Air lens from objective to target...

[Fred Bohl]

The time has come for a discussion and sharing of what optical affects are caused by the lens that is the air between the scope objective and the target. I'll start this off with the following about mirage.

From our good friends at Wikipedia:

A mirage is a naturally occurring optical phenomenon in which light rays bend via refraction to produce a displaced image of distant objects.

An inferior mirage is one in which the mirage image appears to be located below the real object.

Inferior images are not stable. Hot air rises, and cooler air (being more dense) descends, so the layers will mix, giving rise to turbulence. The image will be distorted accordingly. It may be vibrating; it may be vertically extended (towering) or horizontally extended (stooping). If there are several temperature layers, several mirages may mix, perhaps causing double images. In any case, mirages are usually not larger than about half a degree high.

Heat haze, also called heat shimmer, refers to the inferior mirage experienced when viewing objects through a layer of heated air; for example, viewing objects across hot asphalt or through the exhaust gases produced by jet engines. When appearing on roads due to the hot asphalt, it is often referred to as a highway mirage. Convection causes the temperature of the air to vary, and the variation between the hot air at the surface of the road and the denser cool air above it creates a gradient in the refractive index of the air. This produces a blurred shimmering effect, which affects the ability to resolve objects, the effect being increased when the image is magnified through a telescope or telephoto lens.

A superior mirage is one in which the mirage image appears to be located above the real object.

A superior mirage occurs when the air below the line of sight is colder than the air above it. This unusual arrangement is called a temperature inversion, since warm air above cold air is the opposite of the normal temperature gradient of the atmosphere during the daytime. Passing through the temperature inversion, the light rays are bent down, and so the image appears above the true object, hence the name superior. Superior mirages tend to be more stable than inferior mirages, as cold air has no tendency to move up and warm air has no tendency to move down.

Superior mirages are quite common in polar regions, especially over large sheets of ice that have a uniform low temperature. Superior mirages also occur at more moderate latitudes, although in those cases they are weaker and tend to be less smooth and stable. For example, a distant shoreline may appear to tower and look higher (and, thus, perhaps closer) than it really is. Because of the turbulence, there appear to be dancing spikes and towers.

 

[F Class John]

I am part of a pretty large email group for shooting and at one point, one of the shooters who is an atmospheric professional couldn’t handle us using the word mirage anymore and wrote this to the group in hopes we would start calling it scintillation instead....

I have considerable experience in the aerospace field computing atmospheric scintillation, which is, I believe, the correct term for what your gunners see. A good analogy is to think of the atmosphere as consisting of bubbles which move around, depending on winds and thermals. The temperature inside each bubble determines its refractive index, which varies from bubble to bubble. The target image passes through these bubbles, and can be bent (refracted) slightly, causing the observer to see a shifted or distorted image. Because these bubbles move randomly and in varying degrees, with no correlation, the image appears to wiggle around at random jumps and amplitudes. This is what is referred to as scintillation, and is the same thing that drives astronomers nuts. As you well know, when the wind is still, and there are no significant thermals, the scintillation is negligible.

There are computer programs available which allow one to simulate this effect for targeting purpose ( as in guided anti-tank missiles ). The astronomers have developed laser beam technology which allows them to compensate their observed star images to subtract most of the scintillation. Few people could afford that.

Mirage is a related phenomenon , but a static , large angular displacement, which makes the sky appear to lie on the highway and look like water.

 

[Fred Bohl]

Good start John! Let's add some more fuel to the fire see the attached pdf

 

[Fred Bohl]

For those not fluent in the metric system. In the Air Composition.pdf in my last post, for the chart of
atmospheric particulate matter, 1 micrometer = 0.00003937 inches.

Therefore, even with a high power scope, individual particles of atmospheric particulate matter will not be visible. However, their aggregate effect will be similar to the haze caused by water vapor which results in a dimming and defocusing of the target image.

I'm sitting at my desk looking out at the park with a line of young trees (mixed species) about 20 yards away and a dense natural woods (mixed species) about 200 yards away. A line of rain showers passed thru with no ground level winds. The changing rain density demonstrated the corresponding dimming and defocussing of the image of both the nearby trees and more so the woods even though the individual drops were only visible just outside the window.

 

[JEFFPPC]

Ahhhhhh, so what's your point? Big words or little words those that learn to deal with and use it win. Those who don't.....don't

 

[dolek]

Ahhhhhh, so what's your point? Big words or little words those that learn to deal with and use it win. Those who don't.....don't

Winning ? There was no mention of competition . I think you added in some little words .

Great stuff folks,interesting .

 

[Turbulent Turtle]

I apologize for my late arrival here; as an explanation, life sometimes gets in the way of fun. (And 2020 has provided more "fun" than I ever thought I would experience.)

I understand this thread is an offshoot (offspring? off kilter?) of the thread I started where I presented my hypothesis (not even a theory,) that ED and Super ED glass help tame the degradation to the IQ from a scope looking through "mirage".

Leaving optics and lens type aside, I think Fred wants to discuss the phenomenon itself. In another thread here, I posted the following thought:

http://forum.accurateshooter.com/threads/unshootable-mirage.4009016/page-3#post-37819948

The term "mirage" is used incorrectly here, but since everybody calls it mirage, why fight it. The correct term would be "heat haze" or "heat shimmer," but for some reason we like our French terms in shooting: Mirage, Ogive.

A real mirage (the optical illusion kind) is when, for example, you're driving on a long straight road and in the distance, there appears to be water, even a lake, on the road ahead. That is a mirage and you do not need a riflescope to see it.

What we experience in our riflescopes is heat shimmer, which is a gradient in the refractive index of the air caused by the variation in temperature between the air at the ground and the cooler air above. This heat shimmer effect is enhanced by telescopic sights to where you can actually see it, which is why when you reduce the magnification the effect diminishes to where you do not even see it without an optics of some kind.

Also, it's a lot more fun saying "I'm watching the mirage" instead of "I'm watching the heat shimmer." It has more of a touch of erudition to it, don't you think?

In fact, the French Air Farce used a plane designed and made by Dassault starting in the 60s up to until recently (I don't keep track of that,) called the Mirage. There were many models. They didn't pick the name because if was shimmering, the name and the plane were a follow on to the prior model, the Mystere (mystery in English, it looked like a knock off of the F-100 Super Sabre). If you ever watched movies about people lost in deserts, there was always the obligatory scene with a mirage drawing people to it as they were thinking this was a lake or some body of water only to realize it was a mirage all along. I believe there is even a casino in Las Vegas by the name of The Mirage. It's the same concept; you think you're having fun and then your money disappears.


But as I said above, everybody (mis)uses the name, so we'll stick with it.

The shooting mirage is created by the sun. The sun's rays will heat up the ground and the air and of course, this occurs unevenly depending on many factors like the surface being heated, the moisture content, and so on, and so on. As the warm air rises from the ground and with different water vapor content, it affects the light coming from the target to your scope and does funky things to it. This shimmering or haze, or what we call "mirage" is very sensitive to air movement (wind) and starts moving at the faintest whisper of wind.

You really can't see this shimmer with your unaided Mark 1 eyeball. My belief is that the depth of field of the eye is far too great to be bothered by this phenomenon. However, you can certainly see it for discrete occurrences such as the back end of a jet engine or the blacktop of a road. But if you look over a field, it's really not there. When you use optics, things change and I believe this has to do with the much reduced DOF of optics compared to your eyeball. When your optics present a shallow DOF, the shimmering going on at the focus distance does show up and messes with the IQ of your optics. The modifier disk that I mentioned above, increases the DOF of your optics by the simple expedient of stopping down the aperture by one F-number. Photographers know that when you stop down, you increase the depth of field. A lot more of the image is now in focus, or within an acceptable value for the area of confusion, and the effect of the shimmer is somewhat drowned out in the wider DOF. This is at the cost of brightness in the scope, again, the equivalent of one F-stop, or 50% of the light.

 

[JEFFPPC]

Winning ? There was no mention of competition . I think you added in some little words .

Great stuff folks,interesting .

No, to me big words. My purpose and involvement on this is to improve my shooting skills and all that is inolved such as reloading, wind reading, bench manners, and such. Whatever I read on here I try to test and incorporate into the shooting game, what I believe is the purpose of this forum. Winning need not be about first place but learning practical application towards the goal, my goal, of being competitive.

 

[Sundown]

Fred...This is very interesting to me. Inferior and superior (image appears up or down). what do you call the mirage when it it caused by the wind (left or right). I know I've been sighted in to the target, when the wind picks up, and the target moves off my crosshairs, back again when the wind dies down.

 

[Fred Bohl]

Fred...This is very interesting to me. Inferior and superior (image appears up or down). what do you call the mirage when it it caused by the wind (left or right). I know I've been sighted in to the target, when the wind picks up, and the target moves off my crosshairs, back again when the wind dies down.

From my initial post and or friends at Wikipedia:

"Inferior images are not stable. Hot air rises, and cooler air (being more dense) descends, so the layers will mix, giving rise to turbulence. The image will be distorted accordingly. It may be vibrating; it may be vertically extended (towering) or horizontally extended (stooping)."

In my world, the hot air rising and mixing with colder layers causes what we typically call mirage. Absent any wind this results in vertical displacement/distortion of the target we have called boil. Adding a horizontal wind component results in a corresponding tilted displacement/distortion of the target the tilt being proportional to the wind velocity. If the wind velocity is high enough relative to the boil rate then the displacement/distortion of the target will be almost all horizontal.

 

[Fred Bohl]

My purpose and involvement on this is to improve my shooting skills and all that is inolved such as reloading, wind reading, bench manners, and such. Whatever I read on here I try to test and incorporate into the shooting game, what I believe is the purpose of this forum.

I also hope the main benefit of this forum is to improve our shooting skills. Toward that goal, this forum also needs its members to share their individual knowledge and experience. To facilitate that sharing we need to develope an accepted terminology and mutual understanding of the principles on which an advancement of skills can be based.

I started this thread with the intent to try to reach a mutual understanding of the effects of air between the scope objective and the target on the perceived sight picture and how to cope with those effects.

 

[Turbulent Turtle]

The atmospheric effects between the target line and the objective lens are definitely something that needs understanding. Being able to extract some measure of information by observing the mirage, on how the physical effects of the wind will affect the bullet trajectory, is the goal towards which we all strive.

But there is also the optical effect of the atmospherics with which we have to contend. Does the target shift its position when looking through the shimmer? Does this shift occur when the wind is blowing in one direction or another? Does this shift only occur when we're "in a boil?" And if it does occur, by how much?

In another thread, I was trying to get at some type of explanation about how these affects are detected and magnified by the optics we use on our rifles. A lot of people have the belief that magnification picks up more mirage and affects the DOF of the optics. That may be true for telescopes and cameras, but that is not the case for spotting scopes and riflescopes. In a riflescope, of the three optical portions of the unit, only the objective lens portion is affected by the "mirage". That is the portion going from the objective lens to the first focal plane at the front of the erector lens. The other two portions are not affected, they only enhance the effects that have distorted the first focal plane image by magnifying this distortion through the zoom lenses and then the eyepiece.

When people zoom down their scopes, they are not combatting "mirage" they are just reducing the zoom on the already messed up first focal plane image. Earlier in this thread, I talked about a modifier disk, another term for an aperture. This MD is the only way that we have to increase the DOF of the objective lens portion. The goal is to get the best, least distorted picture possible on the first focal plane. If that image is messed up, the zoom lenses and the eyepiece will only zoom in on the mess and make it worse.

 

[Fred Bohl]

TT - Sorry to be slow in my response. I've been digging through some old test data from tests we did using a large aperture Maksutov Telescope, video imaging camera and processed using RegiStax software. Also went thru my field notes on "mirage" and wind flag correlation while shooting.

As to your series of questions: Does the target shift its position when looking through the shimmer?(yes) Does this shift occur when the wind is blowing in one direction or another?(yes) Does this shift only occur when we're "in a boil?" (no but typically less) And if it does occur, by how much?(for ranges out to 600 yards the maximum displacement seen was about 1 inch per hundred yards but the typical was about 0.1 inch per hundred yards - note that this was for sunny day, 10 mph cross wind, flat short grass open field,
44°9'N - 88°30'W - 775 ft altitude, mid August, 2-3 PM, about 80 degrees F, and 70% RH) My answers here are based on our imaging test results. I've given the test conditions in such detail because sadly the results will change for each of the conditions.

I'll wait for some reaction to the image analysis information before getting into actual shooting results.

 

[Depart123]

... And if it does occur, by how much?(for ranges out to 600 yards the maximum displacement seen was about 1 inch per hundred yards but the typical was about 0.1 inch per hundred yards ...

So in other words, a maximum 1 moa shift.

Except I've seen more than double that in certain conditions.

 

[Turbulent Turtle]

TT - Sorry to be slow in my response. I've been digging through some old test data from tests we did using a large aperture Maksutov Telescope, video imaging camera and processed using RegiStax software. Also went thru my field notes on "mirage" and wind flag correlation while shooting.

As to your series of questions: Does the target shift its position when looking through the shimmer?(yes) Does this shift occur when the wind is blowing in one direction or another?(yes) Does this shift only occur when we're "in a boil?" (no but typically less) And if it does occur, by how much?(for ranges out to 600 yards the maximum displacement seen was about 1 inch per hundred yards but the typical was about 0.1 inch per hundred yards - note that this was for sunny day, 10 mph cross wind, flat short grass open field,
44°9'N - 88°30'W - 775 ft altitude, mid August, 2-3 PM, about 80 degrees F, and 70% RH) My answers here are based on our imaging test results. I've given the test conditions in such detail because sadly the results will change for each of the conditions.

I'll wait for some reaction to the image analysis information before getting into actual shooting results.

Yeah, my "questions" were rhetorical, but I do appreciate the information. As for what occurs in a boil, many believe that the target image is higher than the actual target and so I hear people shooting low but straight on. The delta-Z should be more consistent as the "mirage" rise would not be dependent on the absent wind but would be just a function of the sun's heat and the material on the ground that is causing the "mirage".

 

[Depart123]

Yeah, my "questions" were rhetorical, but I do appreciate the information. As for what occurs in a boil, many believe that the target image is higher than the actual target and so I hear people shooting low but straight on. The delta-Z should be more consistent as the "mirage" rise would not be dependent on the absent wind but would be just a function of the sun's heat and the material on the ground that is causing the "mirage".

In my experience, the target image deflects in the direction of the warmer air. With a previous zero, a target image will deflect up if say snow is now on the ground.

If a shooter zero'd say in the cooler early morning, then it got hot, the image would deflect down causing shots to be low.

Similarly, how many times have you seen it when shooters have zero'd in current sunny conditions, and then a cloud comes over and everyone is suddenly shooting high?

In my experience, the reason shooters shoot high in a boil is because conditions had caused them to zero low to begin with.

 

[Dub]

this is all very interesting to me. I do have a hard time wrapping my head around it though...

 

[rr2030]

This thread is interesting and beyond my desire to comprehend. In the meantime, I will continue contemplating why Mrs. Howell, packed all those clothes for a three hour tour.

 

[Fred Bohl]

In my experience, the target image deflects in the direction of the warmer air. With a previous zero, a target image will deflect up if say snow is now on the ground.

If a shooter zero'd say in the cooler early morning, then it got hot, the image would deflect down causing shots to be low.

Similarly, how many times have you seen it when shooters have zero'd in current sunny conditions, and then a cloud comes over and everyone is suddenly shooting high?

In my experience, the reason shooters shoot high in a boil is because conditions had caused them to zero low to begin with.

I concur with your observations. Your simple explanations of your observations are great! This is the kind of clear and useful information that I hoped would be shared in this thread. Thank you for sharing.

 

[Fred Bohl]

Guys I hope the lack of posts to this thread is because everybody was out shooting since last Thursday. Or had family activities for the Labor Day weekend.

I began my interest in "mirage" as an aid versus hinderance to shooting success when I went from 24x scopes to 36x scopes and could actually observe the effects. That was about 30 years ago and I've learned a lot and done a lot of experiments toward using "mirage" to greatly improve my shooting success.

So it was very startling to find from the lack of participation in this thread that most of you must have "mirage" under your control and don't want to share your knowledge or methods with the rest of us.