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Equipment Discussions >> Refractors

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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Reflector/Refractor equivalence formula new [Re: Asbytec]
      #5803965 - 04/17/13 05:43 AM

Norme,
In a nutshell, when the subject is bright enough for the eye to work in photopic mode, the eye can easily realize the full potential of the instrument's resolving power.

As subject brightness decreases, a threshold is crossed where the eye's resolving power is just matched with that of the scope (where the Airy disk cannot quite be resolved.)

Below this threshold, further dimming results in ever worsening visual system resolving power, the same level of detail is there as when the subject is bright--and the MTF chart has not changed one iota--but the observer's eye has very much become the weak link.

A solar filter, if of very good optical quality, does not really alter the MTF (meaningfully.) while observing the Sun, all manner of detail is seen, as expected due to the still sufficiently bright solar disk.

But the Moon is dimmed by the solar filter to such an extent that it's like a DSO in terms of surface brightness. All the detail present without the filter in place is there, as a time exposure with a camera would reveal. And the MTF chart for the scope is unchanged. It's the eye's awful resolving power at this low light level which makes the perception of *any* detail--even along the ultra high contrast terminator--problematic.

As Jon says, this is a most worthy test to conduct. More than any amount of verbiage possibly could, it will drive home just how limited is the eye at low light levels. Which is why a little 60mm f/6 can reveal faint DSO detail in images which cannot be visually detected with scopes of 10X the aperture.


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Paul G
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Reged: 05/08/03

Loc: Freedonia
Re: Reflector/Refractor equivalence formula new [Re: GlennLeDrew]
      #5804563 - 04/17/13 01:34 PM

Quote:

Norme,
In a nutshell, when the subject is bright enough for the eye to work in photopic mode, the eye can easily realize the full potential of the instrument's resolving power.

As subject brightness decreases, a threshold is crossed where the eye's resolving power is just matched with that of the scope (where the Airy disk cannot quite be resolved.)

Below this threshold, further dimming results in ever worsening visual system resolving power, the same level of detail is there as when the subject is bright--and the MTF chart has not changed one iota--but the observer's eye has very much become the weak link.

A solar filter, if of very good optical quality, does not really alter the MTF (meaningfully.) while observing the Sun, all manner of detail is seen, as expected due to the still sufficiently bright solar disk.

But the Moon is dimmed by the solar filter to such an extent that it's like a DSO in terms of surface brightness. All the detail present without the filter in place is there, as a time exposure with a camera would reveal. And the MTF chart for the scope is unchanged. It's the eye's awful resolving power at this low light level which makes the perception of *any* detail--even along the ultra high contrast terminator--problematic.

As Jon says, this is a most worthy test to conduct. More than any amount of verbiage possibly could, it will drive home just how limited is the eye at low light levels. Which is why a little 60mm f/6 can reveal faint DSO detail in images which cannot be visually detected with scopes of 10X the aperture.




Great post. It helps to understand the selective pressures under which our nighttime visual system evolved. Discerning fine detail was not important, as evidenced by the distribution of rods in the retina. Where our night vision excels is picking up movement, particularly with peripheral vision, a critical survival task in an environment filled with large nocturnal predators.

It's not particularly good and differentiating small differences in brightness or contrast, no selective advantage in being able to do so. For example, for a light source to be perceived as being 100% brighter it has to be 900% brighter (response compression).


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Reflector/Refractor equivalence formula new [Re: Paul G]
      #5805777 - 04/17/13 10:51 PM

Glenn, again, thanks for the explanation. Yes, I understand pretty much everything you said.

It seems, however, some folks use the eye's low light lack of resolution as an argument against using MTF as a standard tool for telescope comparisons. To which, I agree. The eye does loose something at low light levels and the MTF does not accurately show what is observed visually. But, the point made is the information (contrast and resolution) is on the focal plane, so MTF can be used even if the eye (which varies widely, anyway) can still be a useful tool even at low light levels.

When checking my scope without the secondary baffle, I did observe the moon's dark side looking for whatever I could see to include stray light and glare. No craters. It was interesting and I suspect the same effect as a solar filter. There were patches of light albedo here and there, but no fine detail

Edited by Asbytec (04/17/13 10:53 PM)


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