Advanced Marksmanship interesting mirage phenomenon and scope question

[taliv]

so i got my S&B back from repair this week (including fixing the parallax) and knight and I were out zeroing this week at 100 yards on 1" dots after that crazy storm front came through. mirage was pretty bad, enough to cause the dots to bounce from one side of my crosshair to another.

so i was moving the parallax all over to make sure it was working, and found an interesting condition. when I adjusted the parallax to 200m setting, the mirage was clearly running left to right. when i adjusted the parallax to 50-70m it was clearly running right to left! can't say i've ever seen this inside 100 yrds before. I think knight said he saw same thing in his scope. wind at my position was left to right.

anyway, i tried to take a video of it with iphone and it's difficult to see. [
<iframe width="560" height="315" src="http://www.youtube.com/embed/6pguN9i9gRM" frameborder="0" allowfullscreen></iframe>
(video doesn't appear to be embedding... link is here http://www.youtube.com/watch?v=6pguN9i9gRM&feature=youtu.be

i'm the camera man there and that's knight's hand in the video (have to give him actor credits here). when he rolls the parallax forward (ccw) that's about 200m and back (cw) goes to about 60m give or take.

anyway, the question is... looking at a 100 yrd berm, parallax/focus at 200m, what wind/mirage are you seeing? and why

 

[Lowlight]

Re: interesting mirage phenomenon and scope question

I dont; see it changing direction, just changing in frequency.
I have watched it full screen several times and it still looks to me as if it is going the same way. Could be the quality or what have you, but it appears to be the same, if anything it is a bit of a boil so it looks to go up, more so than across which being closer the frequency is magnified a bit more, so it has an illusion effect, but I can't see a direction change from one side to the other

 

[dynamic research]

Re: interesting mirage phenomenon and scope question

Another thing to look at. Walk down a range on a warm day and you will find warm spots and cooler spots. These alone will change the look of the mirage. Also look at the end of the range, the back stop. This will hold the hot or cold air depending on shade or sun exposure. The one to be concerned with is the mid range mirage. And as lowlight observed, the boil, this will cause target shimmer from left to right as you explained. There is alot more happining within 100 yards than most people think. Like a my sniper instructor once told me, "What is the wind adjustment for a 15 mph wind at 100 yards? It dosent matter, its not enough to effect the impact". But thats from a snipers point of view..

 

[gstaylorg]

Re: interesting mirage phenomenon and scope question

Slightly unfocusing your scope <span style="font-style: italic">toward</span> you from the target (ie. closer) allows you to see mirage better than if focused directly on your intended target. You will hear a lot of people document that if you accidentally unfocus slightly <span style="font-style: italic">past</span> the target (ie. farther), the mirage bands can appear to change direction by 180 degrees. This is an optical illusion caused by the way the human brain perceives the motion of the waves through the scope. Check out the following link for a more detailed description:

http://en.wikipedia.org/wiki/Spinning_Dancer

In particular, if you look closely at the second image on this link (the blue positron tomography emission scan), you can shift the direction you perceive the image to be turning by focusing on the elbows as they come around at either side.

I became interested in this phenomena because I could never satisfactorily explain to myself the physics of how slightly unfocusing a scope either closer or just past a target could cause the mirage direction to change 180 degrees. For example: a scope focused slightly closer to the observer than a 1000 yd target shows the mirage moving from L to R. By focusing slightly past the 1000 yd target, the mirage can then appear to move from R to L. This made no sense to me as the scope focused slightly <span style="font-style: italic">past</span> the 1000 yd target is <span style="font-style: italic">perfectly</span> focused on, let's say, around 1050 yd. And the same scope would be focused slightly <span style="font-style: italic">short</span> of 1100 yd.

Changing the focus of the scope slightly short or slightly past the target and causing the mirage bands to apparently change direction 180 degrees, therefore, must be an optical illusion caused by the observer's perception, as the focal plane of that scope would exactly focused on some other distance, and focused either slightly short or long of a third distance...<span style="font-style: italic">all at the same time.</span> Of course that's not possible, and the link above explains why our perception can lead to this apparent change in motion of the image.

FWIW, I have on several occasions tried this out at the range and in fact it works. You can focus slightly past a target and the mirage will appear to change direction. But if you do a similar thing as I mentioned above regarding looking at the elbows of the image, you can then get your perception of the mirage to shift back to the correct direction the wind is actually moving. For me, looking at the edges of the field of view was the key to causing the mirage to shift back to the correct direction, even though I was focused past the target.

 

[taliv]

Re: interesting mirage phenomenon and scope question

Thanks!!! I'm surprised I never noticed that before. I guess I just never moved it past the target. As I said the only reason I did it this time was to check that my parallax was working as the scope just got back from repair.

Well learn something every day...


Thanks again

Lowlight yeah I almost didn't post the vid because I couldn't actually see the change either. In fact to me in the vid it just looks shakey and doesn't appear to be going either direction. It was crystal clear in person. I think it was just the iPhone video plus the change in codecs from mov to YouTube.

 

[rideHPD]

Re: interesting mirage phenomenon and scope question

Well this seems like a good enough place to ask this: how do mirage and elevated temps change affect apparent parallax? On a recent shoot out in the desert (temps up to 108F, humidity: what's water?) I noticed that the range on the parallax knob was about 50 yards beyond what the actual distance was that I checked later with google earth. I previously tested the parallax knob to see if the ranging system on it was calibrated without mirage, and it was so at 25, 50, 100 and 200 yards. The actual distances were 170 yards and 570 yards.

 

[kraigWY]

Re: interesting mirage phenomenon and scope question

I learned "reversing" mirage the hard, (spelled embarrassing) way when I attended the National Guard Coache's clinic.

If you focus the scope beyond the target, it will in fact give you the prospective the mirage has changed direction.

You feel like an idiot when they instructors gather the other students behind your target, points out that your shooters groups have moved the other direction as every one else, reason being, I had my spotting scope focused beyond the target instead of the before the target.

Its not fun being the guy used to point out "what not to do".