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General Astronomy >> General Observing and Astronomy

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Jon Isaacs
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Re: Surface Brightness and Performance new [Re: NGC7088]
      #5659624 - 02/03/13 09:49 AM

Quote:

However I still don't understand it in this context. Can you or someone find simpler words to digest?




I hope this is what you are thinking of:

The surface brightness of an object is proportional to the exit pupil, a telescope cannot increase the surface brightness over what is seen naked eye, it can magnify it making it larger but not brighter.

A simple example goes like this:

Consider a generic extended object, say Andromeda. Your eye sees it with it's 7mm exit pupil. You now point your 70mm telescope at Andromeda. At 10x this produces a 7mm exit pupil, the same as your eye. It also collects a hundred more times light than your eye. You have 100 times the light but the image is magnified 10 times so that light is spread out over 100 times the area. 100 times the light, 100 times the area... The result is that the image is larger but the intensity or surface brightness of the object is no greater. If you use exit pupils that are larger than the entrance pupil of the eye, the additional light falls unused...

This is a simple proof that a telescope cannot increase the surface brightness of an extended object, it can only make it larger. It can also make it dimmer. If the image were magnified 20 times instead of 10 times, that 100x the light would have been spread out over 400 times the area and thus the surface brightness would have been reduced by a factor of 4.

This is a simple proof that the surface brightness is proportional to the square of the exit pupil.

I hope this helps someone understand something...

Jon


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GlennLeDrew
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Re: Surface Brightness and Performance new [Re: Jon Isaacs]
      #5659867 - 02/03/13 12:08 PM

And Bill Ferris did indeed mean 2 degrees, and not 2 arcminutes.

He was elaborating on a very important concept regarding human vision. As light levels decrease, our resolving power decreases. Down at the level of faint fuzzies seen against a dark night sky, our resolving power has gone from the daytime limit of 1-2 arcminutes to the atrocious nighttime limit of a couple or few degrees (!)

In other words, just to be detected at all, a dim nebula must subtend on our retina a size equivalent in diameter to several full Moon diameters. Yet at the same time and in the same view we can see those very tiny points of light called stars.

Our visual system is a very marvelous and adept processor of signals. On the fly it processes both bright and dim parts of the image so as to get the most information. When parts of the image are very dim, light receptors on the retina are effectively bunched together in groups, acting like much bigger and hence more light-sensitive 'pixels'. This is a necessary tradeoff--either see nothing except visual system 'noise', or have at least a detection of *something*, even if poorly resolved.

The relationship of the relevant factors are charted out in an image (with user guide, I posted in my Gallery, linked to in my signature below. I forget the image title exactly at the moment, but you'll know it when you see it. At first, you'll probably find it all quite complicated. Save the image and copy the 'user guide' text, then print them out for continued study..,


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Dave Mitsky
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Re: Surface Brightness and Performance new [Re: GlennLeDrew]
      #5659892 - 02/03/13 12:28 PM

There are mentions of etendue, which is a measure of the flux gathering capability of an optical system, in relation to telescopes at these two sites:

http://pan-starrs.ifa.hawaii.edu/public/design-features/wide-field.html

http://www.physics.purdue.edu/research/features/lsst.shtml

"For many applications, the rate at which objects can be detected scales as the etendue of the telescope, which is defined as the product of its collecting area (A) times the field of view (Omega)."

"The ability of a telescope to survey large patches of the sky is given by its étendue. Étendue, the French word for extent, is defined mathematically as the product of the light collecting area and the field of view of the telescope. This quantity measures the number of photons per unit frequency per unit time a telescope will accept."

There's a mathematical treatment of the concept at http://www.physics.csbsju.edu/370/photometry/etendue.pdf

Dave Mitsky


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NGC7088
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Re: Surface Brightness and Performance [Re: Dave Mitsky]
      #5660848 - 02/03/13 10:40 PM

I found the chart. It is complicated. It seems to me to be absolutely more relevant to telescope setup than the site quoted at the beginning of this thread.
An example could help. In a polluted sky (say 18) suppose I wanted to see..say something simple..high in the sky right now, say M1 (Mag 8.4, SB 11, Size 8x4') or M78 (Mag 8, SB 12 size 8x6'). The telescope is 90mm. Another choice is 381mm. How can I use the chart to select the eyepiece/filter combination to produce the best contrast (or at least enough contrast). Right now I cannot see those from here, not in the binoculars, not in the small telescope, I have seen them in the big one in the past but did not set it up today. The sky is possibly worse than 18, to the naked eye it even has a red tinge to it...


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GlennLeDrew
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Re: Surface Brightness and Performance new [Re: NGC7088]
      #5661201 - 02/04/13 06:56 AM

Your surface brightness values in the two examples would seem to be in magnitudes per square arcminute. To convert to magnitudes per square arcsecond, add 8.89. Then, with your sky brightness, go back to the chart and determine the parameters for visibility.

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NGC7088
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Re: Surface Brightness and Performance new [Re: GlennLeDrew]
      #5661343 - 02/04/13 09:24 AM

Ok so M1, would be close to 20, and the sky would have to be darkened to 19 which means a 1.585 increase in magnification but the size has to be increased by 2.37, so if I call all that a 3x increase in magnification now the sky is roughly 20 and the size must now go to 26 but the 3x only got me to 24...I feel like I'm chasing my tail here... Is there a short cut?

Also, thinking about Mr. Ferris's 2 degrees, I don't have a telescope/eyepiece combination that even reach 2 degrees of FOV. How come I can see so many Dso's in that telescope?


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GlennLeDrew
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Re: Surface Brightness and Performance new [Re: NGC7088]
      #5661463 - 02/04/13 10:36 AM

The 2 degrees referred to is the angle subtended on your retina. If, for example, your eyepiece has a 50 degree apparent field, an object subtending 2 degrees on your retina would have an apparent diameter 1/25 the field of view.

Always bear in mind the difference between absolute and apparent angle.


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BillFerris
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Re: Surface Brightness and Performance new [Re: NGC7088]
      #5662238 - 02/04/13 06:13 PM

Quote:

Also, thinking about Mr. Ferris's 2 degrees, I don't have a telescope/eyepiece combination that even reach 2 degrees of FOV. How come I can see so many Dso's in that telescope?




Hi, I wasn't talking about the field of view of the eyepiece. I was talking about the apparent size of a celestial object when viewed through a telescope. For example, a 5 arcminute diameter galaxy observed at 24X magnification will appear in the eyepiece as (5X24=120, 120/60=2) 120 arcminutes across or 2 degrees in size. A really tiny 1 arcminute diameter galaxy observed at 120X will also appear 2 degrees in size at the eyepiece.

Let's consider a bright galaxy like NGC 5194. This is the main component of M51, the Whirlpool galaxy. NGC 5194 is roughly 11 arcminutes across in its largest dimension. It appears as a tiny patch of fuzz in an 8X finderscope. At that magnification, it appears nearly 1.5 degrees in size. When viewed through a telescope at 80X, the galaxy appears 10-times larger than when viewed through the finderscope. It's apparent size is just shy of 15 degrees. NGC 5194 is an obvious, extended object at this magnification. What's more, details within the galaxy also emerge. A brighter core, spiral structure, mottling in the disk. These smaller, finer details--details not seen in the low power finderscope--are now presented with enough scale that the dark adapted eye is able to discern them as having dimension.

Hope this helps.

Bill in Flag


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GlennLeDrew
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Re: Surface Brightness and Performance new [Re: NGC7088]
      #5662312 - 02/04/13 07:03 PM

Quote:

Ok so M1, would be close to 20, and the sky would have to be darkened to 19 which means a 1.585 increase in magnification but the size has to be increased by 2.37, so if I call all that a 3x increase in magnification now the sky is roughly 20 and the size must now go to 26 but the 3x only got me to 24...I feel like I'm chasing my tail here... Is there a short cut?




Your example of an object having a surface brightness of 20 MPSAS in a sky of surface brightness 19 MPSAS. On the chart, you look at the top row, which is for that 19 MPSAS sky. You then look for the circle having the 20 MPSAS number inside it. To the lower left of the circle is the number 19, which tells you that such an object must subtend 19 arcminutes on your retina in order to be detected. If its size is, say, 5 arcminutes, then it must be magnified just about 4X.

The math you need to perform is the magnification required (if necessary) to enlarge the object to the required size on your retina. Read the guide thoroughly for a fuller understanding, including what to do when your scope's exit pupil is smaller than your eye's pupil.


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NGC7088
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Re: Surface Brightness and Performance new [Re: GlennLeDrew]
      #5662501 - 02/04/13 09:16 PM

In the beginning all of us are taught a couple of simple telescopic relations regarding magnification, true field of view and apparent field of view. Lets call the last two:
T's and A's. The simplified equation for T is T=A/M where M is the magnification and A is given by the design of the eyepiece.
Now it turns out that there is a more important and basic relation regarding these same parameters. There seems to be a second set of T's and A's (or is T's or A's?). Well T's and A's are always welcome in my book, but it seems in practice the more T's and A's there are the harder it is to understand them. You would think the originals were enough, but no.
In the new (to me) relation I'm trying to "grab" here the description I'm getting here there is a new A (or can it be called a T?) perceived by the eye. Maybe it can be called an F, since it seems to be the final step after fondling around with all the original T's and A's?.
So F, being the final angular perception in the retina is the one that has to approach 2 degrees so that if one "gets lucky" enough to afford enough contrast then a faint object may be perceived is obtained thus:
F=Original size X magnification
The irony here is that if none of us were told anything "in the beginning" I'm sure all would have concluded this same relation by intuition. How much mumble jumble about T's and A's was necessary to confuse the beginner when at the end this simple relation ends up being the most important?
This and an understanding of variations in contrast. Which I'm avidly pursuing now.
And just wondering, the discoverers of the object had to use the same basic relation in reverse to list the size in the catalog... oh well.
The benefit here is a clear understanding that to get to F you look up the size listed in your catalog and using the T's and A's in your collection try and "get lucky" to obtain it. Or at least you may have enough range so that getting an additional eyepiece or Barlow will get there. Or you need a different telescope. The question remains, how to use the chart to insure the contrast, particularly in a light polluted skies possibly with the use of filters.
I'll get back to my examples in the next posts.
BTW I "nailed" M1 from my polluted backyard today with the binoculars. Did not set any telescopes. Not m78. I can also see some 6 magnitude stars naked eye but I know some 6xrs I did not see. So the sky is slightly better than yesterday, but still bright pink...
I'm too tired now but I'll be back to discuss how "lucky" I get with LeDrew's chart tomorrow.
Please excuse all the pun. Couldn't resist. If anyone state's they're offended I will not do it again. A little fun is always good in such a tough subject I think.


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NGC7088
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Re: Surface Brightness and Performance new [Re: NGC7088]
      #5662520 - 02/04/13 09:28 PM

P. S.
The sad part about it is that the F's are independent from the T's which we were all taught were very important for a "rich field" experience...


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GlennLeDrew
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Re: Surface Brightness and Performance new [Re: NGC7088]
      #5662619 - 02/04/13 10:24 PM

Note that, as I explained in the accompanying text, my chart is just another way to present graphically the information appearing in the RASC's Observer's Handbook, to which I recommend the interested investigator turn. My chart quantizes the data in 1-magnitude steps, and is intended more as a tool for understanding, not necessarily for planning. A graph of continuous curves, as appears in the Handbook, is a better aid for actual work.

My chart would be much more effective if it were possible to represent the range of threshold object size in true proportion. The impact of actually seeng just how much larger an angle a very dim and low contrast object must subtend would impress immediately and viscerally. But given the range of 10 to 1,000 arcminutes, the difference ratio of a factor of 100 is simply to large to accommodate.


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NGC7088
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Re: Surface Brightness and Performance new [Re: GlennLeDrew]
      #5663400 - 02/05/13 11:22 AM

After reading the instructions more carefully let me approach the M78 example:
SB=m+2.5log(2827ab)=almost 21
When the telescope is at 7mm exit pupil the magnification is about 13x. 13 times 8 ( the larger dimension of the object ) is 104 minutes. So I know the size. And assuming the sky is as bad as 18 and that this magnification has not dimmed it (because it's still 7mm exit pupil) the starting point is Sky 18 and object 21. A situation in which the critical size is not in the chart but falls in the second column where the object is dimmer by the sky by 3. The bottom scales indicate however that object should appear brighter than sky by .06m and a brightness ratio of 1.06 Greater than 1. I take it this means the object should be seen if the size is adequate.
Since I don't see it I'll try first to increase magnification to darken the sky.
So using a 25mm (20x) eyepiece reduces the pupil to 4.5 and dims the object and the sky by 1 magnitude.
The magnified size is now 20x8=160' and the position in the chart is 19,22. Still the same column, but now it's clear that the magnified size is larger than the critical size and if I don't see the object is completely my fault. (or some assumptions such as my eye pupil being 7mm to begin with or that the sky was 18 if it was really closer to 17). Yet it's so close that a filter should do the trick.

How did I do?


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GlennLeDrew
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Re: Surface Brightness and Performance new [Re: NGC7088]
      #5663471 - 02/05/13 12:04 PM

You did well! Note that the new magnified size of 160' is hardly more than the threshold size of 155'. You are attempting to operate awfully close to the edge of detection. An object only some 6% brighter than the surrounding sky can make for a difficult detection for a not experienced observer. Brighter by about 10% is a more realistic threshold if a reasonably certain detection is desired.

All DSOs at all times will have a brightness relative to the sky of greater than unity. Even during the day! But the brightness ratio may be something like 1.00000000000001. This is because the light of the object and the sky glow always *add* together, resulting in the object appearing brighter than the surrounding sky, however slightly so. It's all about minimizing sky glow in order to increase contrast.


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NGC7088
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Re: Surface Brightness and Performance new [Re: GlennLeDrew]
      #5664262 - 02/05/13 08:15 PM

The only way to move to the right is filters?
oh sorry I know, travel to a better site or use larger aperture of course. Any other way?

Edited by NGC7088 (02/05/13 08:20 PM)


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GlennLeDrew
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Re: Surface Brightness and Performance new [Re: NGC7088]
      #5664413 - 02/05/13 10:09 PM

As always, a darker sky improves the view. For suitable objects, filters definitely help.

A larger aperture does not improve contrast; it only permits to see smaller objects and finer details.


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BillFerris
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Re: Surface Brightness and Performance new [Re: NGC7088]
      #5664549 - 02/06/13 01:07 AM

Quote:

The only way to move to the right is filters?
oh sorry I know, travel to a better site or use larger aperture of course. Any other way?




Filters offer the advantage of actually improving the contrast of a celestial object versus the sky. Unfortunately, galaxies typically enjoy only modest enhancement from broadband filters. Relocating to do your observing under a darker night sky also enhances the contrast of celestial objects. Unfortunately, the night sky only gets so dark. Hopefully, you live within a reasonable drive time of a true dark sky site. If so, take advantage of it.

Aperture also allows for the observation of lower contrast objects but not in the same way as a filter or moving to a darker site. Choose any size you wish and find a galaxy of that size which lies at the very threshold of visibility in a given aperture. In a larger aperture under the same sky, a galaxy of the same size but lower surface brightness (i.e. lower contrast) will be visible. Increasing aperture doesn't improve the contrast of a galaxy versus the sky but it does lower the threshold contrast at which galaxies become observable.

Bill in Flag


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NGC7088
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Re: Surface Brightness and Performance new [Re: BillFerris]
      #5664713 - 02/06/13 06:53 AM

In order to evaluate in how this approach is different from the one suggested in the link at the beginning of the thread I note the following:
1. The limiting minimum magnification is still at or about 7mm. But there is some flexibility in case someone is aware their own personal eye may run from 8 to 5mm starting point.
2. The limiting maximum magnification is obtained as combination of how "dark" the sky can get starting from what nature has to offer in terms of darkness and how much it can be practically altered with magnification and /or filters.
3. This is also modified with a consideration of threshold magnified object size aiming to conspire with the above conditions to maximize contrast.

In terms of a "how to run a telescope" point of view it looks like a good idea to have eyepieces focal lengths such that can run from minimum magnification to maximum darkness (26 ?) in steps roughly equivalent to -1 magnitude at a time.

In the M78 example increasing magnification further will not help as the required size grows faster than the magnified size. That suggest that a "per aperture" set of graphs could be prepared to show the expectancy of seeing objects of a given minimum magnitude. It would have to be also a "per size range" family of graphs. Don't know really if they would be more useful than the chart as it is.


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NGC7088
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Re: Surface Brightness and Performance new [Re: NGC7088]
      #5664725 - 02/06/13 07:11 AM

Looking around the web it's clear that others have noticed the value of the "contrast" approach.
At http://web.telia.com/~u41105032/ Nils Olof Carlin has three articles about it. The second one:
http://web.telia.com/~u41105032/visual/blackwel.htm
Is the one dealing with extended objects. He seems to disagree about Clark's interpretation in what appears to be the threshold size. To simplify into a rule of thumb Carling suggest, quote:
"To detect a faint object, you can increase magnification till the sky is so dark that you have difficulty seeing the field stop, or till the object has an apparent size of 1 degree, whichever comes first."
I think at this point Mr. Ferris would have said: "2 degrees"
Does Mr. Carlin case hold water?
Looking at the M78 example would suggest not. Anyone care to comment?
Looks like the "contrast approach" is still well in development stage?


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Tony Flanders
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Re: Surface Brightness and Performance new [Re: NGC7088]
      #5664770 - 02/06/13 08:06 AM

Quote:

Nils Olof Carlin ... seems to disagree about Clark's interpretation in what appears to be the threshold size.




My advice is: Don't fret about it. One degree, two degrees -- who's to say? How big is a galaxy, anyway? M33's cataloged size (down to the mag-25 isophote) is 70' x 40'. I doubt that anybody has ever seen that much of the galaxy visually, however. The part that actually makes an impact -- which has surface brightness much higher than that entire 70' x 40' ellipse -- is no more than half that size.

No celestial object is a circle, no celestial object has uniform surface brightness, and no celestial object obeys well-defined rules.

The take-home lesson is that the optimal magnification for viewing deep-sky objects is often surprisingly high. So in each and every case, experiment until you find what's right for you.


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