#1. "RE: Can any experts tell me what this is?" | In response to Reply # 0
Mon 17-Sep-12 01:40 PM | edited Mon 17-Sep-12 01:41 PM by elec164
My vote is for a plane, and nicely captured!
Reason being is that the contrail forms somewhat just behind the plane while meteors the smoke trail seems right at the meteor. Also meteors burning up in the atmosphere usually would apear as a bright source of emitted light whereas this object seems to be unlit and just occluded the moon.
It appears to be a triangular shape on one side with a separation from the cloud trail which seems to be what I would expect a plane to do.
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#3. "RE: Can any experts tell me what this is?" | In response to Reply # 0
I'm mystified but I do not believe it is a plane, and I think your second image demonstrates that...
The first image, with the long streak, was apparently shot at 1/200s. The second image was shot at 1/8 that speed, or 1/25s.
The slow shutter speed well stopped the plane, with a clear image. That suggests the object in the top image, if it were a "small" object like a plane, was traveling at an extremely high rate of speed.
The basic distance formula...
D = distance from subject (any units you wish)
a = angular diameter in arc-seconds
Size in units of D
The width of the moon is about 1800 arc-seconds. The trail is about half the width (just eyeballing it)
The trail therefore spans about 900 arc-seconds...
Assume a distance of a mile, for discussion purposes. S = 23 feet (approximate)
23 * 200 = 4600 feet per second. That is a fantastical speed of about 3000 MPH.
The above assumes a plane or other "small" object, not a contrail.
Double the distance and the speed doubles (work the formula). The object would have had to be quite close in order for the speed to be some "reasonable" sub-sonic value.
If it were a meteor, it would have been quite big because it would have had to be very distant in order not to "be on fire" in the atmosphere so I think we can rule that out.
If it were any small object traveling at a speed anywhere near what I compute, the chances of capturing it in a 1/200s image is minuscule and astronomically small - pun intended . You might know if the streak was visible to you for some time (as a contrail would be, slowly drifting across the face of the moon). That would be a critical clue.
If it were a contrail then the formula is not very helpful because the contrail could be just about any size, up to miles long. If it was a contrail, it would have been extremely dense. The moon shines through thin and even moderate clouds, and images well above black.
I've never photographed contrails in front of the moon so I don't have any experience with the relative density of dense contrails verses how well the moon will shine through. Clouds that hide the moon are quite thick, on the order of thousands of feet thick (cumulus or heavy stratus).
The above does not give me any clue what it is, just a process of elimination to try to figure out what it isn't .
I don't ever recall seeing a similar image with a dark opaque streak through the moon. This begs for a net search to try to find similar images.
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#4. "RE: Can any experts tell me what this is?" | In response to Reply # 3
Mon 17-Sep-12 06:37 PM
I see your reasoning and I can only add my observations.
The in focus plane was shot at 7:59 PM heading west for a landing into LAX. The shot in question was shot about 9 minutes later. The moon had risen and I did raise the lens to compensate, a 300 2.8 on a solid tripod with RRS BH55 ballhead.
The object in question was heading east southeast ... I mention it because that would put the object above the LAX flight path higher and more distant.
Where the plane was visible to the eye, I shot three pics of that plane, I did not see the object in question when shooting. It was a post surprise. My assumption at the time was it was pure chance that I got the shot. Very fast, maybe?
Only other point I can think of was the perceived angle of the object. What optically does it suggest assuming I didn't alter the angle when raising the lens? Is the object heading down or have I raised the camera and tilted slightly? I need to revisit the originals
#5. "RE: Can any experts tell me what this is?" | In response to Reply # 3
Mon 17-Sep-12 07:32 PM
>The first image, with the long streak, was apparently shot at
>1/200s. The second image was shot at 1/8 that speed, or
>The slow shutter speed well stopped the plane, with a clear
>image. That suggests the object in the top image, if it were
>a "small" object like a plane, was traveling at an
>extremely high rate of speed.
Well Neil you are certainly more experienced and knowledgeable about this than I, so I guess I should defer. But Im still thinking plane. I would think the altitude of the plane would influence how this looked as well as focal length used. What kind of setup were you using Harry? The maximum aperture is listed as 0 so you obviously used a setup that didnt pass on the lens information. Im thinking you may have used a telescope with either prime focus or eye-piece projection?
I live in a densely populated urban area within the flight path of 5 airports and the approach path of two of them. I at times have tried to catch a plane in front of the moon, but have yet to get anything to be proud of. The ones Im trying to get are on an approach for landing and are probably at around 2000 to 5000 feet. And the poor attempts I have captured so far sort of look like Harrys second example.
But during the day I can often see jets flying in flight path lanes that are WAY up there. You usually cant hear them, and often you really dont see a plane; all you see is the emitted contrail. Sometimes the contrails stretch the entire sky, and sometimes they quickly dissipate not to far from the source (similar to what is seen in the first shot in question).
I did find this example . While possibly not a great example, it does show how the contrail can occlude the moon in a similar manner in which Harrys first shot shows. But this example the plane looks to be about the same altitude as Harrys second example whereas the first one is flying at a much higher altitude. Its pretty cloudy here now, but when I get clearer skies and fortunate enough for the flight lanes to be overhead, Ill try to image those high fliers with my Bigma and see how much of the frame they fill.
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#6. "RE: Can any experts tell me what this is?" | In response to Reply # 5
Tue 18-Sep-12 03:32 AM
>> I would think the altitude of the plane would influence how this looked as well as focal length used.
The altitude would matter, in the context of distance to the camera. I mentioned that in my discussion of the application of the distance formula. Double the distance and the computed speed doubles.
The point I tried to raise is that the object would have to be very close (a few hundred yards or less) in order for the computed speed to be somewhere around the speed of typical aircraft.
An example... let's assume a jet descending, flaps down, at 150mph, which I think is a reasonable number for a minimum speed.
150mph = 220 feet per second (fps)
Now let's estimate a maximum possible speed, and that would be the speed of sound, 1126 fps. Aviation is not allowed to break the sound barrier in populated areas. Commercial aircraft can do about mach 0.85 or about 950 fps. I doubt a military hot dog pilot would risk going much more than mach 0.90 to 0.95 around LA, so mach 0.85 is within 10% of the max for any conventional aircraft.
At 220 fps, the camera to subject distance to cover 1/4 degree in 1/200s = 252 feet.
At 950 fps, the camera to subject distance to cover 1/4 degree = 1088 feet.
The numbers suggest that if that were a plane, it passed within 252 - 1088 feet from the shooter. If that plane passed within 1100 feet of Harry he would have noticed it . In a big way .
That is why I suggested that if it were a plane, it would have been a mile or two distant, minimum. But then the computed speeds are way too high.
Focal length does not matter here because the distance computation only requires angular diameter of the "subject" (the streak) and distance. If the focal length doubled, the moon is twice as big, the streak is twice as long, and the streak is still half the diameter of the moon (roughly), so it is still roughly 0.25 degrees long.
Similarly, suppose you wanted to photograph a 6 foot tall man in Silhouette against the moon, such that he exactly spanned the diameter of the moon.
At what distance would you shoot that scene?
The distance formula gives you a precise number:
d = subject distance
s = size (6')
a = 1800 arc-seconds (average)
Answer: 688 feet
(this is a useful formula to tuck away. A person(s) in front of the moon is a classic image)
The focal length does not matter. Short focal lengths will deliver a tiny moon and a tiny man. Long focal lengths will deliver a big moon and tall man but in all cases, at a 688 foot working distance from the man, the image results in a man the height of the moon. Same principle.
I'm not sure I am even disagreeing with you, except I do not believe the streak was made by an aircraft sans contrail. While I accept it could be a contrail, it begs the question of the opaqueness of contrails.
One other reason it is not a solid object...
Let's assume a solid object 10% as long as the black trail. If so, then each moon pixel behind that streak was blocked for only 10% of the exposure time. If the object length is 20% of the streak then
the blockage only lasted 20% of the exposure time.
The result would be a "ghost image" similar to a person walking through a long exposure scene (we all did that one when we got our first SLR, right?). The image would record 80-90% moon and only 10-20% (in density) of our mystery object.
The fact that the streak is very dark, black or nearly so, precludes the possibility of an object of small size relative to the length of the trail.
The more I think about it, only a contrail makes sense. And the streak looks like a rapidly dissipating contrail.
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#7. "RE: Can any experts tell me what this is?" | In response to Reply # 5
Tue 18-Sep-12 11:51 AM
Another mathematically derived factoid for reason-ability checking...
If this is an exhaust contrail then the aircraft would have to be at about 20,000 ft altitude or greater because vapor contrails do not form behind lower flying aircraft.
(Numbers vary on the lowest possible altitude but in the real world most aircraft ascend to 30,000 feet or slightly higher and then level off. Fuel economy is greatest at those altitudes because air resistance declines with altitude).
Assuming the camera clock was accurate, the image was taken at 8:07 PM PDT when the moon was 4 degrees 49 minutes in altitude.
Assuming an altitude of 30,000 ft, that puts the aircraft at a distance of 67 miles. Derived from the same distance formula.
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#8. "RE: Can any experts tell me what this is?" | In response to Reply # 7
Tue 18-Sep-12 01:33 PM
Thanks for your time and knowledge on this. I'll have to try and squirrel away the information so I have it when the need arises!
But I think I have discovered the main difference in our approaches. You are working from a premise that the streak was made during the exposure. I'm working from the premise that the streak and plane are static in the image. So in that sense the "a" of the plane in your formula would be zero rendering "S" as zero.
Of course "a" could have been some value fewer than 1degree (seconds would be the correct unit ???) rather than zero (perhaps more likely), but I think that would be negligible I would think.
As to speed, well using the same rough assumptions, a typical large commercial aircraft crusing at about 30,0000 feet would do so at about 500 to 600 MPH.
There is not much detail available on the object, but if I grab the image and upsample it in PS, on the left side of the streak is a triangular shape that is separated from the streak by about the amount I would expect a contrail to lag behind a plane.
And as you suggest, anything moving creating a streak during the exposure that long would be moving rather fast, and anything entering the atmosphere that fast would burn up making the leading edge an emitted light source with the light diminishing down the trail.
As such I still see the subject and streak as being fully existing during the exposure and frozen in capture by the shutter speed.
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#10. "RE: Can any experts tell me what this is?" | In response to Reply # 8
Tue 18-Sep-12 02:23 PM
When I entered the discussion it was not clear to me if there was any consensus on the nature of the image- a fast moving streak or an effectively static contrail.
I wanted to dispense with the idea that it could be a fast moving object streaked in the image. And I think we are all in agreement on that now.
That leaves an aircraft emitting a vapor contrail. The linked image convinced me that the opacity of a contrail is sufficient to block the light of a full moon (I had never previously thought about that idea).
My last post or two attempts to mathematically reconcile a viable dimensional model - size of aircraft and distance, relative to the altitude of the moon at the time and the fact that the aircraft must be quite high (cruising altitude) in order to leave a vapor trail.
For various reasons I do not believe it could be a wingtip vortex contrail from a landing aircraft.
As you can see from my last post I am having trouble coming up with a viable model . But that is what makes this an interesting exercise. And at worst it illustrates that various permutations of the simple distance formula can be used to compute various facts from the study of astronomical images (and any photographic images, for that matter, even macro images).
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#9. "RE: Can any experts tell me what this is?" | In response to Reply # 7
Tue 18-Sep-12 02:11 PM
This is an interesting mathematical puzzle
The well imaged aircraft in the second image spans about 114 arc-seconds, based on counting the pixels across the aircraft verses the diameter of the full moon.
I don't know aircraft well enough to ID the model. A short 737 is about 100 feet in length. A 747 230-250 feet.
Assuming a short 100 foot long aircraft, the distance is 34 miles. Assuming a long 250 foot aircraft the distance is 85 miles.
I previously estimated the distance to the streak in the first image at 67 miles, assuming an aircraft at a cruising altitude of about 30,000 feet.
The problem here is that the aircraft in the first image is at best a spec of a couple of pixels length. If you enlarge the image you can see what appears to be a speck leading the trail, and the contour of the trail does approximate the contour of a typical aircraft exhaust contrail.
There is a fair amount of uncertainty in these computations but I think the aircraft, if that's what it is, should be much larger, by about a factor of 5-10.
It could, of course, be a small military jet, something in the F16 class. Such an aircraft is more like 50 feet in length. But even in that case I would expect a larger speck.
For example, assuming a 50 foot long F16, at a distance of 70 miles, the aircraft would span 27 arc-seconds. The moon in the first image spans 475 pixels (approx.) and the moon, on that day, spanned 33' arc or 1980" arc. That is 4.16" per pixel.
An F16 at 70 miles should span 6-7 pixels on the posted image, assuming its flight path was reasonably perpendicular to the optical axis of the lens (may not be a good assumption???). But given the wingspan of a typical aircraft the observed size may not change much regardless of orientation.
The speck at the head of the trail (left side) spans 2 pixels. We are still off my a factor of 3. In order to reconcile that the small aircraft would have to be at about 210 miles distance, and if it is a commercial aircraft that puts it at 600 miles or more.
Harry, having the original image, you may be able to make a better estimate of the size of the leading "speck" in the first image.
Am I correct that the 2nd image was shot with a TC and then downsized a bit? Or you up-rezed a 300mm image beyond 100% pixels? It doesn't affect the computations here - I'm just curious as to the exact circumstances of the two shots and attempting to reconcile my numbers. At 300mm on a D7000 I calculate the diameter of the moon that night at 604 pixels, which is smaller than your second image of 742 pixels.
Another note, not terribly important. I think the aircraft is traveling from right to left, as the 1st image is oriented. And the moon is orientated just about right for the rising full moon on that night. You are facing east-southeast at an azimuth of about 117d. That would put the aircraft moving in a northerly direction, not east to southeast. Or maybe you interpret the trail differently than I do?
my Nikonians gallery.
#11. "RE: Can any experts tell me what this is?" | In response to Reply # 9
Tue 18-Sep-12 03:15 PM
>This is an interesting mathematical puzzle
>The problem here is that the aircraft in the first image is at
>best a spec of a couple of pixels length. ...
I went back to the original and tried more exposure and sharpening but it's not much improved. I'll post another cropped pic with the above applied at the following Smugmug link.
>Harry, having the original image, you may be able to make a
>better estimate of the size of the leading "speck"
>in the first image.
Both were taken with the same gear, no TC. In Smugmug I chose a larger version on the plane landing. If you go to the gallery you can chose the size on the right side of the picture. http://hsswan.smugmug.com/Other/temp/8943065_GVNXV6#!i=1851441001&k=SvGbDFH
>Am I correct that the 2nd image was shot with a TC and then
>downsized a bit?
>Another note, not terribly important. I think the aircraft is
>traveling from right to left, as the 1st image is oriented.
>And the moon is orientated just about right for the rising
>full moon on that night. You are facing east-southeast at an
>azimuth of about 117d. That would put the aircraft moving in
>a northerly direction, not east to southeast. Or maybe you
>interpret the trail differently than I do?
I agree the object in question is traveling in a southeastern direction, right to left in frame away from me, while the one landing is traveling west. The camera is facing an east southeast direction. BTW, if I turn the camera 45 degrees I'd be showing the Griffith Observatory about 1/2 mile distant and a little further is Mt. Hollywood my true north from the spot. Not germane to this thread but to the forum overall. BTW, thanks for the investigation! It's very insightful in context.
#12. "RE: Can any experts tell me what this is?" | In response to Reply # 11
Tue 18-Sep-12 06:13 PM
Well, your "Original Size" SmugMug moon sizes are another mystery. They are roughly 850 pixels in diameter. Because of the low horizon they are stretched or pulled slightly due to atmospheric refraction or general poor seeing at that low altitude. I get numbers of 863, 840, 821 and 863 for the widths and heights of the two moons. But the widest direction is reversed on the two images. I decided that 850 is the number I would use. That is surely within 5% of the actual number.
That diameter, in pixels, corresponds to almost precisely the value I compute at 420mm (300mm with a 1.4x TC). I revisited my computations and don't see anything wrong with them, certainly not to the tune of almost 40% deviation. That 40% is 10x any error I may have in the measurement due to bad air. So I guess that will be another mystery on top of a mystery
I just checked my formula against one of my full moon images shot just after the 12/21/2010 solstice lunar eclipse with my D300 and 500/4 AFS at 500mm (no TC). My actual image is 838 pixels against a computed size of 855 pixels. That is a margin of error of 2%, which is probably within tolerance of the actual focal length of the lens verses spec (Nikon may have fibbed a bit, to the tune of 10mm).
Your image is the same size or slightly larger than mine. Your camera has 14.9% more pixels in each dimension (16mpx vs 12mpx) but my lens is 60% longer. Thus my confusion...
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#13. "RE: Can any experts tell me what this is?" | In response to Reply # 12
Wed 19-Sep-12 03:49 AM
You know I do have 1.4 TC for that Nikon 300mm ... you may very well be right about that. Usually I don't bother with it because the lens is so sharp I can crop the same and have a better picture without it but that night I may have used it. I just don't remember.
I just took a break and walked out front to sit in the evening cool and I saw a meteor. It came out of the south heading for the San Fernando Valley and must have been seen by the whole basin. It was a short view before going out my line of sight but very cool and strange. Not a normal sighting for me. A meteor this evening, Friday the shuttle.
#14. "RE: Can any experts tell me what this is?" | In response to Reply # 13
Wed 19-Sep-12 02:29 PM
One mystery solved
I always set the non-CPU Lens data when using MF lenses, even in a case like this where a manual exposure without metering might be used. It eliminates the confusion you had.
Good catch on the meteor!
my Nikonians gallery.
#15. "RE: Can any experts tell me what this is?" | In response to Reply # 0
Sheesh, you guys and all that whiz-bang math! Anyone can see the moon is really Pac-Man in disguise and he's about to open his mouth and munch on an alien!
One thing I love about space is that it is an ever changing, mysterious place. I love a good mystery!