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Amalia
Reged: 10/16/04
Posts: 5165
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Quote:
Quote:
Amalia...and you said (in a whisper) that I didn't know the secret. schade.
ok...maybe I joke about the fiery reds and purples, but yes, after various sorts of "pre-stimuli" to the eyes, I've also seen deceptively reddish hues in objects like M42.
I doubt they're deceptive. I think what happens is that you dark-unadapt back closer to the rod/cone break while viewing the moon and see less "grey" from the rods when you then look at M42. You'd probably see less filaments, but what you'd see would be more coloured.
It probably only works for fairly bright objects -- I think the results if you tried it on a galaxy is that you'd just see one "colour": black.
I think you are right.
I also think with the "pre-stimuli" Jeff uses he will also be able to see little pink pigs dancing a waltz with purple-yellow girafs on the Andromeda Galaxy - he just doesn't dare to report it.
Amalia
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sixela
Postmaster
Reged: 12/23/04
Posts: 8967
Loc: Boechout, Belgium
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About the constellation in which those "I just hit my head on the car roof" stars lie...
For you, it would probably be Coma Berenices.
For me and others, probably Corona Borealis. At least by their look in a Tintin comic book.
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400mm f/4.46 David Lukehurst truss Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
Edited by sixela (01/18/05 03:29 PM)
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jmoore
Carpal Tunnel
Reged: 10/01/03
Posts: 1959
Loc: Beaufort, NC
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Quote:
I also think with the "pre-stimuli" Jeff uses he will also be able to see little pink pigs dancing a waltz with purple-yellow girafs on the Andromeda Galaxy - he just doesn't dare to report it.
No comment.  But those little colorful animals do sound familiar...
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Hardin 12"
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 215
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There are many studies in the scientific literuture concernig color vision.
These two are among the best I found:
Change of Color Appearance in Photopic, Mesopic and Scotopic Vision
Jae Chul SHIN, Hirohisa YAGUCHI 1 and Satoshi SHIOIRI 1
Graduate School of Science and Technology, Chiba University, 1-33 Yoyoicho, Inage-ku, Chiba 263-8522, Japan
1 Department of Information and Image Sciences, Chiba University, 1-33 Yoyoicho, Inage-ku, Chiba 263-8522, Japan
(Received October 6, 2003; Accepted February 12, 2004)
OPTICAL REVIEW Vol. 11, No. 4 (2004) 265–271
# 2004 The Optical Society of Japan
The color of night: Surface color perception under dim illuminations
JOEL POKORNY,1 MARGARET LUTZE,1 DINGCAI CAO,1,2 and ANDREW J. ZELE1
1 Depar tment of Ophthalmology and Visual Science, University of Chicago, Chicago, Illinois
2 Depar tment of Health Studies, University of Chicago, Chicago, Illinois
(Received March 16, 2006; Accepted April 7, 2006!
Visual Neuroscience ~2006!, 23, 525–530. Printed in the USA.
Copyright © 2006 Cambridge University Press 0952-52380 06 $16.00
DOI: 10.10170 S0952523806233492
I strongly suggest reading them. Here is a summary.
Color vision is mediated by cones. There are three types of cones. The processing of cone signals is very sophisticated: from conversion to lightness and hue (the r/g and b/y opponents signals) in the retina, to color constancy processes, to contextual interpretation of colors.
Besides cones there are also rods, which are very sensitive to light.
In daylight only cones are active (rods are saturated). Rods become active ad about 0.3 Lux (14 magnitudes per square arc second). As soon as rods provide signals they interfere with the r/g and b/y signals of the cones. At this level there are many mistakes in color recognition.
As the light level further decreases, S cones (those sensitive to blue) become inactive first. Color vision is thereafter mediated by rods and L-cones (those sensitive to red). The number of different hues recognized drops dramatically until, at about 0.01 lux (17.5 mags per square arc seconds) only two hues ("warm", a reddish-gray, and "cold" a blue-green-gray) are discerned. They are only a rough approximation of the real colors. Bright colors, including bright pink, become "grue"-gray, red and dark colors become reddish-gray.
With further decrease of illumination only rods mediate vision, but they use the same nerves -or communication channels- used by cones so that a first source of unreal color perception (illusions) stems from this fact. In addition, the different brightness level are interpreted by the brain as "colors". There is a interesting experiment which shows that any color is seen as "grue"-gray when seen alone, but when diffent levels of brightness are simultaneously seen, the same color (whichever color), if not the brightest, is seen as reddish (not all people have the same amount of illusion).
The second paper is very clear about this.
Bottom line: we can see rudimentary colors in mesopic vision (bright planetaries and maybe the trapezium). I doubt we can see the red at all (L-cones are far little sensitive to H-alpha). We cannot see at all the real colors of the greatest majority of nebulae, but we are often tricked by our visual cognitive system. We interpret moderately bright areas as reddish when close to brighter areas that we see as greenish.
As an example look at the following picture: http://forum.astrofili.org/userpix/1932_orione5_1.jpg
Which color are the wings? Reddish? Wrong. They are gray. This example is an illusion due to the color constancy priocesses that take place in our brain.
Edited by Mauro Da Lio (11/23/07 05:34 PM)
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 5974
Loc: Beatrice, Nebraska
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Mauro Da Lio wrote:
Quote:
Bottom line: we can see rudimentary colors in mesopic vision (bright planetaries and maybe the trapezium). I doubt we can see the red at all (L-cones are far little sensitive to H-alpha).
A few planetary nebulae do show some red color in moderate to large apertures. The most famous example is probably IC 418 in Lepus, which is sometimes known as either the "Pink Planetary", or the "Raspberry Nebula". The outer shell is often distinctly reddish in hue at low to moderate powers, while the central core is more bluish in color. Another example is the tiny planetary Campbell's Hydrogen Star (PK64+5.1) in Cygnus, which, at least in my 9.25 inch SCT, looks about as reddish as an M-class star (looks a little like the companion to Eta Cassiopeiae). As for diffuse nebulae, the parts of M42 which show reddish colors to me tend to be well *away* from central Huygenian region and the Trapezium rather than adjacent to them. These reddish regions tend to be narrow and often located the outer regions of the nebula in places similar to those shown in photographic images. It does take some aperture to see the coloration, but it does show up from time to time depending on how sensitive the observer's eye is to dim red light. Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 215
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Only if surface brightness is above L-cones threshold (say 17 mpsas) you have a chance to see real colors. Otherwise there are solid scientific studies that show that color perceptions at scotopic levels (say 20) happens but are illusions. But, even if the surface brightness in V band is above the typical figure of 17-18, that does not necessarily mean that the red fraction (H-alpha emission) is strong enough to stimulate L.cones. L-cones are weakly sensitive to H-alpha. Thus it might be that reddish percepition on nebulae that are on "average" above the theoretical limit for cones is still an illusiuon.
A proof might be to look at the nebula with a H-alpha filter. If you cannot see it then your L-cones are not stimulated by the H-aplha emission (rods do not see H.alha at all). You only detect OIII and other lines with rods.
As for what concerns M42, its brightness profile si shown here: http://www.clarkvision.com/astro/surface-brightness-profiles/introduction.html . According with it the reddish regions are below the thresholda (especially those farther from trapezium).
I personally saw reddish and greenish in the M42. However, as soon as i concentrated carefully only on the reddish part it became gray.
PS It might be interesting to find the surface brightness of those two planetaries in the "red" (H-alpha) part. I did not find that.
PS you may have a lot of difficulties to discriminate between real colors and illusions.
Edited by Mauro Da Lio (11/24/07 09:21 AM)
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Starman1
Postmaster
Reged: 06/24/03
Posts: 10186
Loc: Los Angeles
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Quote:
I have just read a post in one forum and a guy said that “we can not see any colour even with a giant scope”. But I doubt if it is true or not. Some veteran observers say that it is possible to see nebula colour through huge scopes, even small scopes. One guy claimed that the M42 that he saw was greenish.
But I feel confused at this moment. As a student who studying Biology, I have a concept that we can’t see things with colour. 
Is there anyone kind enough to tell me the truth? 
Scotopic vision is color-blind, so the faintest objects will all be seen in B&W. However, if the object is bright enough, and/or the scope is big enough, mesopic vision is activated (in which the cones are partially activated). Under these conditions, you can defintiely see color.
I've seen (and see on a regular basis):
M42: green near Trapezium, rose and peach hues in the fainter parts, reddish hues in the "arches" and a bluish tint to M43 and nearby NGC1977. In a 28" or larger scope, brilliant reds appear.
M8: rose hues
M20: rose hues in brighter parts
M17: rosy hue in "Swan" portion
M57: (big scopes) faint pinkish hue.
and blue or green tints in lots of planetary nebulae.
So yes, colors are visible, but where/when will depend on:
--size of scope (bigger scopes definitely see more color)
--light sensitivity of eyes (the genetic component)
--darkness of sky
--nature of filter transmission (if filter is used)
--experience at the eyepiece (dedicated double star observers learn to see quite subtle hues, for example)
I did an interesting test on M42 recently, using a filter slide and color filters. It was quite interesting to see which parts of the nebula were dimmed and by how much with each filter. I would defintely recommend a UHC-type filter that has a strong broadcast in the H-Alpha end of the spectrum. The nebula definitely has a strong H-Alpha component to its brightness.
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
Edited by Starman1 (11/24/07 10:24 AM)
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BillFerris
Carpal Tunnel
Reged: 07/17/04
Posts: 2435
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Quote:
Only if surface brightness is above L-cones threshold (say 17 mpsas) you have a chance to see real colors. Otherwise there are solid scientific studies that show that color perceptions at scotopic levels (say 20) happens but are illusions.
At 20 magnitudes per square arcsecond, vision is still in the mesopic range. When observing under a pristine dark sky where the sky surface brightness is in the 21.7-22.0 mags/arcsec.^2 range, vision is mesopic and colors--including red--are still perceptible. Stars, planets and bright nebulae show obvious color. Auroral displays and carbon stars are examples of celestial objects that commonly appear red to observers. And, of course, there's the ubiquitous red flashlight. If it looks red, vision is still mesopic.
If you want dark adaptation to progress fully into the scotopic range, you need to confine yourself to a dark environment for considerable time. High magnifications (2mm or smaller exit pupil) darken the sky enough that the eye will transition into the scotopic range after 15-30 minutes of continuous observing. It's one reason why the best deep-sky views often require high magnification. A piece of fabric over the head or similar aid to block stray ambient light also helps.
Bill in Flag
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Grand Canyon Adventure
Lowering the Threshold
18" Obsession
4.5" Meade 4500
10x50 Swift Audubon
Cosmic Voyage
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suburbanskies
super member
Reged: 12/18/04
Posts: 173
Loc: New Jersey, USA
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Quote:
PS you may have a lot of difficulties to discriminate between real colors and illusions.
Very interesting, Mauro. The red you *think* you see may not be red in reality!
Mark
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johnfdean
professor emeritus
Reged: 06/04/06
Posts: 543
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Definitely color can be seen. Since M42 appears to be the topic, I can certainly see green with hues of pink with my 14".
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Celestron C6 f/5 by Vixen with Polaris GEM
14" Tscope dob f/4.7
80mm Nighthawk on Eq 2
Celestron C-4 f/10 GEM
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 215
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Don and others. *Please read the referenced papers*. You can get them at any University Library.
As for what concerns the size of the scope: it does not increase surface brightness. So If a nebula is under cones threshold it will never get above whatever the size of the scope (see Bill Ferris' linked document).
As for what concerns the limits of mesopic vision I again invite you to *read* scientific papers and the related experiments. Bill, when I say 20 I mean the surface brightness *of an extended object*, not the environmental conditions for being dark adapted. Sure, you see the laser beam red, orange stars, green leds etc because they have very high surface brightness (not related with average environmental illumination).
Please read the scientific literature. Here is the "state of the art" referenced by paper 1.
ASTM. ~1994!. Standards on Color and Appearance Measurement. Phil-
adelphia: American Society for Testing and Materials.
Boynton, R.M. & Olson, C.X. ~1987!. Locating basic colors in the OSA
space. Color Research and Application 12, 94 –105.
Brown, W.R.J. ~1951!. The influence of luminance level on visual sensi-
tivity to color differences. Journal of the Optical Society of America 41,
684 – 688.
Buck, S.L. ~2004!. Rod-cone interactions in human vision. In The Visual
Neurosciences, eds. Chalupa, L.M. & Werner, J.S., pp. 863–878.
Cambridge, MA: MIT Press.
Cao, D., Pokorny, J. & Smith, V.C. ~2005 a !. Associating color appear-
ance with the cone chromaticity space. Vision Research 45, 1929–1934.
Cao, D., Pokorny, J. & Smith, V.C. ~2005 b !. Matching rod percepts with
cone stimuli. Vision Research 45, 2119–2128.
CIE. ~1951!. Proceedings 1951, Vol. 3, p. 37, Paris: Bureau Central de la
CIE.
Hecht, S. & Hsia, Y. ~1945!. Dark adaptation following light adaptation to
red and white lights. Journal Optical Society of America 35, 261–267.
Ishida, T. ~2002!. Color identification data obtained from photopic to
mesopic illuminance levels. Color Research & Application 27, 252–259.
Kohlrausch, A. ~1931!. Tagesehen, Dämmersehen, Adaptation. In Hand-
buch der Normalen und Pthologischen Physiologie, eds. Bethe, A.,
Bergmann, G.V., Embden, G. & Ellinger, A., pp. 1499–1594. Ber-
lin: Springer.
Kuehni, R.G. ~2003!. Color Space and Its Divisions, pp. 1– 408. New
York: Wiley-Interscience.
MacAdam, D.L. ~1978!. Colorimetric Data for samples of OSA uniform
color scales. Journal of the Optical Society of America 68, 121–130.
McCann, J.J. & Benton, J.L. ~1969!. Interaction of the long-wave cones
and the rods to produce color sensations. Journal of the Optical Society
of America 59, 103–107.
Middleton, W.E.K. & Mayo, E.G. ~1952!. The appearance of colors in
twilight. Journal of the Optical Society of America 42, 116 –121.
Nagel, W. ~1924!. Appendix: Adaptation, Twilight Vision and the Duplic-
ity Theory. In Helmholtz’s Treatise on Physiological Optics Translated
from the Third German Edition by JPC Southall, Third German Edi-
tion, Vol. 2, pp. 313–343. Rochester, NY: Optical Society of America.
Purkinje, J. ~1825!. Beobachtungen und Versuche zur Physiologie der
Sinne. Neue Beiträge zur Kenntniss des Sehens in subjectiver Hinsicht,
pp. 1–192. Berlin: Reimer.
Schneider, N. & Von Campenhausen, C. ~1998!. Color and lightness
constancy in different perceptual tasks. Biological Cybernetics 79,
445– 455.
Shin, J.C., Yaguchi, H. & Shioiri, S. ~2004!. Change of color appearance
in photopic, mesopic and scotopic vision. Optical Review 11, 265–271.
Simon, R. ~1904!. Über Fixation im Dämmerungssehn. Zeitschrift für
Psychologie und Physiologie der Sinnersorgane 36, 186 –193.
Smith, V.C. & Pokorny, J. ~1975!. Spectral sensitivity of the foveal cone
photopigments between 400 and 500 nm. Vision Research 15, 161–171.
Verriest, G., Buyssens, A. & Vanderdonck, R. ~1963!. Etude quanti-
tative de l’effet qu’exerce sur les resultats de quelques tests de la
discrimination chromatique une diminution non selective du niveau
d’un eclairage c. Revue d’Optique 3, 105–119.
Wald, G. ~1945!. Human vision and spectrum. Science 101, 653– 658.
Walkey, H.C., Barbur, J.L., Harlow, J.A. & Makous, W. ~2001!.
Measurements of chromatic sensitivity in the mesopic range. Color
Research & Application 26, S36 –S42.
Edited by Mauro Da Lio (11/24/07 06:04 PM)
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 215
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Quote:
...Next summer, try for color in the Dumbbell (M27). when it's at the zenith, the "bow-tie" sometimes appears greenish, and the "football" (the fainter sections at right angles to the "bow-tie") appear rose-colored.
From:
The color of night: Surface color perception under dim illuminations
JOEL POKORNY,1 MARGARET LUTZE,1 DINGCAI CAO,1,2 and ANDREW J. ZELE1
1 Depar tment of Ophthalmology and Visual Science, University of Chicago, Chicago, Illinois
2 Depar tment of Health Studies, University of Chicago, Chicago, Illinois
Visual Neuroscience ~2006!, 23, 525–530. DOI: 10.10170 S0952523806233492
"At the lowest illumination levels ... the reported colors can be associated with the relative scotopic luminance for the samples: samples seen as blue-green-gray are predominantly associated with high relative scotopic luminance, samples seen as red-orange are associated with intermediate relative scotopic luminance values..."
"...some of the visible samples when presented in the context of the other samples were identified as red or orange, whereas when presented individually each was identified as blue-green".
PS it seems to me that the fainter sections of M27 are dominated by OIII and are actually green, whereas it is the brightest part to be red (exactly the opposite of visual perception which tells reddish the fainter part and greenish the brightest) http://www.astrodon.com/oldsite/M27OIIIHAStarsRGB.jpg
Edited by Mauro Da Lio (11/24/07 06:51 PM)
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Starman1
Postmaster
Reged: 06/24/03
Posts: 10186
Loc: Los Angeles
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Mauro,
There is much literature to suggest that what happens when the scope gets larger is that the image simply gets larger, making it more visible.
Lots of literature suggests that the contrast ratio is the same in all scopes, so if color isn't visible in one scope it won't be visible in any.
However,:
1) The contrast of an object with the night sky may be 1.1:1, making the object only 0.1 brighter than the night sky. Double the size of the telescope, and the ratio becomes 4.4:4, for a difference of 0.4 The contrast ratio didn't change, but the delta did. Perhaps our vision sees the delta and not the ratio, because it is definitely the case that fainter objects are more visible in larger scopes.
2) Bigger scopes do see more color. Period. That's empirical, not theoretical. I've looked through scopes of 3" to 32" on the same night, all targeting M42, and seen increased color in larger apertures. Whatever visual analysis says, it has to explain what is empirically verifiable. What I KNOW is true is that fainter objects and details are more visible in larger scopes, and deep sky objects have more color in larger apertures. Any argument to the contrary is simply false. So stating that "If a nebula is under cones threshold it will never get above whatever the size of the scope" is inadequate to explain what I have seen. If the cones are active enough to see color in larger apertures, then I should see it in smaller apertures is the implication, yet this simply is false and verifiably so.
What is open to discussion is WHY that's the case.
3) a nebula filter makes a nebula more visible by increasing contrast. The nebula isn't any brighter with the filter in place, but the background is darker. However, adding a nebula filter to my 12.5" doesn't allow me to see any more of a nebula that can be seen without the filter in a 32" scope. Since the contrast ratio is the same in the big scope as it is in my smaller scope, and since the nebula filter increased the contrast in my scope, the 32" shouldn't have an improved contrast over the 12.5" with no filter, and definitely shouldn't have an improved contrast over the 12.5" with filter.
Eppur su muove. And yet it does.
Bottom line: a larger aperture mimics the contrast enhancement of a smaller aperture used with a filter.
This discussion has gone on over many years and many threads, and resurfaces periodically because the theory does an inadequate job of explaining the empirical facts. I suspect the issue is semantics rather than a hard disagreement, but, for me, it appears the spread of light quantities I mention in point 1 seems to have a significant effect on what is seen. The physico/psychological reason for it is, IMO, poorly explained in the literature, and I have read extensively on the subject. Clark's articles attempt to bring it to a mathematical explanation, but I am left unpersuaded. I SEE an improvement in brightness and contrast and color in larger scopes. Arguing that it doesn't happen gets nowhere.
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 5974
Loc: Beatrice, Nebraska
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Mauro Da Lio wrote:
Quote:
Only if surface brightness is above L-cones threshold (say 17 mpsas) you have a chance to see real colors. Otherwise there are solid scientific studies that show that color perceptions at scotopic levels (say 20) happens but are illusions. But, even if the surface brightness in V band is above the typical figure of 17-18, that does not necessarily mean that the red fraction (H-alpha emission) is strong enough to stimulate L.cones. L-cones are weakly sensitive to H-alpha. Thus it might be that reddish percepition on nebulae that are on "average" above the theoretical limit for cones is still an illusiuon.
A proof might be to look at the nebula with a H-alpha filter. If you cannot see it then your L-cones are not stimulated by the H-aplha emission (rods do not see H.alha at all). You only detect OIII and other lines with rods.
Well, with all due respect to the so-called "scientific" studies, they often do not directly or extensively deal with the exact observation conditions which we as experienced amateur astronomers work with. The studies may be somewhat valid under some conditions but probably not under others. I would have to see any study use specific astronomical targets, instruments, filters, and a large number of experienced observers before I would take its findings as being completely valid.
One particular incident convinced me of the possiblity of seeing reddish colors in some nebulae. Many years ago, Lumicon began producing their first line of Oxygen III filters. The first production run had a single primary passband for the OIII lines in the blue-green part of the spectrum only, but later on during another production run, a red secondary passband was introduced (either accidentally or deliberately). A friend of mine had one of the early OIII's while I got one in the second batch with the "red leak" passband which let through a lot of red light (including the H-alpha line, as I could verify using a spectroscope). We were both observing M42 one evening with our two ten inch Newtonians when I was somewhat startled to see some faint reds in certain parts of the nebula. I had never seen reds in nebulae before in *any* telescope or when using my narrowband filter, but I hadn't tried my OIII out on M42 at the time, as I had only recently purchased it. We could not see any red hues in the nebula using my friend's 10 inch, but in mine, they were faint but definitely present. We then compared our two OIII filters and noted that distant red radio tower beacon lights were visible through my OIII but not in his, so the reason behind the difference in the view was revealed. At the time, Jack Marling was president of Lumicon and we wrote him to tell him what we had uncovered. He wanted the filters back for detailed measurement, so we sent them to him, along with the "recommendation" that the red secondary passband be left in. I had also noted extensive but very faint reddish hues in M8 when I used my OIII. My H-Beta filter also has a red secondary passband, and with it, M42 is not as large as in the OIII filter, but what is visible again shows at least some faint red hues in the outer portions with only the innermost Huygenian region showing much in the way of the bluish color from the H-Beta line. I recently ordered a new 2" Lumicon OIII filter, and the red secondary passband is no longer present, so I don't see reds with it when observing bright nebulae. However, the DGM Optics NPB filter *does* have a nice high-transmission red passband for H-alpha, and with that filter, I once again can see faint reds in some nebulae provided I am using enough aperture and don't kick the power too high.
Some people do have enough red sensitivity to see faint reds while many others don't. I am sort of "in the middle", as I can see reddish hues in some cases but not others. I can, for example, just make out the faint reddish color of Mu Cephei with my unaided eye when it is around 4th magnitude, while some others may not see the color at all. In particular, some children sometimes have a *lot* more red sensitivity. I recall at a dark sky star party when we had an 8 or 9 year old youngster at a 12.5 inch Newtonian looking at M20, and without being prompted or told what he would see, he immediately mentioned the faint reddish color of the primary nebula. Thus, to just dismiss all claims of seeing reddish colors in some nebulae in moderate to large apertures is not only unfair, it runs counter to what repeated observations by some very experienced amateurs have observed. Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
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btschumy
Think Astronomy
Reged: 04/13/04
Posts: 1092
Loc: Austin, TX, USA
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Quote:
I think that most of us must see nebulae greenish, but as it's moslty "monochrome" we often interpret it as grey.
This comment got me thinking... Double star observers often note that it is the contrast with the companion that makes a star's color more vivid. I wonder if this would work with color in deep sky objects as well?
If our visual system interprets dim monochrome as grey, then perhaps introducing another color into the field would allow the monochrome color to be "seen".
A simple test that comes to mind is to view the object through an eyepiece with an illuminated reticle. Perhaps having the red of the reticle would allow the blue or green of a nebula to be more pronounced. Possibly the introduction of a green dot in the field would cause reds to be more pronounced.
Definitely something to try next time I'm out.
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Bill Tschumy
Where is M13? Freeware -- Add a new dimension to your observing.
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 215
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Quote:
...Any argument to the contrary is simply false.
....So stating that "If a nebula is under cones threshold it will never get above whatever the size of the scope" is inadequate to explain what I have seen. If the cones are active enough to see color in larger apertures, then I should see it in smaller apertures is the implication, yet this simply is false and verifiably so.
...The physico/psychological reason for it is, IMO, poorly explained in the literature, and I have read extensively on the subject. Clark's articles attempt to bring it to a mathematical explanation, but I am left unpersuaded. I SEE an improvement in brightness and contrast and color in larger scopes. Arguing that it doesn't happen gets nowhere.
I think you definitely should read the two papers. They explains what you and other people see, and why not al people see the same. I understand that some people may dislike the conlusions, but I think they are solid.
One example is that you see geeen the brightest part of M27 (which in fact is red, and reddish the fainter part, which in fact is greenish). Studies show that when vision is mediated only by rods colours are related to the context: brightest lights are greenish and middle lights are reddish. This exactly fits your experience and explain why you see green the red part of M27 and red the green part (I understant the disappointement, we all tendo to "believe" to our eyes).
When I mean scientific studies I mean works published on scientific journals *after peer review*. When I mean "literature" I mean that.
As for what concerns the size of the scope I think there is a little confusion. One object that is below the threshold of cone detection cannot be seen in color. However being above the threshold is a condition necessary, but not suffcient. You also need a given size for the colour patch to be recognized. That is why the experiments have been carried out with samples that were seen under an apparent angle of 8°. A smaller patch is more difficult do be recognised either as shape and, as colour. Here is where a bigger scope helps: it provides bigger images where colour can be recognised, but only if the brightness is above the cones' threshold.
In other words: if a nebula is below cone thresholds you will never see it in colors, If it is above, you need another condition: a telescope that provide a image large enough to be recognized. This is the concept you mean and which is explained by the "optimal magnification" by Clark. However in the case of colors you have also to consider that as you push magnfication the surface brightness decrease. This may not be aproblem for shape recognition (as long as the surface brightness is within the reach of rods) but may be a problem for cones, because they reach is far shorter. So in an attempt to magnfy the image to improve its perception, you may end in lowering the surface brigthtness below the thresholds.
Edited by Mauro Da Lio (11/25/07 12:02 PM)
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Mr. Bill
Carpal Tunnel
Reged: 02/09/05
Posts: 2334
Loc: Just passing through.....
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M27 definitely appears as an intense greenish blue "hourglass" in my 25x150 Fujinon binoculars.
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Mr. Bill
Oberwerk 100BT 45 degree + Hercules fork mount
15x70 AP binos + Paragon p-mount
120mm f/5 Orion achromat + Moonlite focuser
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/6.5 Antares achromat
150mm f/8 "bent" homemade achromat
8 inch Orion newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery split tube
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Edited by Mr. Bill (11/25/07 06:30 PM)
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 215
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Well, with all due respect to the so-called "scientific" studies, they often do not directly or extensively deal with the exact observation conditions which we as experienced amateur astronomers work with. The studies may be somewhat valid under some conditions but probably not under others. I would have to see any study use specific astronomical targets, instruments, filters, and a large number of experienced observers before I would take its findings as being completely valid.
This is a generic statement. I invite you to read the papers I pointed out and then say what is wrong with the procedure to the point that conclusions are false.
I will post here some resumé of the works so that the procedure is clear and the conclusions may be understood.
The only point that you raise and which might be relevant is not the *astronomical targets* in itself, but the factb that studies were mostly carried out with broadband stimuli. The response to narrowband stimuly might be somewhat different (but I do not expect completely different results). As for what concerns the experience there are two points: experince means a better processing if signals fron the retina. However if NO signals comes fron the cones no real color can be seen (again I do not mean that no nebula at all stimulate cones, however most of them are not bright enough).
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One particular incident convinced me of the possiblity of seeing reddish colors in some nebulae. Many years ago, Lumicon began producing their first line of Oxygen III filters. The first production run had a single primary passband for the OIII lines in the blue-green part of the spectrum only, but later on during another production run, a red secondary passband was introduced (either accidentally or deliberately). A friend of mine had one of the early OIII's while I got one in the second batch with the "red leak" passband which let through a lot of red light (including the H-alpha line, as I could verify using a spectroscope). We were both observing M42 one evening with our two ten inch Newtonians when I was somewhat startled to see some faint reds in certain parts of the nebula. I had never seen reds in nebulae before in *any* telescope or when using my narrowband filter, but I hadn't tried my OIII out on M42 at the time, as I had only recently purchased it. We could not see any red hues in the nebula using my friend's 10 inch, but in mine, they were faint but definitely present. We then compared our two OIII filters and noted that distant red radio tower beacon lights were visible through my OIII but not in his, so the reason behind the difference in the view was revealed. At the time, Jack Marling was president of Lumicon and we wrote him to tell him what we had uncovered. He wanted the filters back for detailed measurement, so we sent them to him, along with the "recommendation" that the red secondary passband be left in. I had also noted extensive but very faint reddish hues in M8 when I used my OIII. My H-Beta filter also has a red secondary passband, and with it, M42 is not as large as in the OIII filter, but what is visible again shows at least some faint red hues in the outer portions with only the innermost Huygenian region showing much in the way of the bluish color from the H-Beta line. I recently ordered a new 2" Lumicon OIII filter, and the red secondary passband is no longer present, so I don't see reds with it when observing bright nebulae. However, the DGM Optics NPB filter *does* have a nice high-transmission red passband for H-alpha, and with that filter, I once again can see faint reds in some nebulae provided I am using enough aperture and don't kick the power too high.
This is very interesting. I did not discard the possibility to see red, I said it is more difficult that seeing green (perhaps there are a very few cases). Your case suggests it may possible to see red in Orion Nebula, but there may be other possible explanations (I bet you can formulate some alternative exlanation, for example considere the bandwith of the two filters and how they might stimulate the L-cones and rods in different proportions -if you want we can better discuss this point... but the discussion is long and involves the point known as "metamerism"). I looked at the Orion Nebula with a red filter: The trapezium is red (yet I am not convinced I see the real red cominga from H-alpha, for the reason above). NGC 7662 is not seen at all in red. The real test I plan to do is look with a H-alpha filter. If the object is not seen then...
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Some people do have enough red sensitivity to see faint reds while many others don't. I am sort of "in the middle", as I can see reddish hues in some cases but not others. I can, for example, just make out the faint reddish color of Mu Cephei with my unaided eye when it is around 4th magnitude, while some others may not see the color at all. In particular, some children sometimes have a *lot* more red sensitivity. I recall at a dark sky star party when we had an 8 or 9 year old youngster at a 12.5 inch Newtonian looking at M20, and without being prompted or told what he would see, he immediately mentioned the faint reddish color of the primary nebula. Thus, to just dismiss all claims of seeing reddish colors in some nebulae in moderate to large apertures is not only unfair, it runs counter to what repeated observations by some very experienced amateurs have observed. Clear skies to you.
As for young people, they have pupil size of 7-8 mm, which provide, for the same surface brightness greater retinal iullumination (google "Troland": Troland is the measurement of retinal illumination: with a pupil size of 8 mm at the naked eye the retina receives 2.56 times the light of a 5 mm diameter pupil from the same surface brightness: that is 1 magnitude more which migh make a big difference since the brightest nebuale are near the limit of coloru perception and 1 magnitude may be enough for meking them viusible). SO the other hand, however, the sensitivity on cone cells to light is muche less likely to vary from people to peole.
Anyway the fact that one perceives red does not make a proof, because there is the proof that red is "percieved" at low illumination levels in some context.
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Starman1
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Posts: 10186
Loc: Los Angeles
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Mauro,
Pictures of M27 distinctly show the "bow-tie" section, the brightest part, as greenish, while the outer section of the long oval show red. This is exactly what I see in large scopes.
So the example you give of opposite color perception, while it may be true, doesn't apply to this object.
Ditto on M42, where photos show exactly the same colors I see visually and in exactly the same places.
And if size of image is a requirement of color perception, as, it is argued, is the faint image visibility, then I can understand the theory of perception.
Just one problem: I have looked through, nearly simultaneously, scopes of two sizes at exactly the same magnification, where the image sizes were identical. The larger scope showed a brighter image, more color, and an improved contrast enabling the visibility of fainter features.
What was different, of course, was exit pupil, since the two different scopes had different f/ratios. Obviously exit pupil plays a role in determining visibility of color, and object extension.
I play the devil's advocate, here, because I enjoy the conversation and I have read extensively on this subject, but why do all the theories of vision, related to the visibility of objects in a telescope, ignore the physical number of photons gathered by the larger aperture? Why isn't the image of an extended object simply easier to see in the larger scope because the larger scope gathers more light? Raise the intensity of the light level, and we all see more and better.
The Moon is bright in a small scope, but it is a LOT brighter in a big scope. At double the power of a scope 1/2 the size, the brightness per unit area calculates to be the same. Yet, the image is brighter. Period. It's easy to calculate why a stellar point is brighter, but not so easy to understand why an extended object like the Moon is brighter. The size of the image is not the explanation that works. Image intensity is simply higher.
Sometimes I think that people who argue about this resemble the old story about the argument about how many teeth are in a horse's mouth. Simply looking through telescopes of various sizes will convince you that the images are brighter, more colorful, and display more contrast in larger scopes.
An image intensifying eyepiece, like the Collins I3 will show more details in a larger scope. Why? Because there are more photons available at the source. Our eyes respond to photons. We see the images as brighter in bigger scopes because there are more photons and because the contrast delta has been stretched.
Maybe. 
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Don Pensack
12.5" Truss Dob, 5" Maksutov
Sustaining Lifetime IDA member, TeleVue junkie
Edited by Starman1 (11/25/07 01:06 PM)
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 215
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Here is a short summary of a little part of "The color of night: Surface color perception under dim illuminations". Just to stimulate the curiosity of reading it entirely. I bet most opinions will change thereafter.
The paper decribes experiments carried out in order to quantify the errors in color detection at dim lights. It also put the results into a well established solid framework of theroretical explanations and furthre data and knowledge (dismissin it without reading is frankly silly).
The results are not in contrast with what people of the astronomical community say to see. The results only points out that most of the colors (of astronomical targest) are not real (this is disappointing, I understand).
The paper starts with an experiment of color recognition at illumination levels of 1, 0.3, 0.1, 0.03, 0.01, 0.003, 0.001, 0.0003 Lux (in astronomical units they correspond to 12.5, 13.75, 15, 16.25, 17.5, 18.75, 20, 21.25 mgnitudes per square arc seconds). These are illumination levels, the luminance of samples is somewhat lower and depends on their reflectance. The paper givers the data for computing both scotopic and fotopic reflectance of all tested sample. The brightes sample are about 1.5 mag less that the illumination level.
24 samples of 8 colors (each colors in three different grades of lightness) were presented simultaneously (in one experiment) or alone (in another) to test persons (who have no kind of color blidness according to alla the other available tests).
The 8 colors were purple (very interesting because it is blue+red), blue, green, gray, yellow, orange, pink, red (the exact colorimetric data are given).
People were asked to indentify the colors of all the samples among the 8 colors. If one somple in three different test got at least two identical names by one test person that name was recorded as the color associated to that sample by that people at that level of illumination (it is the "perceived" color, not necessarily the real color). If the sample got three different identification it was classified as not recognized.
The samples were presented so that they werer seen under an apparent angle of 8° (lerss means that recognition problem, like those pointed out by Don might interfere). In othert words, in stronomical terms, it is like looking with a scope big enough. As an example M97 (there are pepole who say seeing colors on it) has a surface brightness of about 21.6 and a diameter od 200". When seen with a 20" scope at 71x at 7 mm exit pupil it looks 21.9 (account for some light losses) and is 4° diameter (less that the 8° of the samples). Seeen at 142x it is 8° diameter but 23.4 (still some people say they see color there).
The most interesting results are those of the gray. Here are some (but there are so many that I again encourage the reading of the original paper).
At 1 Lux (12.5 mpsas) gray is identified correctly by all testers. But starting at 0.3 Lux 20% of the samples were not recognized (three differend identifications), which is explained by the fact that rods intervention interferes with the correct processing of color by retina (r/g and b/y opponents). In the remaining 80% of the cases gray was indicated as purple, gray, or green (depending on which of the three sample lightness).
At lower light levels different people tend to have their different color perception: blue, green, gray. In the scotopi levels color perceptions remaing and sometimes are also richer and more variegated, Not only "grue" (this is the most comon definition for all bright lights in scotopic region) but also "yellow". At the lowest level there were recordings of reddish for the intermediate level gray. Thi latter point is very interesting. The intermediate level gray is seen as "red" by some observer. However, in another experiment he same gray at the same illumination level is seen as green. The darkest gray at the immediately higher illumination level (same as the medium gray at the lower lavel) is not seen as red. This means only that high level post processing of brightness data occurs in the cognitive process.
Edited by Mauro Da Lio (11/25/07 03:09 PM)
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