than 24 MPSAS, or 20.5 MPSAS, should permit this nebula to be seen without a filter. And indeed, on a particularly transparent night from my former red zone apartment in West Ottawa, I just barely glimpsed the N. Am. neb with an unfiltered 10X50 bino.


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#17 aeajr

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Posted 21 August 2018 - 03:22 AM

Thanks for all the input.   I am going to have to study these for a while.



#18 Waddensky

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Posted 21 August 2018 - 04:59 AM

Generally, integrated magnitude, surface brightness and contrast alone do not give enough information on how well the object will be visible in a given telescope. Apparent size and background brightness are also important here because of the properties of the human eye: larger objects on a brighter background are more easily detected than small objects with the same contrast on a dark background. That's why in larger apertures dimmer extended objects can be seen: you can magnify more (larger object), while maintaining a larger exit pupil (brighter background). Filters don't make an object brighter, they're increasing contrast by dimming the background brightness. There are some excellent apps and online calculators around that can help you to perform these calculations, based on the eye's ability to detect contrast.

 

The 'problem' with surface brightness is that the listed value in catalogues is an avarage value that assumes the brightness is evenly spread across the whole angular area of the object. For most extended objects, this is not the case. I've seen people creating their own visibility index by combining integrated magnitude with surface brightness by adding them up or even multiply them in an attempt to take the uneven brightness distribution into account. Perhaps creating such an index based on your own personal experience is an idea.


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#19 gnowellsct

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Posted 21 August 2018 - 06:48 AM

Well it's a rough analogy but consider taking 100 lit candles and putting them all on one small table outside. Then consider spacing them out over a square mile. If you were to fly overhead at 10,000 feet with a pair of binoculars your odds of seeing the table with 100 candles would be pretty good. But the odds of seeing the same lumens of light emitted over a square mile would be very small.
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#20 gnowellsct

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Posted 21 August 2018 - 07:04 AM

There is of course easy viewing and hard viewing. In my 14 inch mag 15 galaxies are tough under typical sky. Mag 16 and even 17 stars can be viewed with patience.

When I want an easy night I take the approximate limiting magnitude of the aperture as calculated by formula and subtract two. If I'm willing to push I take the same formula and add two.

You can see surface brightness in action by watching a faint star at the limit of your aperture. When the seeing degrades the seeing disk of the star spreads out over a greater area and it becomes too faint to see, and it winks out. When the sky steadies you can see it again. GN
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#21 jpcannavo

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Posted 21 August 2018 - 07:42 AM

When I first started in astronomy, three years ago, I wanted to see galaxies and nebula because the pictures in books and magazines were all so beautiful and colorful.   How disappointed I was to learn that those colors were often false, added for contrast.  I would never see anything like that in the eyepiece.  

 

Not only did I not see colors,  I could not see them at all, even in my 8" scope.   Looking at lists I would see magnitude ratings for galaxies and nebula that seemed to suggest I should be able to see them.  I could see stars of that magnitude and start clusters of that magnitude.  Why couldn't I see galaxies and nebula of that magnitude?

 

That was when I learned about surface brightness and the reason I would like to discuss this topic.  I am not really sure I fully understand this topic and would like the more knowledgeable on CN to help explain this magnitude vs. surface brightness topic and how it relates to what can be seen in an eyepiece. 

 

What I learned was to give up on these targets from my very light polluted home location.   On the www.darksitefinder.com map I am in a dark white area, the second worst.  I don't even try for most nebula and even M31 is barely a gray smudge.   OIII and Nebula filters don't seem to be much help either.

 

Any input on the subject would be welcomed by me and I am sure it would be helpful to those entering the hobby who quickly become frustrated because they can't see things that they feel they should be able to see.

Ed

Going on a bit of tangent, a related and fundamental issue here is so-called conservation of entendue. And I have to say, my first understanding of this was kind of like finding out there is no Santa Claus! Without going into some of the detail I have previously posted, the idea is simply this: Relative to the naked eye, telescopes increase the brightness of point sources of light, but not the surface brightness of extended objects. So for example, if you gaze at an object like m31 with the naked eye, it will present as an extended object with a given, and rather faint, surface brightness. Now here’s the thing, with a telescope - regardless of aperture - that visual surface brightness cannot be increased. Moreover if it where increased, physical conservation onservation laws would be violated! An easy way to observe this is to take a pair of 7x50 binouculars and look at an extended grey wall, or an extended uniformly dim surface at night. The view through the binoculars will be equally dim, no brighter! But, there will be more detail, as magnifying while maintaining surface brightness will increase detail seen. So the sad reality is that there is no Santa Claus, no matter how big the aperture, faint fuzzies remain just as faint, albeit larger and more detailed. By the way, here is an excellent way to simulate the effect of increasing aperture. Get an 8x10 print of a galaxy or nebulae. Hang it on a wall in a large dark room with a dimmer light or other controllable dim light source. Stand at a distance and adjust the light to be dim enough to simulate some telescope in view. Now slowly walk towards the print. The apparent surface brightness will remain unchanged, but as you get closer more detail will come into view as image scale increases. This exactly simulates the effect of increasing telescope aperture while maintaining maximum exit pupil. Hope this adds to a broader understanding of the issues in play for visual work.


Edited by jpcannavo, 21 August 2018 - 07:46 AM.

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#22 Jon Isaacs

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Posted 21 August 2018 - 09:12 AM

Surface brightness is overwhelmingly important, and it's extremely helpful to understand it. But I agree that I rarely bother looking at the published surface-brightness values. There are just too many different aspects to an object's surface-brightness profile to capture in a single number. Besides, the published numbers sometimes seem to be flat-out wrong, no matter how you measure surface brightness.

In general, surface brightness is most important to people who observe under heavy light pollution and to people who observe exotic objects such as aging planetary nebulae and dwarf galaxies.

All the objects in the NGC have reasonably high surface brightness as far as dark-sky observers are concerned. Otherwise, they wouldn't be in the NGC, which was compiled based entirely on visual observations.

 

I rarely look up surface brightness but I suspect that it's for the same reason, you and Kevin rarely look up surface brightness:  we already have a pretty good idea what it is based on the object type.  And when I look up an object, typically I will look at the visual magnitude (integrated brightness) and right there are the dimensions of the object. 

 

I do think it's important to understand the concept and there are times when the surface brightness explains why an object is difficult to see.  A while back I was looking at some galaxies in Sculptor and Sky Safari listed the Sculptor Dwarf at about magnitude 10 and 1 arc minute in diameter, that should be easy.   I had no internet access and it didn't occur to me that dwarf galaxies are generally close and large.  So, when I looked it up on the internet and realized it was about half a degree by half a degree, I realized why I had not seen it.  

 

The average surface brightness is around magnitude 26 rather than the magnitude 19 I had been expecting.  And furthermore, thinking it was small, I had used high magnifications rather than low magnifications.  At the magnifications I had been using, it would have overflowed the field of view.

 

Jon


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#23 Starkid2u

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Posted 21 August 2018 - 10:12 AM

When I first started in astronomy, three years ago, I wanted to see galaxies and nebula because the pictures in books and magazines were all so beautiful and colorful.   How disappointed I was to learn that those colors were often false, added for contrast.  I would never see anything like that in the eyepiece.  

 

Not only did I not see colors,  I could not see them at all, even in my 8" scope.   Looking at lists I would see magnitude ratings for galaxies and nebula that seemed to suggest I should be able to see them.  I could see stars of that magnitude and start clusters of that magnitude.  Why couldn't I see galaxies and nebula of that magnitude?

 

That was when I learned about surface brightness and the reason I would like to discuss this topic.  I am not really sure I fully understand this topic and would like the more knowledgeable on CN to help explain this magnitude vs. surface brightness topic and how it relates to what can be seen in an eyepiece. 

 

What I learned was to give up on these targets from my very light polluted home location.   On the www.darksitefinder.com map I am in a dark white area, the second worst.  I don't even try for most nebula and even M31 is barely a gray smudge.   OIII and Nebula filters don't seem to be much help either.

 

Any input on the subject would be welcomed by me and I am sure it would be helpful to those entering the hobby who quickly become frustrated because they can't see things that they feel they should be able to see.

As I've told you many times before, Ed, you need to attend star parties. For several reasons, not the least of which is to SEE the objects you cannot see from home. M101 is a classic example. Great mag, low surface brightness. Can't see it from anything except dark skies. Another reason is how do you KNOW what you're looking for if you've never seen it before? The question may sound foolish and counter-intuitive but until I went to a star party, I didn't know WHAT I was looking at! Once I saw it and knew what it looked like in the sky, I was more easily able to track the object down at home. A third reason is you're missing out on a ton of knowledge you could gain in just one weekend. And of course, the immersion aspect can't be overstated, either. If you want to speak from a position of strength, that is how you get there. You have clubs on LI and you just missed two of the biggest star parties in the NE part of the country, Stellafane and Black Forest (registration closed). Again. A lot of people here would KILL to go to one of these parties, Ed. You're a few hours drive away from both! The guys in South Jersey put on TWO star parties a year, spring and fall.  That's a hop, skip and a jump (and a couple of tolls) for you. So, when you add it all up, you've got to move to the next level. You've got all winter to think about it, Ed. Save your pennies up and before you know it, you'll be good to go BEFORE the party happens! Best wishes and clear LI skies for you, of course...

 

STARKID2U


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#24 aeajr

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Posted 21 August 2018 - 10:46 AM

STARKID2U,

 

I thank you for your suggestions.   Experience has shown me what targets to pursue at my home and what to hold off till I am at a darker site.   But that does not mean I have fully internalized the reason or the details. 

 

The collected wisdom of this community often shows me that there is more to it than I realize.   So I invite others to teach me and in the process create a resource that I can point others to so that they can understand why they can't see things from their site.

 

FWIW, I am a member of a club, attend group observing sessions and go out with others on a regular basis.  I have observed at my home location and locations that are much darker than my home.   I have seen the difference so I know how to select my targets according to conditions.   

 

I am not trying to solve a problem, I am trying to understand a concept. 

 

You have been a huge help to me and to others and I thank you and everyone.     


Edited by aeajr, 21 August 2018 - 11:08 AM.

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#25 aeajr

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Posted 21 August 2018 - 10:51 AM

Well it's a rough analogy but consider taking 100 lit candles and putting them all on one small table outside. Then consider spacing them out over a square mile. If you were to fly overhead at 10,000 feet with a pair of binoculars your odds of seeing the table with 100 candles would be pretty good. But the odds of seeing the same lumens of light emitted over a square mile would be very small.

 

We often speak of trying to "visualize" a concept.   This is a brilliant analogy and I thank you.   Perhaps others have offered similar illustrations but this one really resonated.

 

If we consider that those candles equate to the stars in a galaxy or globular cluster I think that the analogy holds well.   I plan to use this often when working with newbies.

 

If we think of the light reflecting off the ground to the glow of a nebula there is again a good analogy for the more dispersed light and lower surface brightness.  The same lumens may be involved ( limited by reflection naturally) but the light per unit area, the surface brightness is lower.

 

It may not be technically perfect but it is one I can use to help others understand and cope with this difference.  And it is one I can use to help internalize the concept myself.

 

Great image you created. 


Edited by aeajr, 21 August 2018 - 10:54 AM.

  • paul m schofield likes this


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