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Visual Observation Illusions

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#1 azure1961p

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Posted 31 May 2013 - 09:09 AM

I think its inescapable that if you see enough drawings by amateurs there arises optical illusions that seem to repeat again and again. I thought Id name a few. If anyone wants to add that's fine.

1. Spokes on Saturn, I don't mean the real ones some have truly observed but the ray like projections even modest aperture produces particularly at lower magnifications.

2. False Albedo Contrasts: A dark feature along side a light background and the dark feature appearing darker where it borders the line of contrast.
Similarly a bright features border appearing brighter where it borders dark.

3. Small Blank Disk illusion: A small orb like Uranus having a "Y" pattern across its face or radial forms emanating from its center.

4. False Color : a warm tone next to a neutral color giving it a cool or bluer/green cast. Like wise a cool tone creating a warm tiny to neutral tones areas.

5. Continuity of line or form: Seeing Cassinis extend farther than its actually resolved linear or angular.. This can also be said of a chain of spots, dashes or other irregular forms.

6. False spots or features filling a blank void .

7. Persistence of vision image doubling: this one has a long history. Enckes Minima drawn as a series of lines at times almost looking like LP grooves. In this case the seeing is making multiples of the same image scattering details. The way to avoid it is in realizing when they double or triple that its not a moment of clarity but thermal distortion. It would seem this is an obvious thing to note but its amazing how many people don't recognize it. I was duped into *resolving* a spot on Jupiter as double when the imaging forum the next day proved me wrong. The slight of hand here is that in seeing MORE features the eye is filled into believing its better seeing.

Im trying to avoid direct correlates to these as some observations are fairly contentious here. So Im keeping it abstract.


Pete

#2 azure1961p

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Posted 09 June 2013 - 03:07 AM

No.8: false generalized color . Saturn for example through my 70mm will have a creamy beige look that pervades everything. Even the colorless rings will appear cream/beige and Cassinis can appear as dark brown grey.
This is never the case through the greater resolving power of larger scopes.

No.9: Preconceived expectations materializing pseudo features that might even exist but to no true detection by the observer.

No.10: no prior knowledge or expectations allowing a feature to go undetected lost in an ambiguous threshold greater attention would have
brought to light.

Both 9 and 10 would seem to be mutual hazards of each other.

Pete

Ps: this is a topic with low appeal but im adding to it for sake of rounding it out.

#3 David Knisely

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Posted 09 June 2013 - 01:02 PM

It may be of low appeal, but some of this does have to be said. Not everything that a person may think they "see" is actually there. There are some details which there are physical reasons why they may not be visible in a given aperture, yet they still sometimes get reported in apertures too small for them to be firmly seen. Then, there are reports of detail that observations with larger instruments cannot verify to exist at all. It all comes down to the human eye and brain interpreting what is seen. Clear skies to you.

#4 azure1961p

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Posted 09 June 2013 - 06:13 PM

Thanks Dave.

Pete

#5 Asbytec

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Posted 11 June 2013 - 08:28 PM

Well, once you delve into abilities of individual human eyes, one opens a realm of uncertainty. That realm of uncertainty, though, is bracketed by the physics of the focal plane on one end. Further, observing at the edge of human abilities is complicated by common illusions and exuberance.

But, how do we know if something we observe is an optical illusion? We have to sketch what we see, mach bands included (unless you recognize them as such). Often enough we doubt our observation and miss something that was actually there, and strangely enough we often do the opposite, too. Strangely, we sketch things we see, or think we see, and they are not there or not doable in that aperture.

So, how does one miss a faint observation that actually was there, but often include observations that are not? Good question.

#6 azure1961p

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Posted 11 June 2013 - 10:56 PM

Well its a different ball game today as you know Norme. Its almost certain you can follow up a visual made one night with posted CCD images in one of the imaging forums and chances are very good it'll show more than anyone is seeing visually with some exceptions. Its always illuminating to see the reality in CCD images that may have been merely clues at the eyepiece. I think the imaging forum makes us better observers!!!

Pete

#7 stanislas-jean

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Posted 12 June 2013 - 02:02 AM

The best way to to illusions!
We saw many observers having looks on images and draw details picked from especially when they was not existing (see mars reports for examples).
Training eyes is a long term work and the result of lot of parameters. It is crucial to quantify our own limits by certain ways especially when it is recognised to be at instrumental abilities.
The best example is encke division where so very few observers had captured this in fact. It is so difficult to recognise something actual when the sizing is approaching the dawes limit divided by 10.
Proceed to tests for being convinced of that and compare this to people who hardly support the contruary.
Illusion is a field moving with the observer abilities itselves. It's not to believe something because I am not able to see similar feature and vice versa. Nobody is a reference, absolute, but what we can do is to test ourselves our abilities. Now to find the necessary means for reaching this.
Stanislas-Jean

#8 E_Look

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Posted 12 June 2013 - 12:56 PM

Even with digital data, many people do not understand the theories or concepts of error, or more accurately, uncertainty. Is something there in the image? Of course it depends on the resolution of the optics, but often there'll be questions because of precision. All measurements have inherent uncertainty and the amount depends on the instrument/system and the observer. An easy example might be a discussion of the angular length or diameter of some celestial object. A more disputable one might be the angular size of some feature on a planet's surface, say, like one of those green blotches on Mars' disk. And then, this applies even to its contrast or brightness levels, leading to questions of whether it is really there or not. This is why we often say, "It's (not) within error (of the measurement)."

#9 azure1961p

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Posted 12 June 2013 - 02:14 PM

Hi Ed,

I've always found it a blast that imagers can get high res CCD pix of Saturns Terby White Spot!!! Its a testament to the achievement if technology that the CCD is capable of seeing the same effects of contrast however unreal.

Pete

#10 ngc 9999

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Posted 12 June 2013 - 08:02 PM

What about the surface details on Uranus that Stanislas Jean sees?

#11 azure1961p

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Posted 12 June 2013 - 11:42 PM

I didn't want to bog down in the contentious points that arise with that and I was hoping to have a discussion about these phenomena without singling out anyone . Uranus has some dedicated and seasoned observers on both sides of the aisle that I just dont want to try and hash out here.

I think the examples sited in the OP stand on their own without the need to attach examples to current observations. Thanks though.

Pete


#12 stanislas-jean

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Posted 13 June 2013 - 01:10 AM

The better way for progressing can be the performance of the observation of long distanced cibles, un-known during the test, practiced under different conditions (lighting, seeing, contrast).
This is harsch to see the results at final.
No matter of discussion about for ourselves and a final status at the issue without illusion considerations: feature is here or not, the feature shape is or it is wrong, etc...
A suggestion.
Uranus was on these kind of tests, ashen light was also, thin features was also, etc... This makes representation and simulation of the viewing difficulties.
Stanislas-Jean

#13 BrooksObs

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Posted 13 June 2013 - 09:37 AM

I would elaborate just a bit on the sore spot David K's early post cited concerning the illusion of observers thinking they are seeing details, features, or things that simply are not there. This situation is a lot more common than most wish to admit. In interacting with other observers over many years I've seen this played out again and again, even among seemingly experienced observers.

It is not at all unusual for an observer to "see" what he anticipates he will, or hopes he can. "Glimpsing" a faint comet, or something like a very faint variable star seemingly at the very threshold of detection in the scope (that really isn't visible at all in that aperture) is quite common, particularly among overly enthusiastic observers. I often get impression that certain deep sky observer's observations reflect a strong influence from having seen, or studied, images of an object's appearance prior to going to the eyepiece. Reports of features/detail "seen" with say a 4" that are in fact threshold in a 20" turn up on-line all the time.

One observer can also easily influence what another thinks he sees when observing together. The most striking example of this I ever saw involved Halley's comet when it was first coming into visual range in 1985. At Stellafane the owner of a huge scope of about 30" aperture allowed me to locate and subsequently show the near-threshold comet to a long line of folks just before dawn. There soon came a point where the first light of day swallowed up the comet and I wanted to call it quits and go to bed. However, quite a number of folks still had not had the opportunity to get a look. Now this big scope was undriven, so it was necessary to reset it about every other observer to just keep the comet's field in view. I explained to the group that the comet was now no longer visible, but the folks in line begged for me to set the scope to the correct spot one more time anyway so they might have a chance to try. Well, I did (with dawn brightening fast by now) and took my leave of the group. The scope's owner related to me later that nearly all who subsequently took a look exclaimed, "Oh yes! I can still see the comet!"...in spite of the fact drift had long since carried its location far outside of the eyepiece's field! This is hardly my only story of this nature.

So, my advice is to try to lean just a bit toward the conservative side whenever you "think" that you are glimpsing something that is excedingly faint and vague, or highly unlikely to be detectable in your scope's aperture. Ultimately you'll be glad you did.

BrooksObs

#14 azure1961p

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Posted 13 June 2013 - 02:27 PM

Good points all and a humorous example(s) as well. I think the thing about illusions is that age and experience does t make them disappear - it merely becomes easier to see the pseudo details for what they are. That said Im still fooled from time to time and it serves to reaffirm the need to be bold in ones goals but just as conservative in questioning the validity of things. Ego I think can skew things a bit and as u mention particularly when there is peer pressure how ever slight.

Pete

#15 Widespread

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Posted 13 June 2013 - 02:42 PM

Once, I imagined I saw pink color in the Orion nebula. With a 90mm refractor. :p

I've noticed since then that I can see it in the general sky background. Maybe I'm just an optimist with rose-tinted retinas.

#16 dscarpa

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Posted 13 June 2013 - 05:20 PM

I think seeing conditions are a factor in illusions for high power lunar-planetary in particular. Unless conditions are very good my mind has to cleanup the image to try to get a true representation of the objects appearance. I totally agree that imagers are a major plus for visual LP. Long long ago when it was believed the dark areas on Mars were vegetation they looked pretty green to me. David

#17 Asbytec

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Posted 13 June 2013 - 10:36 PM

"...particularly among overly enthusiastic observers. I often get impression that certain deep sky observer's observations reflect a strong influence from having seen, or studied, images of an object's appearance prior to going to the eyepiece."

That's a good point. I don't have any humorous examples, but I will tell you after much enthusiasm and without knowing the PA and sep of 42 Ori in advance, I was able to 'see' the companion despite the bright first ring. I pegged it to within about 10 deg PA. Remember, Pete?

Was it an illusion? Even though the sightings were fleeting and very hard to lock down, I doubt it. It was a very difficult observation, and it took much time and effort to finally decide where the companion is.

I just think this makes the case for careful observation doing difficult things...things that are, in theory at least, might be possible. If they might be possible, then sometimes they will be possible. But, what about those things that are not possible? Can training overcome the physics of the focal plane? I don't think so, but what are those physics, specifically? Are they hard and fast barriers that cannot be breached? Often, probably so...

Dscarpa, another good point on seeing and our minds cleaning up the images. I have been blessed with diffraction limited seeing. Such seeing has allowed me to observe Osiris (bright crater on Ganymede) and the 'apparent' elongation of Io in a modest 6" CAT. Again, with careful study and attention, I am pretty sure those are not illusions. That bright spot on Ganymede was indeed there, and Io was indeed elongated (in appearance, not reality) when compared to Europa's circular disc.

I have never seen ashen light nor markings on Uranus, though. Have others?

#18 David Knisely

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Posted 14 June 2013 - 12:42 AM

Once, I imagined I saw pink color in the Orion nebula. With a 90mm refractor. :p

I've noticed since then that I can see it in the general sky background. Maybe I'm just an optimist with rose-tinted retinas.


Actually, a few people can see faint reds in M42 at the right powers and under the right conditions. I have seen a few hints of them in my 100mm f/6 refractor using the DGM Optics NPB filter, but they are only hints. Otherwise, it takes my 10 inch and 14 inch Newtonians to show much of the faint reddish hues in M42 and M8, although again, the NPB filter with its red secondary passband helps to bring out the reds a little. Clear skies to you.

#19 David Knisely

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Posted 14 June 2013 - 12:51 AM

BrooksObs wrote:

It is not at all unusual for an observer to "see" what he anticipates he will, or hopes he can. "Glimpsing" a faint comet, or something like a very faint variable star seemingly at the very threshold of detection in the scope (that really isn't visible at all in that aperture) is quite common, particularly among overly enthusiastic observers. I often get impression that certain deep sky observer's observations reflect a strong influence from having seen, or studied, images of an object's appearance prior to going to the eyepiece. Reports of features/detail "seen" with say a 4" that are in fact threshold in a 20" turn up on-line all the time.


One has to be a little more careful when discounting some claims of faint objects being visible in smaller apertures than most people see them in. Visual sensitivity varies widely (as well as observing experience, which can help nearly as much). I have seen different observers have as much as nearly two full stellar magnitudes of sensitivity difference when it comes to their faint limits of observation. When I first started out with my old 8 inch Newtonian, I could rarely get much past 14.2, but with some experience (and learning that higher powers help push your magnitude limit), I got close to 15th magnitude. Indeed, one night, I managed the magnitude 15.1 central star in M57 with a 9.25 inch SCT, and occasionally could see stars in the region of the Ring down to 15.3. This didn't happen all the time, but it did happen, so conditions as well as aperture all factor in to make a magnitude "limit" less than hard and fast. Indeed, despite there being some faint targets in the Herschel 400 observing list, Jay Reynolds Freeman managed to catch all of them in a 55mm refractor (although many were probably quite marginal). There are limits to what can be seen, but often, they may be less hard and fast than many people might make them out to be. Clear skies to you.

#20 dscarpa

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Posted 14 June 2013 - 10:56 AM

In addition to resolution contrast is also very important. My WO ZS-110 and IM715D have lots of the latter which seems to give them punch above their weight class. Viewing Uranus on a night of very good seeing using my C9.25 at 380X it looked like the poles were ever so slightly darker than the rest of the disk. The mak at the same power didn't show it. As to Venus no ashen light but I've seen very faint dusky markings and an irregular terminator. The ZS is my favorite scope for our sister planet. David

#21 azure1961p

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Posted 14 June 2013 - 11:38 AM

I nice testimonials to target/aperture. Both Neptune and Uranus are particularly unforgiving of smaller apertures. Neptune at 350x through my C6 is not at all as nice as through my 8" and at least at 350x in that scope.

Venus Ive learned not to bother with if not for seeing issues than notorious fertile ground for optical illusions. I still look now and again, but its tempered with a lot of respect for its magician like nature.

I'm certain beyond a shadow of a doubt if some one images the number 5 on Venus in photoshop, and pushed it as real - you'd have a line of folks with number 5 drawings of this planet.

Pete

#22 bassplayer142

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Posted 14 June 2013 - 11:39 AM

Interesting topic. I believe the only answer to solving this is more aperture! :cool:

#23 David Gray

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Posted 14 June 2013 - 01:08 PM

Too much going on with me at present so can't get too involved with this. A worthwhile thread which details many of the pitfalls that an experienced observer should constantly address, to the point of it becoming second-nature/automatic.

Venus: what concerns me with some drawings I see of this planet's diaphanous features is when they are executed under reported seeing conditions that can even render Jupiter's features challenging - i.e. such as IV-V seeing! As I have alluded to here: http://alpo-j.asahik...12/v121019z.htm
Likewise with Uranus!

#24 t.r.

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Posted 18 June 2013 - 08:34 AM

Perhaps a little authoritative knowledge about the human visual system can help drive home the point of illusions and in particular, human variance...

https://www.hf.faa.g...manVisSys2a.htm






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