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Equipment Discussions >> Reflectors

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jpcannavo
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Reged: 02/21/05

Loc: Long Island New York
Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5916676 - 06/12/13 10:40 AM

I think there are some muddles here.

Contrast sensitivity, or its inverse: contrast threshold (1/contrast sensitivity), is a function of the various properties of the real image on the retina and the retina itself. Now, wrt the issue here, the relationship between CS (i.e. contrast sensitivity) and spatial frequency (where high spatial frequencies = small angular visual detail, and low spatial frequencies =large angular visual detail ) yields a function.

As it turns out, the CS function reaches a maximum at about 6 cycles per degree. In other words, the ability to detect small low contrast features increases with increasing apparent angular target size - or perhaps more precisely, real image angular size on the retina - up to about 10 minutes of arc or so.

Now this function is determined by various neurophysiological processing (spatial summation etc) by the retina and nervous system. And, subject to that function, CS for a potential visual detail is determined by its angular extent at the retina (or equivalently apparent visual angular extent).

Now realize that the image on the retina "knows nothing" (if you will) about the aperture, exit pupil or other manner of its origin. To say, then, that exit pupil or aperture changes contrast threshold is, I think, misleading. I prefer to think of the issue here as follows, and as a consequence see no mystery or confusion.

Consider two scopes one 2" one 20" both observing the same galaxy, and both at 10x per inch. i.e. 20x and 200x respectively. Now the luminance (surface brightness) of the various features of the real image of the galaxy on the retina is identical in each telescope. And, the luminance of the retinal real image of the background sky is identical for both scopes. So contrast parameters of both retinal real images are identical. But, the image provided by the 20" scope is 10x larger. Now realize that aperture has not changed the above CS function at the retina (nor would exit pupil). The CS function is physiological fixed. Moreover, aperture has not changed the CS at any given region on the retina. The CS of any given retinal region is still a function of the spatial frequency of detail on that region. (And the same would hold if we varied exit pupil, CS would still only be a function of spatial frequency on that region). This is why I feel speaking of aperture or exit pupil changing CS is misleading.

Now if we assume that the actual visual target - the galaxy - has a relatively smooth gradation of (nearly) ever increasing fine detail, as any good astrophoto shows it does, then the 20" scopes increase of image scale by a factor of 10 has now brought detail 10 times as small into the visual detection range, where detection here is a function of contrast sensitivity. And for a given area of the 2 dimensional extended object (the galaxy) we could argue that 10x10 =100 times more detail is visible in the 20" scope. (for those more familiar with the topic at hand, i.e. HR Blackwell 1946, I am admittedly ignoring here the falloff for lower spatial frequencies)

This is how I understand aperture increasing visual detail. As for mystery, the only one here is the CS vs. spatial frequency function itself. And the answers to that mystery take us far afield, and into the realm of visual neurophysiology.

Hope this helps.

Joe


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auriga
Pooh-Bah


Reged: 03/02/06

Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5917237 - 06/12/13 03:41 PM

Hi,
Recently I saw a post in this forum that states the issue as follows (paraphrased by me, hopefully accurately):

For extended objects: the brightness of an image of an extended object depends entirely on the size of exit pupil.

But for a given exit pupil, say 5mm, a larger aperture scope will give a higher magnification and therefore a larger image.

The eye discerns larger images better than smaller images.

Therefore for a given exit pupil, say 5mm, more will be seen in the larger aperture, since the magnification is higher, and the eye sees larger images better, even though the brightness of image of the object remains the same since the exit pupil remains the same.

On the other hand, if the magnification in the two telescopes is made equal, the larger aperture will show more since its exit pupil will be larger at equal magnification than the exit pupil in the smaller scope, and so the image of the object will be brighter.

At least that is how I understand it so far. Let me know if this is off base.

Bill


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Big dob for objects with structure and detail? new [Re: jpcannavo]
      #5917274 - 06/12/13 04:09 PM

Quote:

I think there are some muddles here.

Consider two scopes one 2" one 20" both observing the same galaxy, and both at 10x per inch. i.e. 20x and 200x respectively. Now the luminance (surface brightness) of the various features of the real image of the galaxy on the retina is identical in each telescope. And, the luminance of the retinal real image of the background sky is identical for both scopes. So contrast parameters of both retinal real images are identical. But, the image provided by the 20" scope is 10x larger. Now realize that aperture has not changed the above CS function at the retina (nor would exit pupil). The CS function is physiological fixed. Moreover, aperture has not changed the CS at any given region on the retina. The CS of any given retinal region is still a function of the spatial frequency of detail on that region. (And the same would hold if we varied exit pupil, CS would still only be a function of spatial frequency on that region). This is why I feel speaking of aperture or exit pupil changing CS is misleading.

Now if we assume that the actual visual target - the galaxy - has a relatively smooth gradation of (nearly) ever increasing fine detail, as any good astrophoto shows it does, then the 20" scopes increase of image scale by a factor of 10 has now brought detail 10 times as small onto the rising (positive slope) portion of the CS vs image scale function. In other words, fine detail 10 times as small now reaches the visual detection threshold as limited by contrast differences. And for a given area of the 2 dimensional extended object (the galaxy) we could argue that 10x10 =100 times more detail is visible in the 20" scope. (for those more familiar with the topic at hand, i.e. HR Blackwell 1946, I am admittedly ignoring here the falloff for lower spatial frequencies)

This is how I understand aperture increasing visual detail. As for mystery, the only one here is the CS vs. spatial frequency function itself. And the answers to that mystery take us far afield, and into the realm of visual neurophysiology.

Hope this helps.

Joe




Joe,

I like reading your posts and until now I haven't disagreed with anything you've written, but the statement I bolded above doesn't seem right to me. And if it isn't right, the logic that follows isn't either.

If the luminance of the image of the galaxy on the retina is identical in each telescope, it should also be unchanged using only the naked eye. If this were so, you could look at the sun through a telescope and suffer no ill effects given you only look for a few seconds, just like you can do so with your naked eye. Who is willing to back up this theorom by trying this???

Rob


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robininni
scholastic sledgehammer


Reged: 04/18/11

Loc: Stephenville, TX
Re: Big dob for objects with structure and detail? new [Re: auriga]
      #5917279 - 06/12/13 04:12 PM

Quote:

Hi,
I have never understood these explanations. My initial assumption was that my failure to understand it is because I am stupid, a belief no doubt widely shared by many.

Recently another possibility has occurred to me: the explanations are correct but are stated in such a way as to be clear only to those who already understand the issues.

As a writer I can tell you that explaining something complex in a simple and lucid way can be very difficult and often requires an unusual turn of mind that can isolate the main issue.

Recently I saw a post in this forum by a Jon Isaacs who does have this turn of mind.

Jon states the issue as follows (paraphrased by me, hopefully accurately): the brightness of an image of an object depends entirely on the size of exit pupil.

But for a given exit pupil, say 5mm, a larger aperture scope will give a higher magnification and therefore a larger image.

The eye discerns larger images better than smaller images.

Therefore for a given exit pupil, say 5mm, in a larger aperture the brightness of the object will say the same as in a smaller aperture at that exit pupil, but more will be seen in the larger aperture since the magnification is higher and the eye sees larger images better.

On the other hand, if the magnification in the two telescopes is made equal, the larger aperture will show more since its exit pupil will be larger at equal magnification than the exit pupil in the smaller scope.

At least that is how I understand it so far. Let me know if this is off base.

Bill




Bill, what John said is exactly true. This is also exactly what Glenn stated earlier in this thread (although not word for word). This is the only correct explanation of this subject. End of story.

Rob


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BillFerris
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Re: Big dob for objects with structure and detail? new [Re: jpcannavo]
      #5917363 - 06/12/13 05:08 PM

Quote:

I hate retyping: an earlier post is somewhat informative here: Aperture and extended detail




Here's an excerpt from the above which can serve as a good jumping off point for discussion...

Quote:

as you get closer to the picture in the dimmed room (for us older folks we assume well corrected vision!), the brightness of the object does not change, yet detectable visual detail clearly increases! (one can argue in fact that the amount of visible detail increases with the square of aperture). If you try this, you will appreciate what aperture does for deep sky observing. You can also simulate the expected gain with different aperture jumps. Of course, you can just get out there and look through a bunch of scopes! But experiments like this are fun nonetheless.




The statement, "as you get closer to the picture in the dimmed room ... the brightness of the object does not change," may be correct for surface brightness but is incorrect when we consider apparent total brightness. This is something most everybody ignores--with one notable exception --in these discussions. In order for object surface brightness to remain constant as apparent size increases, the object's apparent integrated magnitude must increase. In short, all objects become brighter as aperture increases.

Since so many amateurs use the terms, surface brightness and brightness interchangeably, I find it helpful to clearly identify this increase in brightness as an increase in apparent integrated--or apparent total--magnitude. This change in brightness is directly the result of the larger or more intense light packet being delivered to the eye by the larger aperture. If one thinks of light as information, you can describe this as more information being delivered to the eye. If you think about it in terms of human visual performance, that larger light packet allows the observer to detect more subtle contrasts.

This is critical to understanding visual observing. If one defines object contrast as the ratio of surface brightness of an extended object to the surface brightness of the surrounding field, then object contrast remains fixed in all apertures and at all magnifications. As a result, object contrast remains constant. Increasing aperture cannot improve object contrast. This can be discouraging when one considers the reality that the dark adapted eye is primarily a detector of contrasts. Its resolving power is atrocious. In a dark enough environment, color is impossible to detect. Our biggest asset when using a telescope at high power under a dark sky is our ability to detect faint light sources. The fully dark adapted eye is thousands of times more sensitive to light. As deep sky observers, we need to leverage this to detect subtle changes in contrast.

We cannot use aperture or magnification to change the contrast of an object versus the surrounding sky. However, by increasing aperture, we do become better at detecting subtle contrasts. In other words, threshold contrast is lowered. Pick any size for a galaxy...1', 5', 10', whatever. The faintest galaxy of that size visible in an 8 inch scope will be brighter than the faintest galaxy of the same size visible in a 16 inch scope. And since the two galaxies are of the same size, the galaxy with the fainter integrated magnitude will also have a lower surface brightness and, by extension, be a lower contrast object. Threshold contrast has been lowered. This is just one of many scenarios in which one can apply the concept of threshold contrast to better understand why we see what we see. This relationship between aperture and threshold contrast not only explains how we're able to see fainter galaxies but also why the same galaxy observed in two different apertures appears more obvious and impressive in the larger scope. Again, that galaxy's contrast versus the surrounding sky will be identical in both scopes. However, it will be farther above the threshold of visibility in the larger aperture. In this manner without changing object contrast, we can explain and understand how a faint galaxy emerges to the eye and how bright galaxy's appear more impressive.

It also provides a framework for understanding the mistaken perception embraced by visual observers for many years that increasing aperture literally improved object contrast. Prior generations of observers made the very natural and straight-line inference that, if an object looks more contrasty or obvious to the eye, then it must be more contrasty. Of course, that was a mistaken conclusion. The galaxy doesn't display greater contrast versus the surrounding sky when viewed in a bigger scope. However, it does have greater separation from the threshold of visibility and this creates an impression that contrast has improved.

It's been said that discussions of threshold contrast and the role it plays in the observation of faint extended objects does not take into account the improved resolution offered by a larger aperture. On the contrary, I would say that the tangible gains of presenting objects as brighter (integrated magnitude) and more detailed (improved resolution) combine to result in a lowered threshold contrast for detection. It has also been said that discussions of threshold contrast can give the mistaken impression that increasing aperture improves object contrast. I don't share this view. However, if some people do come away with that impression, it is not because of some problem inherent in the concept. It's because we need to do a better job of presenting, explaining and talking about it. The response shouldn't be to not talk about threshold contrast. The response should be to find more innovative and creative ways to communicate with and educate the amateur astronomy community about this important concept.

Bill in Flag


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GlennLeDrew
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Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5917440 - 06/12/13 05:56 PM

Joe,
Part of what I was trying to impress, as you explicitly stated, is that the eye doesn't 'know' a priori what the aperture or exit pupil are.

In your previous post you dealt with the case of a specific object at the same exit pupil through different apertures. This is the most straightforward of all the possibilities to grapple with. The conclusion is as valid for a dim object as it is for a bright one, and is completely expected by (most) anyone:
- the image surface brightness is the same
- the contrast sensitivity is the same
- the linear detail perceived scales as the aperture

One 'muddying' factor here is clearly contrast sensitivity, which has a direct bearing on threshold contrast, and which in turn is dependent on image surface brightness. If we progressively stop down a scope, the image dims and contrast sensitivity worsens. Where at full aperture we might perceive a brightness difference as small as 10% above sky brightness, when well stopped down this might be as bad as 50%, or even 100% above sky brightness.

Another 'muddying' factor is the role of image scale, at least when we restrict to dealing with a single object. Altering the image size makes deconstructing the contribution of the various elements difficult to assess, on an intuitive level at any rate.

To aid in *deconstructing* the various contributions was why I presented the case of two differing apertures taking in otherwise identical galaxies, each lying at a distance (scaling with aperture) which results in their both subtending the same angle on the retina at given exit pupil. The view in each scope is indistinguishable. Contrast sensitivity and threshold contrast are unchanged, no matter the aperture.

And so the concept of threshold contrast as seems to be espoused would seem to be inextricably entwined with the role of image scale, as well as instrumental resolving power. All the more so when every example employed deals exclusively with what happens at the threshold of detection of detail for a *specific* object. The result is then really the statement of the fundamental principle that a bigger aperture employed on any given object delivers a more detailed image. In this context, contrast sensitivity and threshold context are completely redundant.

To fully understand the effects of all the variables is most difficult when the variables are not isolated and dealt with in turn. An example of a common myth resulting from an under-appreciation of the variables involved.

If the observer is unaware of the fact that as the image dims his resolving power worsens due to reduced contrast sensitivity, what does he conclude after bumping up the magnification on a small galaxy and more detail appears? The contrast has improved because the sky has darkened! (This myth is pernicious in the extreme; it's still bandied about to a surprising extent, and I despair of ever seeing it relegated to the status of polio.)

Later on he learns that it's really the case that the increase in image scale has afforded the perception of detail faster than the rate at which image dimming reduces contrast sensitivity.


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GlennLeDrew
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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5917481 - 06/12/13 06:20 PM

Oops... Removed this post due to a misinterpretation... Sorry!

Edited by GlennLeDrew (06/12/13 06:23 PM)


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jpcannavo
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Reged: 02/21/05

Loc: Long Island New York
Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5917968 - 06/12/13 11:13 PM

Rob
As strange as it seems, the luminance (i.e. surface brightness or light intensity per unit area) of the real image of the sun on the retina is no greater when looking through a scope than when looking with the naked eye. In fact, depending on exit pupil, it may be smaller!! (for the few second or two your eye would last!). BUT the total integrated brightness - to use bill's terminology - will be much much greater on the retina with the scope, since the real image is much much larger. As such, far more heating power (wattage) is being delivered and your eye cooks very quickly. There is no need to do the experiment, the photometry gives the answer.

This concept - the optical invariant, conservation of etendue etc. - that aperture cannot increase luminance is very counter intuitive, but it is a critical concept in optical engineering.

Let me end with a very very rough analogy. Imagine being hit with one drop of boiling water (analogous to glancing at the sun naked eye). Not much damage, probably no band-aid needed. Now imagine an entire kettle of boiling water spilling on you (like glancing at the sun with a scope). Now the water in the kettle is no hotter (very roughly analogous here to luminance) than the drop, but the kettle load (far more total heat) will land you hospitalized in a burn unit in critical condition. (admittedly, this is so rough an analogy as to verge on metaphor, but it has heuristic value nonetheless).
Hope this helps.
Joe


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jpcannavo
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Re: Big dob for objects with structure and detail? new [Re: BillFerris]
      #5918074 - 06/13/13 12:47 AM Attachment (8 downloads)

Quote:

The statement, "as you get closer to the picture in the dimmed room ... the brightness of the object does not change," may be correct for surface brightness but is incorrect when we consider apparent total brightness. This is something most everybody ignores--with one notable exception --in these discussions. In order for object surface brightness to remain constant as apparent size increases, the object's apparent integrated magnitude must increase. In short, all objects become brighter as aperture increases.




Bill, just to be clear, and before you put yourself in a special category, I am perfectly clear on the relevant photometric distinctions here.

I would also be cautious with an analysis that attempts to explain the relevance of integrated brightness for detection in terms of "more information being delivered to the eye". While this model seems to work at one end of the contrast sensitivity function, where contrast sensitivity increases with image scale, it fails to predict that a maximum occurs (at some mid spatial frequency) with the other end of the function then falling off at lower frequencies.

CSF

As for all the other elaboration, I think we are all more or less converging on the same explanation, but perhaps with varying degrees of clarity. Speaking of clarity, I do like the above referenced explanation by Jon Isaacs. He kind of cuts right to the chase. informed by the directness of his account, I edited my prior post a bit.

Dark and steady skies!


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GlennLeDrew
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Re: Big dob for objects with structure and detail? new [Re: jpcannavo]
      #5918101 - 06/13/13 01:16 AM

Joe,
I enjoyed reading the Wiki article on CSF. It did seem to confine to the photopic regime, where 5-7 cycles/degree are optimally detected. But at low light levels, as I understand it, this can decrease to a fraction of a cycle/degree (i.e., several degrees/cycle.) Unless I'm conflating minimum size for detection (which can be several degrees) with a cyclical variation in a much larger field.

This is another 'confounding' factor when dealing with the matter of DSO observation.


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dave brock
professor emeritus


Reged: 06/06/08

Loc: Hamilton, New Zealand
Re: Big dob for objects with structure and detail? new [Re: robininni]
      #5918184 - 06/13/13 03:36 AM

Quote:

.... I know the difference in views between a 20" and 24" can be pretty big and so I don't know if I want to downsize from the 25" to a 20".

Rob




Hi Rob.
I won't get into the discussion of contrasts, brightness etc. but will just say that in my experience the difference in views between two scopes is much more evident when changing down as opposed to changing up. When I built my 16" I was a little disappointed at the gain over my existing 12". I decided to keep both for a while before deciding which one to sell. I ended up using the 16" only (after the many nights of setting both up for side by side comparisons) for a month or two and when I eventually used the 12" again it was clear the 16" was the keeper. Same with the 16" - 20".

Dave


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jpcannavo
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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5918421 - 06/13/13 09:33 AM

Glenn
Interesting stuff! Look around, there is a deep literature base here. The classic work is HR Blackwell 1946 !

Its neat how this topic dovetails with photometry and visual deep sky observing.

One point I was trying to nail down (perhaps with limited success) was the importance of appreciating the Contrast Sensitivity Function (herein CSF) and how it provides the explanation, in this context of this discussion, of Jon Isaacs straight forward (if somewhat understandably simplified) statement above that "The eye discerns larger images better than smaller images". But look at the function and note that this is only true up to a point!

The other point I was trying to emphasize (again with limited success!) is the fact that the CSF is only a property of the retina/visual system itself - extra ocular optics do not add additional variables to this function.

Speaking of aperture exit pupil changing contrast sensitivity/threshold, is a somewhat different matter, and I am somewhat hesitant - but will give this more thought. Note however that it is critical here to distinguish between contrast sensitivity/threshold, and the contrast sensitivity/threshold function! (i.e. the distinction between a function f and the value of f somewhere in its domain).

On the other hand, the visual detectability of a given target is very much a function of aperture, exit pupil - and one might add sky darkness, transparency, seeing etc. - as well as retinal/visual system properties. (we can speak of a multi-valued visual detectability function!) But again, none of these extra retinal/visual system variables change the CSF function. They instead determine the point that an object falls on that 2-dimensional plot of that function , thereby determing whether and object becomes visible or not.


There are many rich digressions here. For example the final phenomenologic visual representation, and how that leads to judgments about contrast in the virtual image at the EP. This all makes me wish I had more time. Bt every time my wife catches me at the computer she looks at me and I imagine her thinking: "did he finish doing such and such for our move out of NYC ?"

Dark and steady!

Joe


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jpcannavo
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Re: Big dob for objects with structure and detail? new [Re: dave brock]
      #5918534 - 06/13/13 11:05 AM

Dave
At RMSS this past week I was setup next to an excellent lockwood 12.5". The differences with my 16 initially seemed subtle, but then became more pronounced as the back and forth views continued through the night. Some day I look to make the 16 to 20 jump, but only after I get more dark western sky observing under my belt. (can you tell I am psyched about moving to Colorado!)
Joe


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turtle86
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Re: Big dob for objects with structure and detail? new [Re: auriga]
      #5918833 - 06/13/13 01:47 PM

Quote:

Hi,
Recently I saw a post in this forum that states the issue as follows (paraphrased by me, hopefully accurately):

For extended objects: the brightness of an image of an extended object depends entirely on the size of exit pupil.

But for a given exit pupil, say 5mm, a larger aperture scope will give a higher magnification and therefore a larger image.

The eye discerns larger images better than smaller images.

Therefore for a given exit pupil, say 5mm, more will be seen in the larger aperture, since the magnification is higher, and the eye sees larger images better, even though the brightness of image of the object remains the same since the exit pupil remains the same.

On the other hand, if the magnification in the two telescopes is made equal, the larger aperture will show more since its exit pupil will be larger at equal magnification than the exit pupil in the smaller scope, and so the image of the object will be brighter.

At least that is how I understand it so far. Let me know if this is off base.

Bill




That's basically my understanding too. Roger Clark's Visual Astronomy of the Deep Sky does a great job explaining how the eye in night vision mode discerns larger images better than smaller images. Larger images mean greater detail, but since greater detail requires greater mag and smaller exit pupil in a given scope, larger images come at the cost of lower contrast. For me, part of the fun and challenge of visual astronomy is finding that "sweet spot" where I can see the most detail in a given deep sky object before it melts into the background due to loss of contrast. M51 is a favorite object and it really is astonishing just how much mag it can take in my 18" scope. I always encourage people I observe with to try several different mags on a object. There's nothing to lose, and you never know just what you might see.


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GlennLeDrew
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Re: Big dob for objects with structure and detail? new [Re: turtle86]
      #5919258 - 06/13/13 06:06 PM

On the matter of brightness and contrast as regards minimum size for detection... In my Gallery (link in my sig) is a chart I made up, based on material in the RASC Observer's Handbook, which in turn is largely based on Bkackwell's 1946 study. I also provide a fairly lengthy 'user guide', if you need help getting to sleep.

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jpcannavo
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Re: Big dob for objects with structure and detail? new [Re: turtle86]
      #5919733 - 06/13/13 11:01 PM

M51 and magnification! At RMSS in Colorado this past week I was getting great image scale with an 8mm Ethos on my F5 16 (250X). I actually think a 6mm would have been perfect!

Joe


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turtle86
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Re: Big dob for objects with structure and detail? new [Re: jpcannavo]
      #5919762 - 06/13/13 11:26 PM

Quote:

M51 and magnification! At RMSS in Colorado this past week I was getting great image scale with an 8mm Ethos on my F5 16 (250X). I actually think a 6mm would have been perfect!

Joe




Definitely worth trying! Seems that a lot of the brighter Messier galaxies can really take the higher mag--M33, M64, M65-66, M81-82 and M104 come to mind. Can't remember if I've tried the 6mm Ethos on M51 myself but agree that it looks magnificent with the 8mm Ethos. The 6mm Ethos is great for planetary nebulae and busting open globs.


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GlennLeDrew
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Re: Big dob for objects with structure and detail? new [Re: turtle86]
      #5920068 - 06/14/13 06:58 AM

It's not necessarily the higher *integrated magnitude* objects which can take magnification. It's those objects with reasonably high *surface brightness* which bear high powers.

Example...

The California nebula has an intrinsic integrated brightness of about 7th magnitude. It cannot take high magnification because its surface brightness is *barely* brighter than the sky. (Intrinsically, it's about 25 MPSAS, but sky glow adds to this, resulting in an apprent total brightness of 4th mag or brighter.)

Many a 12th magnitude (and fainter) planetary can withstand quite high magnification due to the rather bright 14-15 MPSAS (over 100X brighter than the sky.)

High surface brightness makes for high contrast, and so smaller exit pupils can be utilized without the object becoming lost in visual system noise.

To drive home the point that integrated brightness becomes increasingly meaningless (useless) as object size increases, consider the sky itself. At a pristine site, where zenithal surface brightness is 22 MPSAS, the integrated brightness of the whole celestial dome (just sky glow, no stars) is easily -7 magnitude, or 10 times brighter than Venus.

I'd bet that most any sky observer, if told that the sky glow at the darkest site amounted to -7 magnitude, would think the bearer of the factoid insane. And so it is with large, low surface brightness nebulae; they have surprisingly bright integrated magnitudes. But these values give no idea whatsoever of the difficulty of detection.


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turtle86
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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5920622 - 06/14/13 01:44 PM

Sure seems that with low surface brightness objects, the quality of the sky conditions matters more than the aperture. I'd hazard to guess that from a "black" observing site, the California and Horsehead Nebulas don't need a big Dob to be seen. I do most of my observing at Chiefland, which is moderately dark but with a noticeable light dome to the north and a smaller one to the east. In my 18", the California and Horsehead Nebulas aren't too hard to see with a 31mm Nagler and h-beta filter, at least on a night when the humidity isn't too bad. But if the humidity goes over something like 80%, trying to pull these faint extended objects out of the muck can be a daunting task indeed.

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Re: Big dob for objects with structure and detail? new [Re: GlennLeDrew]
      #5920752 - 06/14/13 02:57 PM

Glenn, Rob.
Yeah.
Surface brightness - i.e. luminance - is a key parameter (as of course are others). It would be neat to see some data quantifying various deep sky objects (galaxies and nebulae) along these lines. I actually think someone has, but can't remember where...?
Joe


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