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For extended objects, magnification does not improve contrast

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#1 GlennLeDrew

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Posted 14 April 2017 - 11:22 AM

(The following concerns extended objects only, not point sources.)

 

This myth of contrast increasing for extended objects with increased magnification due to the darkened sky is still making the rounds. It's a particularly pernicious, antibiotic-resistent bacterium of an anti-fact.

 

Boosting magnification shrinks the exit pupil, which in turn dims both sky and object equally. Optically, contrast does not alter in the slightest. The increased image scale affords improved detection of detail, which very powerfully gives the *illusion* of improved contrast.

 

In fact, the scene dimming via exit pupil shrinkage actually *reduces* contrast because visual system noise becomes relatively larger. But in spite of this dimming and worse visual noise, the image scale increase involves more retinal cells which can extract more information...

 

...until a point is reached where a reversal, or turnover occurs, and visual system noise's contribution exceeds the gains afforded by increased image scale. Depending on sky and object surface brightness/contrast, this turnover occurs over a good range in exit pupil diameter. The brightest planetary nebulae can withstand surprisingly small exit pupildims, whereas the very low surface brightness nebulae in a dark sky demand at least moderately large exit pupils.

 

The persistence of the myth of boosted contrast for nebulae and galaxies via increased magnification must derive in part because of the *power* of the illusion imparted by the gains afforded by the increase in image scale. Even if an observer has heard or read of the true workings, the impression gained by his own eyes would seem to deny this, and instead support the contrast increase misperception.

 

Some might say, "If the net effect is like a boost in contrast, why get wrapped up in the details of why?" Well, this goes to understanding and differentiating between the external optics and the human visual system. I think it's important to know which does what, for it then allows us to more effectively use our equipment.

 

I've been battling this myth for decades now, and wonder if I'll live to see its expiration/eradication. I'd rather go out feeling like a St. George that a Don Quixote. ;)


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#2 Jon Isaacs

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Posted 14 April 2017 - 11:37 AM

:waytogo:

 

Glenn:

 

Excellent post, concise and to the point.  I particularly like this paragraph:

 

"Some might say, "If the net effect is like a boost in contrast, why get wrapped up in the details of why?" Well, this goes to understanding and differentiating between the external optics and the human visual system. I think it's important to know which does what, for it then allows us to more effectively use our equipment."

 

I think this is very important.  And I will say, contrast is a very simple concept but it took me a long time to really understand what contrast was because of all the vague and technically incorrect usages. It is a most abused and misused concept with things a bright sky attributed to a large central obstruction because the CO "reduces" contrast.  

 

Thanks.. Your post should be a sticky.. 

 

Jon


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#3 Astrojensen

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Posted 14 April 2017 - 11:44 AM

 

Some might say, "If the net effect is like a boost in contrast, why get wrapped up in the details of why?" Well, this goes to understanding and differentiating between the external optics and the human visual system. I think it's important to know which does what, for it then allows us to more effectively use our equipment.

That is exactly right. It's only when one begins to fully understand the situation, one can truly begin to use magnification to its full advantage. 

 

 

I've been battling this myth for decades now, and wonder if I'll live to see its expiration/eradication. I'd rather go out feeling like a St. George that a Don Quixote.

Sadly, I think the final victory is still far away, since the old myths live on in old guidebooks for decades to come and it's one of those concepts, that beginners find very hard to grasp. 

 

 

Clear skies!

Thomas, Denmark


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#4 walt99

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Posted 14 April 2017 - 11:52 AM

Al Nagler said :

 

"The best view occurs with the highest power that comfortably includes the target object. Higher powers darken the background sky, reveal fainter stars and show more detail. The resulting smaller exit pupil also minimizes the effects of eyesight defects."

 

Coming from this high authority,  I have believed it,  and never seen it challenged.  Are you saying Nagler is incorrect ?


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#5 Starman1

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Posted 14 April 2017 - 12:04 PM

And size is a big player in the equation

The background in my scope is quite black at about 228x, but small planetaries, though quite visible at that power,

display a lot more detail at 388-493x.  It's not an improvement in contrast at all, merely size.

If I go to 776x, they are harder to see because they have dimmed too much.

 

Part of learning to use a telescope is learning how to balance magnification (size) with decreasing brightness (exit pupil).

There is always a "telescopic eutectic point" where the two conflicting curves cross, and it's different on every object and in each scope.

 

The term contrast has always had a nebulous definition (pun intended).  Look at the long discussions on the Eyepieces Forum where

the term refers to about a dozen different eyepiece parameters all acting together.

 

Getting people to understand how telescopes work is always a battle.

I've been doing this for 54 years, but didn't understand what the exit pupil was until a couple years ago.  I knew how to calculate it, and how it worked, but not what it really was.

How can an eyepiece be afocal and have an exit pupil "focus" at the same time?  Hint: the exit pupil is an image of the primary.

So why expect beginners or casual observers to understand exit pupil, magnification, and scope size with regard to brightness?


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#6 jallbery

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Posted 14 April 2017 - 12:05 PM

Al Nagler said :

 

"The best view occurs with the highest power that comfortably includes the target object. Higher powers darken the background sky, reveal fainter stars and show more detail. The resulting smaller exit pupil also minimizes the effects of eyesight defects."

 

Coming from this high authority,  I have believed it,  and never seen it challenged.  Are you saying Nagler is incorrect 

I think what Glenn is saying that extended objects are dimmed by the same factor as the background sky, so the contrast factor remains the same.  Note that Mr. Nagler says that the increased magnification reveals fainter STARS not fainter details in extended objects.  The esteemed Mr. Nagler is not above cherry picking his details for marketing purposes (and the source of the quote is most definitely in the realm of marketing literature).


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#7 NiteGuy

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Posted 14 April 2017 - 12:06 PM

I've always used the words "increased contrast"  with increased magnification because I'm too impatient (lazy?) to use 6 paragraphs to describe it. I guess it wouldn't be too much trouble for me to add one more word and refer to it as "apparent increased contrast", what do you think?


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#8 Starman1

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Posted 14 April 2017 - 12:11 PM

Al Nagler said :

 

"The best view occurs with the highest power that comfortably includes the target object. Higher powers darken the background sky, reveal fainter stars and show more detail. The resulting smaller exit pupil also minimizes the effects of eyesight defects."

 

Coming from this high authority,  I have believed it,  and never seen it challenged.  Are you saying Nagler is incorrect ?

There is nothing false in what he said.

He just didn't add, "within reason".

Also, one could INTERPRET that to mean that contrast was improved.  He did not say that, though.  Just that increased magnification improves the view, and it usually does.

But not always.

Because TOO-HIGH powers result in more eye aberrations being visible and a too-low brightness on the objects.

Also, when the Airy disc is magnified to where it has a visible size, further magnification cannot reveal fainter stars.  Stars then behave as extended objects and get dimmer

with increasing magnification.

 

Still, beginners often use too low magnifications on most objects because they haven't yet learned how to see faint objects.  Larger exit pupils make for brighter fields.

It takes a bit of training of the eye to view darker fields of view.


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#9 GlennLeDrew

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Posted 14 April 2017 - 12:38 PM

Al Nagler said :

 

"The best view occurs with the highest power that comfortably includes the target object. Higher powers darken the background sky, reveal fainter stars and show more detail. The resulting smaller exit pupil also minimizes the effects of eyesight defects."

 

Coming from this high authority,  I have believed it,  and never seen it challenged.  Are you saying Nagler is incorrect ?

Al's comment quoted here is much too brief to permit to properly parse the meaning. For stars (which I made clear in my OP to not be addressing), a darker sky does indeed boost contrast because they retain brightness while the sky darkens. Until the exit pupil gets down to about 1mm, whereupon the resolved Airy disk thereafter behaves as an extended object.

 

He doesn't expressly address the behavior for extended objects, and so one can infer nothing in this regard. I don't see why Al might ascribe to the false notion of contrast actually increasing *because* of sky darkening, as opposed to it actually being *because* of the increased image scale. I'll give him the benefit of the doubt, and grant that where applicable the concept of "perceived" contrast is implied.

 

It's unfortunate when 'authority' figures lazily pass over in brevity or silence matters which already are a source of confusion because of the power of illusion. They know that not all amateurs are fully informed in optics and visual perception. And so at least a modicum of effort should be expended on clarity.


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#10 GlennLeDrew

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Posted 14 April 2017 - 12:52 PM

I've always used the words "increased contrast"  with increased magnification because I'm too impatient (lazy?) to use 6 paragraphs to describe it. I guess it wouldn't be too much trouble for me to add one more word and refer to it as "apparent increased contrast", what do you think?

I think "perceived" contrast covers it nicely.

 

My objection arises from the incompleteness of stating that contrast improves because of the darker sky. Period. Nothing more. This can lead to the misperception that the sky darkens more rapidly than does the object, or perhaps even that the object doesn't dim at all.

 

It suffices to say that "contrast is perceived to improve due to the increased image scale despite the image dimming." One sentence, not a few paragraphs (unless desiring to address the full gamut of sky and object brightness, and exit pupil range.) A darkened sky has absolutely nothing to do with it. Indeed, a dimmer scene is disadvantageous because of the greater contribution of visual system noise.


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#11 walt99

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Posted 14 April 2017 - 01:38 PM

 

I've always used the words "increased contrast"  with increased magnification because I'm too impatient (lazy?) to use 6 paragraphs to describe it. I guess it wouldn't be too much trouble for me to add one more word and refer to it as "apparent increased contrast", what do you think?

I think "perceived" contrast covers it nicely.

 

My objection arises from the incompleteness of stating that contrast improves because of the darker sky. Period. Nothing more. This can lead to the misperception that the sky darkens more rapidly than does the object, or perhaps even that the object doesn't dim at all.

 

It suffices to say that "contrast is perceived to improve due to the increased image scale despite the image dimming." One sentence, not a few paragraphs (unless desiring to address the full gamut of sky and object brightness, and exit pupil range.) A darkened sky has absolutely nothing to do with it. Indeed, a dimmer scene is disadvantageous because of the greater contribution of visual system noise.

 

That is indeed the misperception I've been deluded by .   Thanks for correcting it.



#12 Jim Nelson

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Posted 14 April 2017 - 03:31 PM

This is a battle I gave up fighting long ago...You've stated the case very clearly, though!

 

There's a fairly complicated relationship between image scale and image brightness and the greatest ease of detecting an extended object or feature (if you can get a copy, Clark's "Visual Astronomy of the Deep Sky" covers this at length), but the basics aren't too hard to understand: low contrast objects need a larger image scale for us to see well, which more than makes up for the dimmer image at higher magnification...up to a point.  

 

You *can* improve contrast, and that's an enormous help - that's what filters are for!  


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#13 GlennLeDrew

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Posted 14 April 2017 - 04:32 PM

It was Roger Clark's book, "Visual Astronomy of the Deep Sky", purchased back in '89 or so, which helped very much in cementing these concepts. I lent it out in '92 or '93 and haven't seen it since.


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#14 Jon Isaacs

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Posted 14 April 2017 - 05:44 PM

I rather dislike the term "perceived contrast" because it just sidesteps the question of what and why your are seeing what you are seeing and just adds confusion to understanding what is really happening.  

 

In this situation, it's a question of the words used to describe what one is seeing.. what one is perceiving is one thing, how one describes that perception is another.  When I view the Veil in a 16 inch with a 6 mm exit pupil, just exactly what I am perceiving that is different than when I view it in a 4 inch with a 6 mm exit pupil?

 

I believe and find personally that if I really sit back and closely observe such comparisons, I can grasp what it is I am actually seeing and i can begin to use more accurate words to describe what one is perceiving..

 

There are a number of factors to consider, not only the image scale and image brightness but also the gradients in contrast and brightness.  Increasing the image size is not always productive, sometimes the reduced gradients in contrast/brightness are counterproductive..  

 

One comment:. Al Nagler did not use the term contrast in his statement. As a general guideline, very often increasing the magnification is the best strategy.  

 

Jon


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#15 theApex

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Posted 14 April 2017 - 07:27 PM

There's a fairly complicated relationship between image scale and image brightness and the greatest ease of detecting an extended object or feature (if you can get a copy, Clark's "Visual Astronomy of the Deep Sky" covers this at length), but the basics aren't too hard to understand: low contrast objects need a larger image scale for us to see well, which more than makes up for the dimmer image at higher magnification...up to a point.  

 

It was Roger Clark's book, "Visual Astronomy of the Deep Sky", purchased back in '89 or so, which helped very much in cementing these concepts. I lent it out in '92 or '93 and haven't seen it since.

I confess I had been in the impression some of its precepts held true till I read his errata-like website (or is it someone else's?):

"the Optimum Detection Magnifications in Appendix F of Visual Astronomy (in which I used method 1 in a way I didn't realize until these discussions) is wrong. Do not use it!"

 

For those in the unknown, Appendix F was V.A.D.S.'s core value! So there you have it, from the horse's mouth, "do not use it!"

 

Although, regarding the increase on an extended object's apparent size, he is still adamant, and has coined a new term, OMV (Optimum Magnified Visual Angle) and regards that book's Appendix E as correct "to the best of my knowledge", as he puts it.


Edited by theApex, 14 April 2017 - 07:30 PM.


#16 Organic Astrochemist

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Posted 14 April 2017 - 08:12 PM

Excellent discussion.
I don't think there is nearly enough consideration for how the eye works in amateur astronomy.

I would propose one modification however

The following concerns extended objects only THAT CAN BE SEEN AT LOW POWER, not point sources.)

If an object subtends less than the critical angle and therefore cannot be seen, there will be no contrast between object and sky. Increasing the magnification can cause the object to be seen and the perception of contrast between object and sky. I think this could be reasonably described as magnification increasing contrast. Observing under light polluted skies, it happens all the time.
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#17 Lard Greystoke

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Posted 14 April 2017 - 09:44 PM

. I'd rather go out feeling like a St. George that a Don Quixote. wink.gif

In this world everyone goes out like Don Quixote.  Leaves something for future generations to do, anyway.



#18 Philler

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Posted 14 April 2017 - 10:07 PM

(The following concerns extended objects only, not point sources.)
 
This myth of contrast increasing for extended objects with increased magnification due to the darkened sky is still making the rounds. It's a particularly pernicious, antibiotic-resistent bacterium of an anti-fact.
 
Boosting magnification shrinks the exit pupil, which in turn dims both sky and object equally. Optically, contrast does not alter in the slightest. The increased image scale affords improved detection of detail, which very powerfully gives the *illusion* of improved contrast.
 
In fact, the scene dimming via exit pupil shrinkage actually *reduces* contrast because visual system noise becomes relatively larger. But in spite of this dimming and worse visual noise, the image scale increase involves more retinal cells which can extract more information...
 
...until a point is reached where a reversal, or turnover occurs, and visual system noise's contribution exceeds the gains afforded by increased image scale. Depending on sky and object surface brightness/contrast, this turnover occurs over a good range in exit pupil diameter. The brightest planetary nebulae can withstand surprisingly small exit pupildims, whereas the very low surface brightness nebulae in a dark sky demand at least moderately large exit pupils.
 
The persistence of the myth of boosted contrast for nebulae and galaxies via increased magnification must derive in part because of the *power* of the illusion imparted by the gains afforded by the increase in image scale. Even if an observer has heard or read of the true workings, the impression gained by his own eyes would seem to deny this, and instead support the contrast increase misperception.
 
Some might say, "If the net effect is like a boost in contrast, why get wrapped up in the details of why?" Well, this goes to understanding and differentiating between the external optics and the human visual system. I think it's important to know which does what, for it then allows us to more effectively use our equipment.


 
I've been battling this myth for decades now, and wonder if I'll live to see its expiration/eradication. I'd rather go out feeling like a St. George that a Don Quixote. wink.gif


Glenn, I sometimes wonder when I see sketches and descriptions of like galaxies posted and a lot of the time it is at some "umteen-galzillion" power, like that's the norm and not the exception. It seems like it's always at like 300x to 600x power and sometimes more.. They must be observing on some remote mountain tops. Usually, my best observations with my 10" and 12.5" Dobs come using from like 90x to 170x. This is where I get my sharpest and most detailed views. Now I will try even higher powers to try to more closely examine the nucleus and core areas. But like 200x and 300x are the exception, not the norm even from dark transparent skies for me.. Now, with PN's sometimes it seems like you can really boost up the powers.

I was getting some pretty good views of M 81 with my 10" from my dark sky site one night and I thought I would try all my ep's on it. Turns out my lowly 20mm Meade (China made) SP at 58x showed by far superior detail (arms, core, and nucleus) to all the rest of my TV Plossls.

Edited by Philler, 14 April 2017 - 11:46 PM.

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#19 Philler

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Posted 14 April 2017 - 10:37 PM

Al Nagler said :
 
"The best view occurs with the highest power that comfortably includes the target object. Higher powers darken the background sky, reveal fainter stars and show more detail. The resulting smaller exit pupil also minimizes the effects of eyesight defects."
 
Coming from this high authority,  I have believed it,  and never seen it challenged.  Are you saying Nagler is incorrect ?


Wouldn't that almost be like telling God he's wrong? How could anyone take issue with the infallible Al Nagler? (LOL)
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#20 GlennLeDrew

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Posted 15 April 2017 - 02:07 AM

Jon,

In differentiating between a real contrast boost (as when sky glow dims or a filter is employed) and the illusion of a contrast boost, some single-word descriptor for the latter would be handy. For me, given that it's the *perception* of a gain in contrast, "perceived" seems to fit the bill. Or if we want to really drive home the point, we could call it "illusory contrast". ;)

 

Of course, we're unlikely to implement a universally used term here. As I noted earlier, it suffices to state that magnification induces the perception of increased contrast due to the extra information gleaned via the increased image scale.


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#21 GlennLeDrew

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Posted 15 April 2017 - 02:27 AM

Philler,

Regarding your comments about galaxy sketches and magnification employed...

 

Due consideration for the exit pupil is a *must*. The rather high magnifications reported might have been afforded by a fairly large aperture. Depending on sky brightness and the contrast of the object, some limit to smallness of the useable exit pupil will come into effect. The fainter outer part will be best seen with a not too small exit pupil. The brightest core and knots can withstand a notably smaller exit pupil, where the faint halo could well be quite invisible (the apparent diameter being now smaller.)

 

Chances are that in most instances a variety of magnifications were employed. To me it seems valid enough to report only the maximum used. It pretty much goes without saying that lower powers will have been used during the session. Unless the intent is to represent the object as seen at just the one magnification, as opposed to the optimal appearance synthesized over some range of magnification (which I suspect is the philosophy of most sketchers.)


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#22 Astrojensen

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Posted 15 April 2017 - 05:19 AM

The following concerns extended objects only THAT CAN BE SEEN AT LOW POWER, not point sources.)

If an object subtends less than the critical angle and therefore cannot be seen, there will be no contrast between object and sky. Increasing the magnification can cause the object to be seen and the perception of contrast between object and sky. I think this could be reasonably described as magnification increasing contrast. Observing under light polluted skies, it happens all the time.

The problem here is that even though you cannot see it, the contrast between the object and the sky IS there, even with the naked eye. Increasing the magnification does NOT increase the contrast between the object and the background sky, because that is a constant. But by making the object larger by magnifying it, while keeping the contrast constant, allows the eye to detect the object.  

 

It CAN be described as magnification increasing contrast, but that is simply wrong from a physical point of view. It should, ideally, be described as magnification allowing detection through sufficient object size.  

 

 

Clear skies!

Thomas, Denmark


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#23 Jim Nelson

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Posted 15 April 2017 - 07:23 AM

 

There's a fairly complicated relationship between image scale and image brightness and the greatest ease of detecting an extended object or feature (if you can get a copy, Clark's "Visual Astronomy of the Deep Sky" covers this at length), but the basics aren't too hard to understand: low contrast objects need a larger image scale for us to see well, which more than makes up for the dimmer image at higher magnification...up to a point.  

 

It was Roger Clark's book, "Visual Astronomy of the Deep Sky", purchased back in '89 or so, which helped very much in cementing these concepts. I lent it out in '92 or '93 and haven't seen it since.

I confess I had been in the impression some of its precepts held true till I read his errata-like website (or is it someone else's?):

"the Optimum Detection Magnifications in Appendix F of Visual Astronomy (in which I used method 1 in a way I didn't realize until these discussions) is wrong. Do not use it!"

 

For those in the unknown, Appendix F was V.A.D.S.'s core value! So there you have it, from the horse's mouth, "do not use it!"

 

Although, regarding the increase on an extended object's apparent size, he is still adamant, and has coined a new term, OMV (Optimum Magnified Visual Angle) and regards that book's Appendix E as correct "to the best of my knowledge", as he puts it.

 

I would emphasize that the foundational concepts of the text are absolutely true, non-controversial and well established within general visual science (In my prior life I have research experience in visual psychophysics, specifically on the detection of low-contrast stimuli of varying spatial frequencies - or "size"), even if the mathematical specifics and some assumptions are off. The *specifics* of the practical application of finding the "optimum detection magnification" has actually never been of much interest to me, as the actual practical application of the concepts in real-life is about understanding what happens with a change in magnification, not pre-calculating some ideal power to use on an object. There are better and more up-to-date sources for calculating an "optimum power".



#24 Jim Nelson

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Posted 15 April 2017 - 07:31 AM

 

 

There are a number of factors to consider, not only the image scale and image brightness but also the gradients in contrast and brightness.  Increasing the image size is not always productive, sometimes the reduced gradients in contrast/brightness are counterproductive..  

 

That's actually a good point - increased image scale, even with sufficient image brightness, isn't always a good thing. Image scale can be *too large*, as well as too small, although for *typical* astronomical objects the brightness is the limiting factor in finding the "best view".



#25 azure1961p

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Posted 15 April 2017 - 07:56 AM

Well said Glenn.

 

How many times I've heard or read:

 

My refractor is great on deep sky because if the black velvety background.

 

Or in an old Questar:

 

 well suited for deep sky due to the black background caused by its high contrast of the Questar. (paraphrased).

 

They never mention little optics needing to go to a higher power per inch to reach suitable image scale and the dimming  (velvety background) involved in the process. This never makes it to print.

 

 

At least in the 70s and 80s AND 90s this notion was rampant. Unitron and Questar I think are partly the cause .

 

Pete


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