Point of aim changing with temperature?

TimK

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I purchased a used high end scope from someone here on the hide. On three occasions it appeared to have lost zero. I put it in my tracking jig, and it seems to track just fine. While testing it, I thought I noticed that the point of aim was moving around as it warmed up in the sun. All three instances where it lost zero were at matches where it got very hot over the course of the day. I ran another test where I clamped the jig to a large concrete post and let it sit in the sun. The target was 50 yards away in shade, so no mirage. The point of aim moved about 0.3 mils. The times in matches that it lost zero were more like 0.5 mils.

Is this possible, or am I being fooled by some trick of optics that I don't understand?
 

nfoley

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Search "light refraction, terrain, mirage" on youtube. Interesting video showing changes in poa with changing light conditions.
I couldn't figure out how to post that video here.
 
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308pirate

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  • Apr 25, 2017
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    Something tells me that there's a much simpler explanation to what you're seeing.

    Occam's razor
     

    dragman

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    Take a bench rifle to the range shot 1 perfect bullseye. push the gun pack into battery with crosshairs on the same spot and leave it for an hour. When you look through the scope it will be off the spot but be assured if you pull the trigger it will go in the same hole as the first shot. defiantly a light mirage thing.
     
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    Formosan

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    Differing thermal expansion of different materials? Scope tube is aluminum, are the rings and base?
     

    sinister

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    A 20-degree Fahrenheit temperature change is worth a 1-minute elevation change.

    Think Florida in the Spring and Fall, or in a desert. Frost on the ground in the morning, high 80s or higher in the afternoon sun.
     

    Vodoun daVinci

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    Target displacement due to mirage...it's killing me as well. After 6 shots my groups continue to rise until I let the barrel cool completely to ambient or the temp I zeroed at. The rising heated air displaces the target image upwards so when I aim, I am actually aiming where I see the target and not where it is. Lower magnification helps as does a mirage shield on the barrel. My target displacement is mostly a function of barrel heat...the hot air rising off the barrel does not seem to distort the image. It just pushes it upward making the target appear higher than it is in reality. My 3rd group is about .5" high and my 5th group is a full 1" above where I'm aiming...about 1 MOA at 100 yards.

    Very frustrating for me anyway but the OP is not the only one.

    VooDoo
     
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    THEIS

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    Hi,

    @sinister
    Sir, would you happen to have the pdf or ppt that NDIA released a decade or so ago in regards to the Army testing and cataloging MV changes from cooling/heating the cartridges? It was one of the most scientific derived test of MV changes relating to POI changes from manipulating the temperature of the cartridges.
    Within the same test was the "location" of the propellant within the case itself to catalog the MV changes.

    Sincerely,
    Theis
     
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    TimK

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    It's not mirage. There was zero mirage during my controlled test. Parallax was adjusted perfectly. At both matches where the scope appeared to fail I had a backup rifle with a Schmidt. It was subjected to the same temperatures and performed flawlessly. The scope is a Minox.
     
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    TacticalDillhole

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    A 20-degree Fahrenheit temperature change is worth a 1-minute elevation change.

    Think Florida in the Spring and Fall, or in a desert. Frost on the ground in the morning, high 80s or higher in the afternoon sun.
    That’s because of velocity changes not what’s going on here.