This topic contains 6 replies, has 1 voice, and was last updated by  chutesnreloads 3 years, 9 months ago.

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  • #22464
     Sgt. Mike 
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    A mirage is a naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky. In contrast to a hallucination, a mirage is a real optical phenomenon that can be captured on camera, since light rays are actually refracted to form the false image at the observer’s location. What the image appears to represent, however, is determined by the interpretive faculties of the person.
    While a good definition from Wiki not what we are concerned with so let’s please look that mirage effects in our endeavors.
    The target rifle shooter can, when conditions are difficult, fire many sighting shots to verify his judgment and determine the correct sight settings before beginning to fire his record score.
    The long range rifle game shooter therefore needs the ability to “read” wind conditions at long range, and before taking that shot at long range. The shooter must take advantage of all the indicators available -trees, grass, dust, smoke and mirage and be able to interpret them. Since the long range rifle shooter has only one or two shots before game takes flight, the ability to read mirage can make the difference between merely a good shot and a possible a miss.

    A mirage condition is not a handicap, since it offers a very accurate method of perceiving small wind changes which, if time permits, may be waited out. Not all wind changes can be waited out; therefore the shooter needs some understanding of the ways in which various mirage conditions can affect the target image.

    The term “mirage” as used by the shooter does not refer to a true mirage, but to heat waves and the refraction of light as it is bent passing through air layers of different density. Light which passes obliquely from one wind medium to another it undergoes an abrupt change in direction, whenever its velocity in the second medium is different from the velocity in the first wind medium; the shooter will see a “mirage”.

    A familiar example is a stick projecting from water. The water surface is the plane separating the two (2) mediums, (air and water), and the stick has the appearance of being bent at the water surface. The same phenomenon occurs as the light from intended target to shooter passes obliquely downward through an atmosphere at uniform temperature. The bending is due to this refraction; which is barely perceptible at 1000 yards and negligible at 500 and 600 yards, but nevertheless it is present. The density of air, and therefore its refraction, varies with its temperature. A condition of cool air overlaying warm air next to the ground is the cause of heat waves or “mirage”. The warm air, having a lower index of refraction, is mixed with the cooler air above by convection, irregularly bending the light transmitting the target image to the shooter’s eye. (Fig. 1) shows greatly exaggerated, the vertical displacement of the target image by heat waves. An elevation correction is evidently necessary in order to center a shot on the target.

    (Fig. 1)
    Heat waves are easily seen with the unaided eye on a hot, bright day and can be seen with spotting scope on all but the coldest days. To observe heat waves, the scope should be focused on a point about midway to the target. This will cause the target to appear slightly out of focus, but since the long range rifle shooter generally does not try to spot bullet holes, the lack in target clarity is more than compensated by clarity of the heat waves.

    Classifying mirage density
    the sight correction necessary to compensate for mirage, at a given range, will increase as density of the mirage increases. Therefore the shooter should be able to distinguish different densities of mirage, which should be recorded in a book for future reference. An individual just starting long range shooting should not attempt to classify the mirage into more than three (3) categories, namely: light, intermediate and heavy. Let’s just keep it simple as you the long range shooter become more experienced then better classification can be made.
    The light mirage is associated with a cool or cloudy day, when the sun cannot heat the ground, and is seen through the scope as a series of fine, faint lines. Target distortion is minimal. This mirage is very useful to the shooter in detecting slight wind changes which require the merest pinch of windage adjustment; while the mirage correction, which will be shown later in this post is practically negligible. An intermediate mirage will be present on the perfect shooting day with 70 to 75 degree temperature and normal relative humidity (45 to 55 percent). The mirage is barely perceptible with the unaided eye, but is easily seen through the spotting /rifle scope (iron sights as recall the term barely perceptible) as distinct lines. Target distortion begins to be apparent and each major change in wind velocity will also require a correction for the change in mirage. Conditions are more difficult than those brought about by a light mirage, but are not the most difficult. A heavy mirage will occur on hot, sultry days (60 to 75 percent humidity) when heat waves can be seen easily with the unaided eye, and appear as very dense lines viewed through the spotting scope. Target distortion is extreme, details are difficult to locate, and any change in the wind velocity will require that the shooter take into consideration the mirage corrections. (Fig 2) shows the appearance of the three (3) mirage
    categories.

    In addition to the three categories of mirage that the beginning shooter should learn to recognize, there are four (4) distinct classes of mirage in each category from each side of the target, plus the vertical mirage. Hence there is a minimum of twenty-five (25) mirage conditions that the beginner may use in detecting wind changes – or that he must cope with, depending on his state of mind.

    It may be stated here that the shooter’s mental state may be the most difficult condition that the beginning shooter must overcome. Wind and mirage affect all shooters on long range shots more or less equally, and in many cases it is the shooter who creates the impossible conditions. Conditions do change sometimes it is best to just wait, but when game or target is on the move or time is short hopefully this primer will aid the shooter.

  • #22466
     Sgt. Mike 
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    Wind causes changes!
    How do the four (4) different classes of mirage come about? Wind blows the heat waves as they rise, causing them to move vertically, obliquely, or laterally across the target. The amount that they vary from the vertical depends on the cross wind velocity, within limits from zero (0) to about twelve
    (12) miles per hour (m.p.h.). As each mirage class is described, you must keep in mind that the description also applies to each category, and to a wind from right (3 o’clock) or from the left (9 o’clock).

    The first class to be considered is the boiling mirage, with heat waves rising vertically as in
    (Fig 3). A boiling mirage is present when there is no measurable wind, and when the wind
    blowing from the shooter directly toward the target (6 o’clock wind) or from the target toward the
    shooter (12 o’clock wind). This mirage class condition requires that the rear sight be lowered, to
    center the shot group within the target. The amount of correction will depend on the category
    (density) of the mirage and vision of the individual shooter. An accurate record of mirage category, mirage classification, wind, temperature, light, sling tension, etc., combined with knowing the rifle’s no wind zero and the zero for that distance, will enable the beginning shooter to refine the values given below to fit his particular location.

    Although the boiling mirage presents little problems in itself, the near 6 o’clock wind or near 12 o’clock wind which appears as a boiling mirage to the inexperienced shooter can present quite a problem when it is fishtailing from about 11:30 to 12:30 or 5:30 to 6:30.

    Wind correction is generally rated as a fraction of the wind’s 3:00 or 9:00 o’clock effect, with the 11:00, 1:00, 5:00, and 7:00 o’clock winds being half value. The 11:30, 12:30, 5:30, and 6:30 o’clock winds could be rated as eight value each, while the wind fishtailing from one side of 6-12 o’clock line to the other gives a combined effect of quarter (1/4) value. A small change in wind direction is very difficult to detect by feel, but secondary heat wave lines will begin to show up in the mirage and have the appearance of just leaning away from vertical, signaling the shooter that a change in wind direction has occurred.
    A Slow Mirage
    A slow mirage; the second classification, exist during a light air of one to 3 m.p.h. blowing from
    3 o’clock or 9 o’clock. Heat waves will be slightly inclined as they move across the target from
    7 o’clock to one o’clock of from 5 o’clock to 11 o’clock (Fig 4).

    Target displacement with the slow mirage requires both an elevation and windage correction. As with the boiling mirage, the rear sight is lowered to correct for vertical component of the apparent displacement. The horizontal component requires a wind correction which will be into the wind, in addition to the correction for wind drift of the bullet.Since the heat waves are crossing from 7 o’clock to one (1) o’clock or 5 o’clock to 11 o’clock, they are making an angle of 30 degrees with the vertical, and their vertical and horizontal components.

    can be computed in relation to the total apparent displacement:
    p = total apparent displacement
    v = vertical component of D
    H = horizontal component of D
    Then,
    V = D cosine 30 degrees = 0.87 D, and
    H = sine 30 degrees = 0.50 D

    Experience has shown the total displacement due to heavy mirage amounts to be ≈ 1 ½ minutes. The vertical correction due to heavy-slow mirage will be 1.31 or 1 ¼ minutes, and the horizontal correction will be 0.75 or 3/4 of a minute.

  • #22467
     Sgt. Mike 
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    As the 3 or 9 o’clock wind rises to a light breeze of 4 to 7 miles per hour, the heat waves will
    make a greater angle with the vertical and will have the appearance of crossing from 8 o’clock
    to 2 o’clock or 4 o’clock to 10 o’clock. This gives the third classification, is a medium mirage
    (Fig 5).

    The medium mirage will also require both elevation and windage corrections of different
    amounts than for slow mirage. Changing the angle from 30 degrees to 60 degrees because of
    the increased inclination, the corrections become:

    V = D cosine 60 degrees = 0.50 D
    and
    H = D sine 60 degrees = 0.87 D

    Going back to the total displacement of 1 ½ minutes caused by the heavy mirage, the vertical correction for a heavy-medium mirage will be 0.75 of 3/4 minute and the horizontal correction will be 1.31 or 1 ¼ minutes. Again the rear sight is lowered and the windage correction is added to that required for wind drift.
    The Effect
    Notice the mirage effect with the same heavy mirage during a wind increasing from one to 3 m.p.h. (slow mirage) to 4 to 7 m.p.h. wind is applied, the rear sight must be lowered 1 ¼ minutes and 3/4 minute windage added for horizontal component of the mirage. During the string, the mirage picture changes from heavy-slow to heavy-medium which signals the increase in wind velocity. The necessary wind correction is made and an additional ½ minute applied to give a total horizontal correction of 1/1/4 minutes for mirage. Adjusting for the vertical component of the mirage is
    accomplished in a similar manner. The sight was originally lowered 1 ¼ minutes and the mirage picture now indicates that only 3/4 minute is needed, so the sight is raised the difference, which is ½ minute. The beginner shooter should practice mentally so that he is able to make the necessary changes without becoming confused.

    A fast mirage, the fourth classification, will be visible when the wind velocity reaches the
    gentle breeze stage of 8 to 12 M.P.H. from 3 o’clock or 9 o’clock. The heat waves will move
    horizontally across the face of the target (Fig.6) and the apparent target displacement will be
    only horizontal. The fast mirage thus requires only additional windage correction. Wind
    velocities over 12 M.P.H. are indicated by the heat waves having the appearance of being
    stretched straight, and letups can be detected by slight waves beginning to appear on the
    mirage lines.

    Boiling with lateral movement
    The last and most difficult mirage picture to identify is the “boiling mirage with lateral left or
    right secondary heat waves” ( Fig.7), which is an indication of the wind coming in from
    between 9 and 12, 9 and 6, 3 and 12, or 3 and 6 o’clock. An easy recorded notation is the word “boiling” with an arrow drawn through it to indicate the direction of the secondary heat waves. The primary and secondary heat waves are caused by the 2 components of the wind, a 6-12 o’clock and a 3-9 o’clock component, the latter being the more important in detecting wind shifts. The secondary heat wave lines should be read in the same manner as the previously mentioned classes, with the secondary lines indicating that both vertical and horizontal corrections will be necessary to compensate accurately for the mirage displacement. The new shooter should be on the lookout for the appearance of these secondary waves while firing in the other mirage classes.

    Mirage can also be used to determine the true wind direction. Traverse the scope until a “boil” is seen, then the wind is parallel to the axis of the scope. Turning the scope through 90 degrees will be equivalent to observing a 3 or 9 o’clock wind. The mirage classification gives the wind velocity up to about 12 m.p.h.

    The beginning shooter can possibly keep abreast of the mirage changes, by plotting them in a book. You can draw one or 2 wavy lines through the circle to indicate the mirage classification. A category change can easily be shown by putting the initial L, I, or H in the circle. However, do not spend to much valuable time at this. It should not require more than 3 to 4 seconds to note a mirage in this manner and the target will, no doubt be gone during that time. If more time is being consumed, additional practice is necessary and several trips into the country, between hunts, with a spotting scope or binos can be beneficial.

  • #22468
     Sgt. Mike 
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    (Fig. 8 is a tabulation of corrections in minutes that will aid the new shooter in learning the effect that mirage can have on the target image. The new shooter, wishing to apply the methods of doping the mirage that have been described, must keep in mind that intensity of the mirage seen will very according to each person’s vision.
    The tabulation of corrections was extracted, mostly, from the experience of competitive shooters but are still apply to the hunter shooting at distances. Values given will have to be modified somewhat by you the shooters for mirage pictures that are distinct to you, at your geographical location.
    As I normally am a competitive shooter and have revamped this article to the hunter I am in hope that you the hunter will understand the value In this video, SFC Lewis explains how to use a spotting scope to monitor mirage, and to watch trace. SFC Lewis is a former Army Marksmanship Unit member, U.S. Army Sniper School instructor, and current U.S. Army Reserve Service Rifle Shooting Team member. In discussing how precision shooters can employ spotting scopes, Lewis compares the use of a spotting scope for competition shooters vs. military snipers. NOTE: You may wish to turn up the audio volume, during the actual interview segment of this video. Having shot with SFC Lewis in the foreground and SFC Buol in the background on a recurring basis I can attest these men knows their craft which is one shot one kill and that neatly fits into the hunters perspective
    http://youtu.be/fjrJcA2gTIw

    Barrel mirage
    Normally this type of mirage is induced by repeated firing during cooler weather such as sight-ins before going hunting. To the Iron sight shooter this normally is not an issue to the person requiring a scope it does affect their performance. Because this is a manmade effect it can be controlled to an acceptable degree by the use of a sunshade the length that is long enough to control the effect or a barrel shield to disrupt the mirage effect.
    Homemade sunshade
    http://criterionbarrels.com/build-your-own-mirage-shield-for-less-than-a-buck

    Homemade scope sunshade
    http://www.thehuntinglife.com/forums/topic/192349-diy-scope-sunshade/

    Now because there is SO much information on the Net I have used a write up from here http://southtexasshooting.org/multimedia/text/mirage.html
    and edited it for the hunter and long range shooter perspective in order to assit those whom might be interested in shooting long ranges

  • #24495
     Sgt. Mike 
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    Mirage Shields — They Aren’t All the Same

    Linked this from a article located on Accurate shooter.com here is the link to the article :
    http://bulletin.accurateshooter.com/2014/08/tech-tip-mirage-shields-they-arent-all-the-same/

    Stacking sun shade will work to reduce barrel mirage during cool days and your barrel starts to warm up first clue is no matter what you twist in the scope it just is not clear .
    When this happens put on a Mirage band or stack up on the sunshades.
    Hope this helps.

  • #24506
     Artful 
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    It can’t hurt, but when you can’t unfuzz the target on a good day with no mirage – That’s the challenge

  • #24538
     chutesnreloads 
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    Oh good…..I’m not the only one with fuzzy targets :p

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