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#1 drollere

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Posted 29 July 2014 - 04:22 PM

i'm interested to take an informal poll on the topic of visual magnitude limits.

(1) looking directly at the star, at what magnitude does the airy disk of the star become so small that the rings disappear and the star no longer presents the appearance of a disk but instead appears as a fuzzy point?

(2) at what magnitude does the star seem to disappear if looked at directly, although still easily visible with averted vision?

(3) what is the calculated limit magnitude for your aperture?

my own interpretation (dredged up in part from memory) is that (1) is about 5 magnitudes above (3) and (2) is about 3 magnitudes above (3).

in my 305mm SCT, rated limit at v.mag. 15, the airy disks disappear somewhere around 10, and the direct images around 12.

any guidance?

#2 Asbytec

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Posted 29 July 2014 - 05:33 PM

It's an interesting question, I tried it earlier this year.

I believe the answers in the 150mm aperture were IIRC (1) about 8th magnitude the ring is pretty much gone, (2) hard to observe a star directly around 10th magnitude and (3) LM is about 13.5 in practice. I'd have to redo the test to be sure of those figures.

So, (1) about 5 magnitudes and (2) 3 magnitudes brighter than limiting magnitude, pretty consistent with your results.

#3 WRAK

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Posted 30 July 2014 - 01:59 AM

(3) The calculated magnitude limit for my most used aperture is +13.4mag for a 140mm APO. Due to light pollution and usually mediocre transparency my observed TML is in average only about +12.2mag with a range from ~11.6 to ~12.8mag (I use the UCAC4 catalog here as reference but as we all know all magnitudes given for faint stars are quite shaky in all catalogs)

(2) My recorded TML for an observing session is usually somehow between direct and averted vision with no clear distinction as it depends on the available faint stars around my reference star for this session used also to determine seeing on the Pickering scale - the difference here for me is rather small, may be ~0.5mag or even less and depends heavily on bright stars nearby or not

(1) I don't understand here "at what magnitude does the airy disk of the star become so small that the rings disappear" as the size of the Airy disk does not change with magnitude - but my limit to get clear diffraction rings is ~6.5mag without CO but this might again be caused by my usually not this good seeing conditions. May be this changes with application of some CO (it has to) but I never checked this in a systematic way.

Wilfried

#4 drollere

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Posted 30 July 2014 - 01:00 PM

(3) The calculated magnitude limit for my most used aperture is +13.4mag for a 140mm APO. Due to light pollution and usually mediocre transparency my observed TML is in average only about +12.2mag with a range from ~11.6 to ~12.8mag

this is a valid concern, but variability in the NELM will appear as variability in anything the limit magnitude affects, so will appear as variability in relation to a fixed (calculated) telescope limiting magnitude.

(2) My recorded TML for an observing session is usually somehow between direct and averted vision with no clear distinction as it depends on the available faint stars around my reference star for this session used

i don't understand this, because the calculated limit magnitude for your telescope theoretically defines the magnitude at which stars are not visible even with averted vision. the question is the magnitude at which that transition occurs. i found it useful to use wide, faint double star systems in WDS where the glare from each star does not much affect the sky brightness and is possible to measure the magnitudes separately.

(1) I don't understand here "at what magnitude does the airy disk of the star become so small that the rings disappear" as the size of the Airy disk does not change with magnitude - but my limit to get clear diffraction rings is ~6.5mag without CO.

i'm aware of the claim that the airy disk diameter does not change because it is a mathematical quantity that depends on aperture alone. however, the visual airy disk does indeed change with magnitude: this was described clearly by william and john herschel, and george airy actually tried to explain it. you can read about all that here:

http://arxiv.org/ftp...3/1003.4918.pdf

my definition may have been obscure. even after the rings around the airy disk disappear, it is still possible (in my experience) to see the star as a tiny disk. but at some fainter magnitude this disk appearance disappears too, although the star still appears as a fuzzy point of light if looked at directly. this is the other magnitude limit. your value of 6.5 is 5.5 magnitudes brighter than your average limit magnitude of ~12.

#5 Starman1

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Posted 30 July 2014 - 04:02 PM

i'm interested to take an informal poll on the topic of visual magnitude limits.

(1) looking directly at the star, at what magnitude does the airy disk of the star become so small that the rings disappear and the star no longer presents the appearance of a disk but instead appears as a fuzzy point?

(2) at what magnitude does the star seem to disappear if looked at directly, although still easily visible with averted vision?

(3) what is the calculated limit magnitude for your aperture?

my own interpretation (dredged up in part from memory) is that (1) is about 5 magnitudes above (3) and (2) is about 3 magnitudes above (3).

in my 305mm SCT, rated limit at v.mag. 15, the airy disks disappear somewhere around 10, and the direct images around 12.

any guidance?

This depends on the darkness of your sky, the experience, the cleanliness of the optics, the color index of the star, the environment in which the star is placed, the magnification, the seeing, etc.
That all being said, here are my experiences with magnitude chart areas in 2 scopes:
Star appears as dot, no ring:
8"SCT--about m.14
12.5"newt--about m.15 (at 13.8, first ring still easily visible)
No direct vision, but averted vision OK:
8"--about m14.8. IIRC
12.5"--about m.16.5-16.6
Calculated limit, for me, in my circumstances:
8"SCT--about m.15.3
12.5"newt--about m.17.5
Actual limits reached:
8" SCT--15.6 (using charts from R. Clark)
12.5" newt--17.35 (using photometry of NGC206, verified with HB mag. of M14)

Seeing plays an incredible role in the above. And since the limits were reached at exit pupils of 1mm and smaller, the Airy disc and ring(s) were seen to deeper magnitudes than they would have been at lower powers.

#6 Starman1

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Posted 30 July 2014 - 04:21 PM

Some charts to use:
https://www.flickr.c...ker/5954756749/
http://www.cloudynig...agSequence1.jpg

Look at the spread among observers in faintest star seen:
http://www.unihedron...MPSASvsNELM.jpg

This discusses the probability of detection:
http://www.twcac.org...piecedarkly.htm

This discusses other aspects of observing at the limit. I recommend the book:
http://www.clarkvisi...visastro/omva1/

#7 drollere

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Posted 30 July 2014 - 05:01 PM

so don, if i read you correctly, you (1) can visualize an airy disk down to within 1.5 to 2.5 magnitudes of your telescopic limit aperture, and (2) can visualize stars with direct fixation to within 0.5 to 1.0 magnitudes of the limit aperture?

rod illuminance sensitivity is about 60 times that of foveal cones at the "peak" cone sensitivity around 550 nm. on that basis i'd expect a magnitude difference of ~4 between the telescopic limit magnitude and the magnitude where foveal vision becomes possible. yet i experience the difference as closer to 3, and you put it at 1.

#8 Starman1

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Posted 30 July 2014 - 06:48 PM

so don, if i read you correctly, you (1) can visualize an airy disk down to within 1.5 to 2.5 magnitudes of your telescopic limit aperture, and (2) can visualize stars with direct fixation to within 0.5 to 1.0 magnitudes of the limit aperture?

rod illuminance sensitivity is about 60 times that of foveal cones at the "peak" cone sensitivity around 550 nm. on that basis i'd expect a magnitude difference of ~4 between the telescopic limit magnitude and the magnitude where foveal vision becomes possible. yet i experience the difference as closer to 3, and you put it at 1.

I have noticed that at small exit pupils, i.e. below 1mm, the first diffraction ring is still visible--especially on close double stars, or those in the center of a star cluster--down to a very faint level.

I have also noticed that stars that are exceedingly faint, once found with averted vision, can be held some of the time with direct vision.

I would categorize increasing faintness as being, from easy to hard, and spreading over a couple of magnitudes :
--visible with direct vision 100% of the time
--visible with direct vision part of the time and averted vision all of the time
--visible occasionally with direct vision and visible with averted vision nearly 100% of the time. This is the level not far away from the limit with averted vision.
--not seen with direct vision, visible with averted vision most of the time
--visible with averted vision some of the time
--visible with averted vision occasionally.

I have not seen a level where the star is always visible with averted vision but is never occasionally visible with direct vision. If it's that bright, it is occasionally seen with direct vision if you look for it. A good example is the central star in M57. When it's visible 100% of the time with averted vision, it is always visible some of the time with direct vision.

I don't think I've actually reached that last level with my 12.5" because I don't have many targets I can use to see exactly how faint I can go. There is a big jump to the next fainter stars in NGC206 from the ones I've seen, and it requires seeing I don't see every time I observe--not to mention needing the star cloud to be nearly overhead.

Also, from the recent paper on theshold viewing and human vision, it seems that scintillation of the atmosphere can momentarily brighten or diminish a stellar target by almost a full magnitude. That would mean the stars that are so faint they're only visible a small percentage of the time are probably only visible because of atmospheric scintillation.
Therefore, an accurate determination of a limit in a scope probably relies on perfect seeing as well, since it seems you might see fainter stars occasionally in less steady air.

There are rods in the fovea centralis in the retina, though they are not dense there. When you look at exceedingly faint stars and see them in the center of direct vision, who says you are using cones? If i see no color and I am completely scotopic, I should not be using cones at all on a star that faint.

#9 drollere

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Posted 05 August 2014 - 12:19 AM

 

I have noticed that at small exit pupils, i.e. below 1mm, the first diffraction ring is still visible--especially on close double stars, or those in the center of a star cluster--down to a very faint level.

I have also noticed that stars that are exceedingly faint, once found with averted vision, can be held some of the time with direct vision.

I have not seen a level where the star is always visible with averted vision but is never occasionally visible with direct vision. If it's that bright, it is occasionally seen with direct vision if you look for it. A good example is the central star in M57. When it's visible 100% of the time with averted vision, it is always visible some of the time with direct vision.

Also, from the recent paper on theshold viewing and human vision, it seems that scintillation of the atmosphere can momentarily brighten or diminish a stellar target by almost a full magnitude. That would mean the stars that are so faint they're only visible a small percentage of the time are probably only visible because of atmospheric scintillation.

There are rods in the fovea centralis in the retina, though they are not dense there. When you look at exceedingly faint stars and see them in the center of direct vision, who says you are using cones? If i see no color and I am completely scotopic, I should not be using cones at all on a star that faint.

 

i must misunderstand some of the points you are trying to make, but other claims i am pretty sure are just incorrect.

 

yes, a small exit pupil is usually necessary to visualize clearly the airy disk and diffraction rings around it. so my interest is in the magnitude at which the airy disk changes to a fuzzy spot with direct vision, then with an image only visible with averted vision, at constant (high) magnification.

 

yes, a star below the foveal threshold and found with averted vision can with attention be visualized with foveal vision; i call the process "foveal coaxing" and have a gif illustrating it on my web page.

 

i understand your points about the vagueness or ambiguity of the threshold between direct and averted vision, but provided one does not use special attention or "coaxing" i do think there is a fairly clear boundary magnitude at which "first glance" direct vision will blank out a star but averted vision will immediately show it. for example, in my TEC 140, limit of about 13.4, it happens around 10.3. scintillation may be a factor, but will still allow estimation of an average within the variance. your reports imply that the variance is really very large.

 

there are no rods in the fovea centralis, or indeed in the fovea anywhere. you may be thinking of the claim that there are no B (short wavelength) cones in the fovea centralis, which is generally true but apparently does occur in some individuals.

 

color perception is not a function of receptor alone but of the entire visual processing system, including pathways and cortex. if you can see a light with direct fixation then it is being recorded by the cones, regardless of whether you can or cannot see color. color sensation is strongly dependent on illuminance level, and all colors fade to achromatic values before the cones stop responding to light entirely. scotopic adaptation is again a state of the whole visual system, not a specific threshold illuminance in the receptor cells alone. i'd be pleased if you would point me to sources that document otherwise.



#10 Starman1

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Posted 05 August 2014 - 01:12 AM

OK, threshold and scintillation:

http://arxiv.org/pdf/1405.4209.pdf

There was a long thread on this paper on CN, but the link is now broken and it will take me a while to find it.  It may be several pages deep in the Forum it's on, whichever forum that is (I don't remember).  But read his description of how scintillation makes a big difference in momentary visibility of a point object.

 

Only the center 1 degree of the Fovea is devoid of rods:

http://www.aoa.org/o...ht-vision?sso=y

 

Threshold of mesopic vision:

http://www.darksky.o...ments/is136.pdf

 

The limit stars to which I refer are below the threshold of mesopic vision.  Hence, no cones are involved.

The normal wandering of the eye assures that some rods will be involved.

I maintain that when a star is faint enough to be invisible 100% of the time with direct vision, it is not visible 100% of the time with averted vision either.  Some of the faint stars around M57 have shown me this.  I also did a substantial number of limit tests with an 8", and discovered the same thing.  That would be the 4th category in my visibility gradient in my last post.

 

Yes, I am implying the variance is large.  And due to many factors:

--scintillation

--color index of the star

--cleanliness of the optics

--low light scatter

--steadiness of the air (seeing)

--transparency of the air and contrast of the star with the background sky

--darkness of the sky

--visual acuity

--optical quality

--age and lens transparency (and even the spectrum of the light passing through the aging lens.

--altitude of the star from the horizon (related to both extinction and chromatic blur from refraction)

and more......

 

If Crumey is correct about scintillation, and ones eye's sensitivity varies a bit, that would go a long way toward explaining why I've found a range of over a half magnitude in a "limit" observation (ranging from 16.8 to 17.35 in my 12.5" in 21.4mpsas skies.  The factors I mention also all come into play.

I have, however, not done as much probing of the limits in the 12.5" as I did in the 8", simply because there is a dearth of charts with stars from 16 to 19 to use as limit charts.

Roger Clark's "Visual Astronomy of the Deep Sky" has excellent chrts down to about magnitude 16, but does not go deeper.



#11 drollere

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Posted 07 August 2014 - 01:38 AM

don, we seem to be having a sidebar here, but i wanted to thank you for the crumey citation. this interested me because i was just using the example of contrast sensitivity in relation to adaptation level in another topic.

 

let's agree to agree that the central 1º of the apparent visual field is not reported to the brain by any rod receptors whatsoever, which is what the AOA page states. therefore, if you are looking directly at a star image, what you see is being reported by the cones. when the cones fail to respond, the image disappears: this is the second threshold i was asking for.

 

"normal wandering of the eye" is very easy to stabilize within much less than a 1º tolerance: 1º is a quarter held at arm's length. the point is you can be aware of when you are holding a star in direct fixation, and it's only in that mode that you would see it if it is above the cone threshold.

 

unfortunately the page by the DSO strikes me as misleading. although it does point out that dark adaptation involves changes other than in the retina, it claims that "Below the intensity of moonlight, the cones cease to function and the rods alone are responsible for what is pure scotopic vision." this is incorrect, if it is misinterpreted to mean that the cones do not function during dark adaptation: they certainly do, which is why we can not only see a first magnitude star with direct fixation even though we are fully dark adapted, we can also see color in betelgeuse or mars.

 

this is the error that several folks on CN make when they speak of a specific *light* as being either scotopic or mesopic. lights are neither one nor the other: only the visual system is either one or another, and whether a specific light is visible depends on the specific luminance of the light and where on the retina it is imaged. mesopic or scotopic is a state of adaptation of the visual system to the entire light environment.

 

typically, unless you have recently looked at the moon or a computer screen, your eye is entirely in scotopic vision when you either visualize a star directly, or see it with averted vision only, or cannot see it at all.

 

i think you have made the problem more complicated than necessary. after all, your quote your limit magnitude variation between 16.8 to 17.35, which is a range of 0.55, and for my purposes the rough average of perhaps 17 would do just fine.


Edited by drollere, 07 August 2014 - 01:42 AM.







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