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OK Sketchers, try this

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#1 John Jarosz

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Posted 15 September 2012 - 07:22 AM

APOD for Sep 15, 2012

#2 GlennLeDrew

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Posted 17 September 2012 - 06:43 PM

In M57, The absolute brightness of the red H-alpha emission is weaker than the dominant blue-green O-III. And given the eye's very much weaker sensitivity to this red light, visually its intensity is almost certainly less than 1/20 that of the O-III. The color of the latter is not far above the threshold of detection at maximal eye pupil diameters, and so it's doubtful that the red is actually truly detectable.

And aperture here only helps to make color easier to see by virtue of permitting a large exit pupil while providing a comfortably large size. Once an eye-matching exit pupil is employed, further increases in aperture do not increase image surface brightness.

Further, before the advent of color imaging, were there reports of seeing red in nebulae, even with the largest telescopes any astronomers would have peered through? Beside the demonstrated illusion that in comparison dim grey patches appear reddish beside less-dim grey patches which are greenish, our familiarity with the photographic appearance today must be at least an unconscious source of bias.

#3 mdowns

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Posted 18 September 2012 - 06:34 AM

In the past,when viewing m42 from a reasonable site,and with transparent skies, I could always trace out vast,ghostly amounts of red.This was with a 16" I had back then.My observing buddies could never see it,though to me,it was very obvious. Also,when compared to those same friends,I would see both yellow and red in faint stars that they could not. I don't doubt for a second that as you mentioned,our observations may be biased by photographs. However, some just might be more sensitive to different, sublte changes in light.Interestingly,I never saw the red in M42 with 10,12.5 and 13 " scopes but with the 16 it was great on transparent evenings.As for M57,never a trace of red in my experience,but then I've never seen it through a 40". :tonofbricks:

#4 IVM

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Posted 18 September 2012 - 09:52 AM

It is a good sketch no doubt, but with sketches like this, where does texture end and where do real details begin? It is very hard to tell because of the technique.

Frankly I think texture is exaggerated in most deep-sky sketches, mostly because it is the limitation of the common techniques - they produce texture whether the artist wants it or not, and it is difficult to smooth it out enough without sometimes actually physically destroying the sketch. So here we have a lot of fine texture which I am certain is not seen visually (I haven't looked through an f/3 40" like this, but have looked through a professional 24" RC that I had to myself for a couple nights). There is a fairly gradual transition as far as size from this clearly artifactual texture to the many little wisps. The thicker wisps should be seen. The multitude of the thinner ones should be regarded as an extension of the texture-producing technique.

The people in charge of APOD (hats off to them - the site is huge in science education and inspiration - but let me say something here) are professional guys. And it is well known that the vast majority of professional astronomers have no idea how deep-sky objects look visually and what visual observing actually entails. So, the sketch is good, great in fact - to those who understand the conventions used and how these texturing conventions have interfered with presenting real detail clearly. But its appearance on APOD (otherwise a photography website) will contribute to the unrealistic notion of the general public that is APOD's audience about how celestial objects look to the eye. They will think that the problems of deep-sky observers producing boring, bleak sketches are lack of technique and lack of aperture ;)

#5 GlennLeDrew

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Posted 19 September 2012 - 02:40 AM

Interesting, and worthy, perspectives, IVM. Food for thought!

About red in M42. The only bit I've detected was in the bright, inner Huygenian region, where surface brightness is highest.

#6 fred-burgeot

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Posted 20 September 2012 - 02:41 PM

Hi,

I am the author of the sketch.
All that I have drawn have been seen, color and texture.
There is no artefact due to the technique, the wisps directed toward the central star are intentional. I don't pretend having represented each of them at its real place, but it was this impression at the eyepiece. Some of my companions have seen it too.

Glenn, as in the subject «sketches with a 40''», you're trying to refute my observation by all means, but never observational. The theory obeys the observation, not the reverse. You should practice with a big telescope before having so much convictions.

You should be happy that a drawing was published as an APOD, instead of trying to demonstrate that the choice was wrong.
The only thing that this publication can contribute, is to remind that we can observe with the eye, not only the CCD sensor.

IVM, a 24'' clearly shows less than a 40''. The resolution is almost halved and the brightness almost divided by three. It is like comparing a 24'' and a 14''. And it's not only theory, I have already observed in a 24'' ;-))

Fred.

#7 banatop

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Posted 20 September 2012 - 03:35 PM

Hi Fred,
your drawing of M57 is magnificent. As I said in the other thread 'Sketches with a 40" :
"You and your circle of friends are some of only a few people on earth who know what can be seen through the eyepiece of a 40" Reflector. For everyone else it's just theory."
Congratulations on your many fine observations and thanks for making them available to us all through your drawings,which I for one, prefer to any theory.
Amicalement, John

#8 IVM

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Posted 20 September 2012 - 05:23 PM

Fred, welcome to Cloudynights, and let me say I envy you your observations with the huge scope - and your drawing technique. I may prefer a different (less textured) technique for nebulosity, but I do envy your skill with your chosen technique.

That said, there will be friendly and perhaps at times robust critique of techniques in general - whether you use them or someone else. And there will be discussion of how much exactly can large aperture help - again, irrespective of who looks through the scope and what they say they see. Also there will be discussion of how exact a notion of the Universe the public without access to telescopes acquires through our drawings and photos. I am glad that you weighed in in this thread because otherwise we were (I was) here sparring with the shadow, so to speak.

I do not doubt your words. That said, I doubt I would see such fine texture and details through a 40", and my "problem" would not be eyesight. I almost never see any texture in nebulous DSOs. What I know about the physical limitations and capabilities of the instrument does affect what I perceive looking into the ocular - probably on the subconscious level already. Reality is neither theory nor perception. I like to attune my perception to reality using theory as a crutch if necessary. Theory we are talking about in this thread is quantitative principles of optical instrumentation and observational astronomy, not some arguable hypotheses.

On the technical level, please note that the 24" that I mentioned was a professional Ritchey-Chretien (retired professional, of course). I admire large short-focus (f/3 in this case IIRC) Dobsonians and their gutsy owners. But these instruments cannot achieve or approach the theoretical resolution of their aperture due to the limitations of the mirror figure and of the focal ratio (that would be collimation and eyepiece aberrations). More importantly, both a 24" of any design and a 40" are limited by the atmosphere.

I accept your argument about light grasp. Indeed light grasp may be of primary importance for DSO details. I think that the effective resolution on DSOs does often scale as the aperture because magnification giving the same surface brightness scales as the aperture. However, the details that cannot be resolved due to aberrations and atmosphere will remain unresolved regardless of brightness.

The texture may be perceived by the observer, but individual grains in it are not of physical origin but of perceptual. This is how I regard the texture in your sketch. The perceived texture may also be anisotropic due to the overall shape of the object and brightness distribution in it. This is how I regard the finer threads of nebulosity in your sketch (radial in the middle and circumferential on the margin). I do not doubt that you perceived them - not after you have explained now that they did not arise merely from the drawing technique.

Again, welcome to Cloudynights and I hope to see more of your sketches here in the future. They are beautiful and, as you have now clarified, reflect with precision how you see the Universe using some of the rarest and most powerful instruments on this planet.

#9 Diabolo

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Posted 21 September 2012 - 12:32 AM

On the technical level, please note that the 24" that I mentioned was a professional Ritchey-Chretien (retired professional, of course). I admire large short-focus (f/3 in this case IIRC) Dobsonians and their gutsy owners. But these instruments cannot achieve or approach the theoretical resolution of their aperture due to the limitations of the mirror figure and of the focal ratio (that would be collimation and eyepiece aberrations). More importantly, both a 24" of any design and a 40" are limited by the atmosphere.



It's not exactly a classical dobs. Like small pro scope is the mirror cell is an astatic one and the optical quality is really good. I haved try to take pics with it and unfortunately, I'm everything but good. I was lucky for the first shots on the moon, dmk31 camera, powermate4x. (Sorry to put pics links in the sketching section of the forum)

Rupes Recta


Arzachel


Lucky we were, during the night Fred Burgeot draw, the seeing was really good. That help a lot !
Just bought a Basler640, hope I will catch some plantes but really, it's not my cup of tea. Those links only to show the resolution of the OTA.

#10 IVM

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Posted 21 September 2012 - 09:30 AM

Thanks, I looked up that photo in your first post. This is a great-looking scope (both of them actually). I understand that it has a sophisticated mirror cell - not like in a Dob. Gee, we should probably remember that in true original Dobs the mirror was glued to a piece of plywood and that was part of what made the design groundbreaking. The word "Dob" is used nowadays for fairly sophisticated instruments in comparison. But of course their cells are not astatic like yours. At one time I wanted to build a cell like that myself... it is not easy! I am glad to hear that the quality is good.

#11 Diabolo

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Posted 27 September 2012 - 11:30 AM

Thanks IVM, I agree, yes, dobson scope still evolve, year after year, that the bright side of Atm ;)

#12 GlennLeDrew

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Posted 28 September 2012 - 03:40 PM

My objections are based on:
- My own views of M57 through a 24". Yes, the image at given exit pupil diameter is smaller, which is a handicap; better to have a larger extent when the subject is faint.
- The knowledge the the eye's sensitivity to the H-alpha wavelength is no better than 10% that of the O-III wavelengths, and is worse still when dark adapted.
- The knowledge that in M57 the H-alpha emission has at best 1/2 the intensity as O-III, meaning that visually the intensity is no better than 1/20 that of O-III. Not only is the red weak, but it is of very low contrast color wise, at the 5% level, and has the green dominance to 'fight' against.
- The well known (?) illusion of dimmer regions of a colorless image take in a reddish hue.
- My frequent observation of auroras, which visually showed only green, but in photos I'd taken and developed later revealed surprising red structure.

This last point is what really drives home to me just how insensitive the eye is to red. To be seen as such, the red atomic oxygen and molecular nitrogen components of auroral emission have to have a surface brightness rather higher than the H-alpha component in M57.

If one were to fly out to M57, from immediately beside it there would still be insufficient surface brightness to see the red. (The view would be like that delivered by a 35m telescope at 5,000X--and 7mm exit pupil for maximum image surface brightness.)

My last reason for doubt is that I've not encountered any report of red seen in M57 by other observers using big telescopes before the advent of color imaging, or photography especially. Surely the experience and equipment of the day were up to the task. If an unbiased observation mentions no red, it's to me a more reliable indicator than a modern observation of same after long familiarity with its colorful aspects.






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