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Increased aperture Vs Perceived Brightness

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#1 Abhat

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Posted 18 August 2014 - 11:19 AM

Couple of days ago I had my XT6i out instead of Z8 to take a quick peak at nebulae (Lagoon & Swan) in Sagittarius.

 

A few days before that I had seen the same nebulae through Z8. To my surprise the nebulae did not look that different in XT6i. The seeing conditions were similar. The magnification was 80X. I was using DGM NPB Nebula filter in both cases. I had my expectations low as 6" gathers lot less light than 8". However my eyes did not see the difference that way. I was pleased with what I was seeing through a 6" scope.

 

The question is, does the increase/decrease in aperture creates proportionate difference at the eyepiece or your eyes adapt and the difference is not as significant as the math behind it.



#2 cpper

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Posted 18 August 2014 - 12:10 PM

You said the seeing conditions were similar, how about the transparency ?

What eyepiece were you using with the Z8 and what with the XT6i ?


Edited by cpper, 18 August 2014 - 12:11 PM.


#3 Fuzzyguy

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Posted 18 August 2014 - 12:27 PM

I recently purchased an 80 ED refractor and I was pleased with the brightness. I was also expecting things to be less bright in the 3" vs my 8" SCT, but that was not the case. Of coarse I was looking at large objects as opposed to small fainter DSO's, but the North America and Veil nebulas were very evident in the little scope. Exit pupil was large though (~4mm) with the 24mm EP and that may have had as much to do with it as anything else.



#4 GOLGO13

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Posted 18 August 2014 - 12:55 PM

A 6 and 8 inch are fairly close in aperture. However, I'm guessing if you did the comparison at the exact same time you'd see the difference. However, the jump from 6 to 10 would be much more noticeable.

 

I had a somewhat similar experience comparing my C6 to a Meade 8 inch. I felt my image was a bit sharper in the C6...but the C8 was just a bit brighter, but not by much. I suspect my C6 was working better for some reason...could have been collimation, cooldown, conditions, etc.

 

Same as when I compared my 4 inch refractor to my XT6 I had (donated it recently to a friend who is lucky he just moved to dark skies). The 4 inch refractor was sharper but the XT6 was just a bit brighter. Not by a large amount but certainly there. If mags are the same then it's simple exit pupil (depending on the object) or light gathering if stars. The XT6 was very good though...and a whole heck of a lot cheaper.

 

I find it's a few steps higher where you really see the difference. Going from 6 to 10 inch...10 to 14...etc.



#5 GlennLeDrew

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Posted 18 August 2014 - 01:06 PM

Abhat,

You've just made an important discovery. Image surface brightness scales as the area of the exit pupil. A 2" aperture or a 20" aperture working at the same exit pupil present images of sky and other extended objects at the same surface brightness. The 10X larger aperture merely provides a 10X larger image. (And stars 100 times fainter, of course, because the light collecting area is 100X greater.)

 

Furthermore, no telescope can deliver surface brightness higher than can be seen by the unaided eye. Indeed, telescopic surface brightness is always a little less due to non-perfect transmission of light.

 

If the exit pupil remains fixed, extended objects in the night sky *appear* to have higher surface brightness as aperture  increases only because of the increase in area occupied on the retina. This makes for higher total, or integrated brightness, and more importantly more resolvable detail (information.)

 

Lastly, even the ideal telescope can never improve contrast; real telescooes always degrade contrast. The best we can ever hope for is the preservation of contrast. And so boosting magnification does not 'darken the sky and thereby improve contrast.' Sky and extended object are dimmed equally as magnification increases, and contrast remains constant. The increase in image scale delivers more information, which is *perceived* as a gain in contrast.


Edited by GlennLeDrew, 18 August 2014 - 01:07 PM.


#6 Tony Flanders

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Posted 18 August 2014 - 01:52 PM

Even the ideal telescope can never improve contrast; real telescooes always degrade contrast. The best we can ever hope for is the preservation of contrast. And so boosting magnification does not 'darken the sky and thereby improve contrast.' Sky and extended object are dimmed equally as magnification increases, and contrast remains constant. The increase in image scale delivers more information, which is *perceived* as a gain in contrast.


I guess I would have to quarrel with the use of the word "contrast" here. It is true that if you define contrast as the ratio of brightness between your target and the background, then telescopes cannot improve contrast.

But who says that's the proper definition of "contrast?" To my mind, what really matters is perceived contrast, not this artificial definition which cannot in practice be measured without fancy instruments. And telescopes most definitely can and do increase perceived contrast.

#7 Tony Flanders

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Posted 18 August 2014 - 01:55 PM

A few days before that I had seen the same nebulae through Z8. To my surprise the nebulae did not look that different in XT6i.


I have a theory about this. I find that perceptions of objects don't increase continuously as you increase aperture and/or magnification; instead, they progress by quantum jumps.

So, for instance, Saturn's rings look pretty much the same at 20X, 40X, 60X, and so on ... until all of a suddent you see Cassini's Division, at which point they look totally different.

Likewise, M51 looks pretty much the same in apertures of 3 inches, 4 inches, 6 inches, and so on ... until all of a suddent you see the spiral arms, and then it looks totally different.

#8 GOLGO13

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Posted 18 August 2014 - 02:11 PM

interesting concept Tony. Maybe looking at M13 at 200x would really show that difference between a 6 inch newt and an 8 inch newt.

 

That's usually when I see the big differences between aperture.



#9 GlennLeDrew

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Posted 18 August 2014 - 02:31 PM

Tony,

First we need to understand how the instrument works in order that we can differentiate between it and and the operration of the observer's visual system. Too often performance is ascribed to the instrument where it's really the interplay between specific conditions of the object under observation and its image as processed in the visual cortex which is responsible.

 

Perceived contrast changes depend sensitively on object size, brightness and contrast. Under some conditions the changes are rather profound, under others hardly notable. But underneath all this variability imposed by the organic visual system the instrument remains a constant, predictable factor as dictated by the laws of optics.



#10 Abhat

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Posted 18 August 2014 - 03:18 PM

interesting concept Tony. Maybe looking at M13 at 200x would really show that difference between a 6 inch newt and an 8 inch newt.

 

That's usually when I see the big differences between aperture.

 

That's a good point. I think higher magnifications is where the increased aperture comes in handy. For certain DSO and Planetary Details that magnification can make a big difference in seeing critical details. I have always been able to see M13's propeller in  Z8 especially at higher magnifications but in XT6i I do struggle to see it.



#11 Starman1

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Posted 18 August 2014 - 03:54 PM

If we're talking about a profound difference in the visual appearance of an object in a scope, I go by the "1 Magnitude Rule".

I see a profoundly different view of an object if I change the aperture enough to gain a full magnitude.

That's 6" to 10" to 16" to 25"

Or, 5" to 8" to 12.5" to 20" to 32"

I find it interesting that these progressions also conform to a lot of common sizes for telescopes.

Sure, of course any increase in aperture will make some difference.

But if you want a profound difference, make it a magnitude gain.

Or more.



#12 Pinbout

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Posted 18 August 2014 - 10:33 PM

 

The increase in image scale delivers more information, which is *perceived* as a gain in contrast.

 

 

its not gain in contrast.

 

the contrast slope becomes slower, wider. the ration between white and black is still whatever, but the slope between them is lengthened.

 

the MTF isnt the tool to express this. you need a different chart like gama or average gradient to express the longer slower slope that holds the increase in information.

.

.

.

and when you go from a large telescope to a smaller telescope you speed up, or compress the contrast slope so it looses information.



#13 Jon Isaacs

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Posted 19 August 2014 - 12:07 AM

 

Even the ideal telescope can never improve contrast; real telescooes always degrade contrast. The best we can ever hope for is the preservation of contrast. And so boosting magnification does not 'darken the sky and thereby improve contrast.' Sky and extended object are dimmed equally as magnification increases, and contrast remains constant. The increase in image scale delivers more information, which is *perceived* as a gain in contrast.


I guess I would have to quarrel with the use of the word "contrast" here. It is true that if you define contrast as the ratio of brightness between your target and the background, then telescopes cannot improve contrast.

But who says that's the proper definition of "contrast?" To my mind, what really matters is perceived contrast, not this artificial definition which cannot in practice be measured without fancy instruments. And telescopes most definitely can and do increase perceived contrast.

 

 

Contrast does have a specific meaning and not an artificial definition, the ratio of two brightnesses.  If you want to use some phrase such as "perceived contrast", that is reasonable but you really can't have it both ways because "perceived contrast" is still contrast, the ratio of two brightnesses.. But difference is, rather than being the ratio of two brightnesses, it's the observers interpretation of what that must look like.  But in reality, when you increase magnification, the reason it is more easily perceived is not because the actual contrast has changed but rather because of the larger image size on the retina covering more rods. 

 

To my mind what really matters in understanding what you are seeing and why you are seeing it.  It's worth understanding that increasing the magnification may make an object more easily seen because the image covers more of the retina. With understanding can come reinterpretation of ones perceptions so they more accurately reflect what one is actually seeing. 

 

Personally it took me many years to understand the concept of contrast because it was misused so often, there's just a lot of confusion.  Getting the basics straight from the beginning, that's the way to keep simple concepts simple.

 

Jon



#14 havasman

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Posted 19 August 2014 - 02:07 AM

With understanding can come reinterpretation of ones perceptions

That pretty much covers a lot of ground very elegantly.


Edited by havasman, 19 August 2014 - 02:10 AM.


#15 GlennLeDrew

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Posted 19 August 2014 - 04:45 AM

Danny,

In what respect does the 'contrast slope' vary with aperture? Do you mean spatially, as related to image scale on the retina? If so, how would a sharp-edged disk of uniform surface brightness be considered? At exit pupils larger than about 1mm, all views are of a sharp transition from darker sky to brighter disk, and so there is no 'slope' in the brightness transition. Yet the perceived contrast can vary as its size is manipulated via magnification/aperture change.



#16 Illinois

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Posted 19 August 2014 - 07:20 AM

I compare to my 6 inch and 10 inch. Nebula and galaxies are noticeable that 10 inch is better. I sold my 10 inch and bought 16 inch!



#17 cpper

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Posted 19 August 2014 - 08:59 AM

I read some time ago this article : http://starizona.com...ing_theory.aspx.  Very nice and informative, I learned a lot. An on topic paragraph:

 

"A more complex theory for the best deep-sky magnification is based on the human eye's response to contrast.  The contrast between the sky background and a faint celestial object is critical for observation.  An object whose surface brightness is such that the contrast between it and the sky is below the eye's detection threshold will be invisible.  Observers have noted that low-contrast objects are often easier to detect at higher magnifications.  It is often assumed that this is due to higher power increasing contrast, but this is not true.  The relative contrast between the object and the sky background is unchanged by magnification (each is affected equally).  However, the eye is more sensitive to low-contrast objects when they appear larger."



#18 Pinbout

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Posted 19 August 2014 - 09:03 AM

 

If so, how would a sharp-edged disk of uniform surface brightness be considered?

 

 

that's a demonstration of visual acuity not contrast sensitivity.


Edited by Pinbout, 19 August 2014 - 01:24 PM.


#19 Chuck Hards

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Posted 19 August 2014 - 09:20 AM

Couple of days ago I had my XT6i out instead of Z8 to take a quick peak at nebulae (Lagoon & Swan) in Sagittarius.

 

A few days before that I had seen the same nebulae through Z8. To my surprise the nebulae did not look that different in XT6i. The seeing conditions were similar. The magnification was 80X. I was using DGM NPB Nebula filter in both cases. I had my expectations low as 6" gathers lot less light than 8". However my eyes did not see the difference that way. I was pleased with what I was seeing through a 6" scope.

 

The question is, does the increase/decrease in aperture creates proportionate difference at the eyepiece or your eyes adapt and the difference is not as significant as the math behind it.

 

 

Coincidentally, I was looking at both the Lagoon and Swan nebulae on Saturday night with a 70" Dob.  You may be surprised to hear that the images were not that much brighter than with our club's 32" Cassegrain, with a similar focal-length eyepiece.  The 70" has over 100" longer effective focal length than the 32", so the image was larger as the light was spread out over a larger area, but was still bright due to the increased aperture.  There was more detail visible in the 70", however.  A Lumicon broadband filter was used to combat some light pollution and the increased contrast made the image more pleasing to the eye, but I don't know that more detail was seen with the filter.



#20 Jon Isaacs

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Posted 19 August 2014 - 09:20 AM

I compare to my 6 inch and 10 inch. Nebula and galaxies are noticeable that 10 inch is better. I sold my 10 inch and bought 16 inch!

 

Small nebulae and galaxies show more detail in a larger scope for the reasons discussed above, the combination of brightness and magnification. A larger scope,  within the limits of the entrance pupil of the eye, (eye's pupil at at least as large as the exit pupil),  the image can be brighter at the same magnification, the image can be larger at the same image brightness or some combination of the two. But for large nebulae, they may be more easily seen in a small scope. 

 

A couple of definitions:

 

Surface brightness: Light per unit area, intensity.. .Stars and extended objects are given visual magnitudes, this is the total amount of light from the object or star. For a star, this is a useful number because a star is a point.  For an extended object like an planet, galaxy or nebulae, isn't so useful since the light does not come from a single point, it comes from an area of the sky, the larger the size of the object, the more the light is spread out and the less intense the light is coming from that object. The Andromeda galaxy has a visual magnitude of 3.4 but that light is spread out over an area of about 3 degrees x 1 degrees, clearly that is much more difficult to see than a magnitude 3.4 star.  The units of surface brightness are magnitudes per square arc-minute or magnitudes per square arc-second.  Andromeda has a surface brightness of about 13.5 magnitudes per square arc-minutes.  The surface brightness is a much better measure of how easily seen an object will be.  

 

Exit pupil:  The beam of light leaving the eyepiece, what you observe.   The exit pupil can be calculated a couple of ways, I think the most intuitive is the aperture divided by the magnification. The greater the magnification, the smaller that beam will be. The brightness of an extended object depends only on the size of the exit pupil, the smaller the exit pupil, the dimmer both the object and the night sky will be.  This makes intuitive sense because the smaller the exit pupil represents less light entering your eye. At the higher magnification, the object is larger, the same amount of light is spread out over a larger area, it must be dimmer.. Since the amount of light is related to the area of the exit pupil, the relative brightness of two images is the ratio of the square of the exit pupils.  A 200 mm scope at 200x produces a 1mm exit pupil, a 200mm scope at 100x produces a 2mm exit pupil, the image of an extended object like a galaxy is 4 times brighter..  

 

If the exit pupil is larger than the observers dilated eye, the observers entrance pupil, then that added light does no good, the light does not enter the eye.  The size of your dilated eye is an individual thing, generally as one grows older, the eye becomes less flexible and is unable to open as far as it does when one is young. Typically a maximum entrance pupil, the observer's dilate eye, is assumed to be 7mm but it varies between about 5mm and 8mm.. You can measure it.. 

 

Contrast: Contrast is the ratio of two brightnesses.  In deep sky observing, one mostly thinks of contrast as the brightness of the object compared to the brightness of the background sky but it can also be used to compare different parts of an object.  A good example of this are the cloud bands on Jupiter, they are bright but they are relatively low contrast, their brightnesses are quite similar. 

 

Since the brightness of an extended object is directly related to the size of the exit pupil, increasing or decreasing the magnification does not change the contrast since both the object and the background sky are changed in proportion.  A telescope cannot increase the contrast of an extended object.. 

 

The contrast of a star against the background sky is a different issue. A star is a point source of light, when you magnify it (up to about 25X/inch) it does not dim, it's still just a point. The background sky does become dimmer, you are spreading the light out over a larger area..  With the brightness of the star unchanged but the background sky dimmer, the ratio of the two brightnesses is increased and the contrast of the star against the background sky is increased.  This is useful to know, increasing the magnification does allow one to see stars more easily, to see faint stars that could not otherwise be seen. 

 

Another conclusion:

 

The surface brightness of the object is proportional to the exit pupil, this is independent of the aperture.  Since the largest possible exit pupil is the size of your dilated eye, the surface brightness of an image can be not brighter than it is naked eye.. Telescopes magnify objects but they do not make the brighter.

 

Looking at an extended object, this certainly can be counter-intuitive. Saturday night I was enjoying the Veil nebula in my my 4 inch scope and my 25 inch, the exit pupils were both about 6mm.. the views were quite different, with the big scope, I was looking at a small portion of the Veil, a patch about 0.7 degrees in diameter, object was large and details unseen in the smaller scope were quite apparent.  In the small scope, the entire nebulae was seen and nicely framed.  It was much smaller.. At first, it seems like the image in the large scope is much more intense but looking back and forth between the two, it is possible to realize that they are equally bright, one is just larger. 

 

In some situations, that larger image scale can actually decrease the contrast, make an object more difficult to see, the object can overflow the field of of view and one has no background sky for comparison..  

 

There is lots to know.. I believe that if one understands basic concepts and how they affect the image, then one can make wiser choices in both purchasing telescopes and eyepieces as well as what to use in a particular situation.  Knowing that increasing the magnification does increase the contrast of a star against an extended object suggests that if one wants to see a super nova in a galaxy or the central star in a planetary nebulae, increasing the magnification is a very powerful tool..

 

Some stuff to consider, I hope this helps those just starting out in this wonderful hobby understand a bit more about their equipment and what they are seeing.

 

Jon Isaacs



#21 Pinbout

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Posted 19 August 2014 - 01:23 PM

http://www.emory.edu...maps/Filcon.jpg

 

its known:

aperture determines the airy disc size which determines the maximum spatial frequency.

 

I would say, similar to film, aperture sizes have different contrast slopes [latitude] because the higher resolution of larger apertures allow for smaller spatial frequencies, cycles per degree,  the  contrast latitude determines the slope of the average gradient seen.

 

wider latitude = more gradients = slower slope = more detail.

 

again you can't use MFT because  the max spatial frequency is constrained in the relative unit 1. which is why you can't use MFT to compare contrast of different apertures.

 

 

oh look at this...it really doesn't have anything to do with this but its fun to move the bar and see the graphic move... :grin:

 

http://zeiss-campus....indexflash.html


Edited by Pinbout, 19 August 2014 - 01:23 PM.


#22 GlennLeDrew

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Posted 19 August 2014 - 04:07 PM

The sharp-edged disk I referred to is assumed to be resolved, subtending anywhere from a few arcminutes up to several or even tens of degrees. It is simply a hypothetical DSO having uniform surface brightness and an abrupt edge. Its graphed brightness profile would present as a flat-topped plateau with vertical sides. If of similar brightness/contrast as a typical galaxy's outer parts, it would exhibit similar changes in perceived contrast as for a non sharp-edged object.

 

Numerous nebulae at least in parts rather emulate a reasonably uniform surface brightness with fairly abrupt cut-off at the edge. Examples include NGC 281, NGC 2175, the California and the North America.


Edited by GlennLeDrew, 19 August 2014 - 04:11 PM.


#23 Pinbout

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Posted 19 August 2014 - 07:22 PM

Ok, take the limit as we approach 100% contrast.

 

Light is a wave,  sinusoidal,  not square wave. There are no picket fences, only gradations.

 

Come on, you didnt even try the Zeiss interactive...I thought it was fun.  :p


Edited by Pinbout, 20 August 2014 - 04:59 AM.


#24 Jon Isaacs

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Posted 19 August 2014 - 09:01 PM

 

 

If so, how would a sharp-edged disk of uniform surface brightness be considered?

 

 

that's a demonstration of visual acuity not contrast sensitivity.

 

 

For dim objects, DSOs.. the dark adapted eye has very poor visual acuity. The contrast differences that depend on aperture are on a very fine scale, much finer than the eye can see at low light levels. One only has to look at an image of a deepsky object taken with an 80mm refractor to realize just how much detail is there to be seen that the eye does not capture.  

 

An interesting demonstration of the poor acuity of the human eye at low light levels is to view the moon though a standard 100,000:1 solar filter. This reduces the surface brightness of the moon from that familiar source of light pollution we well know to an object with a surface brightness that corresponds to a relatively bright DSO..  The details are still there to be seen but at those low light levels, the moon looks like a blob..

 

Jon



#25 Pinbout

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Posted 20 August 2014 - 05:12 AM

The eye is very sensitive to contrst changes. Not very sensitive to absolute luminance.

 

And that 80mm scope is modulating the light waves it receives. It is lowering the amplitude and compressing the contrast of the object.

 

 

you can't use MTF to compare contrast of different apertures cause the spatial frequency unit on the x-axis is a % of that specific apertures max spatial frequency, which is defined by the size of the airy disc.

 

if you change the x-axis to cycles per degree instead of percentages you could compare contrasts with different apertures, and you'll see the larger aperatures have a longer contrast curve.

 

I only modfied the MTF because a lack of a better tool.


Edited by Pinbout, 20 August 2014 - 06:47 AM.



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