Spoke to an accomplished PRS shooter and employee of a major optics company yesterday(I don't wish to give names without permission). He anchored a spotting scope on a 1000 yard range, and covered it to shield it from sun and wind, and recorded with time-lapse.The results showed the reticle changing position dramatically on the F-class sized target, moving mostly in the vertical, and moving enough to cover most of the target. Can anyone explain this phenomenon? Thx.
Reticle changing direction on target over time.
- Thread starter dunraven
- Start date
I don't know about f-class but in short range BR I say the damned target frames are moving. I would have had better score or groups if they'd just stand still.
Yes... When the sun gets "higher / brighter" as the day wears on, the image moves up. Obviously the target does not move, however, the image does! Myself and a few shooters from Houston used to go quite often and shoot 600 and 1000 yards in the morning, preparing for a match that following Sunday OR to test loads. In each case as the sun rose and the "brilliance" on the target got brighter, the shots would rise, sometime rather dramatically! Conversely, if there was fairly heavy cloud cover, that phenomenon would not happen..
Very interesting! This past week, I was testing 6.5 loads @ 400 yds. I was shooting 5 shot groups of three charge weights, round robin and cooling barrel to ambient temperature between shots. Starting temp was 80*, rising to 93*, 1/2 moa wind.
After pulling targets, I noticed the last 2 shots on each target were 1/2 inch higher than the previous three shot. I wrote it off to temp difference and/or my hold being different. Now I'm wondering?????
After pulling targets, I noticed the last 2 shots on each target were 1/2 inch higher than the previous three shot. I wrote it off to temp difference and/or my hold being different. Now I'm wondering?????
In 1000 yard BR I have seen rises of 3 to 4 inches when the sun pops out from behind a cloud. It can ruin a group real quick unless your gun tracks absolute and you dont move it. Matt
I have seen the same thing, although shooting for groups to define a load at 1000.. If you have a "partly cloudy" day, with huge Cumulous clouds and they are moving at a good clip, with bouts of bright sunshine in between cloud cover, the "vertical" at 1000 will ruin the score of even the best shooters!
Mirage.
^^^^This^^^^
This.
Ned Ludd
Silver $$ Contributor
According to Snell's Law, light can be differentially refracted according to the incident angles it enters/leaves a specific medium, as well as the density of that medium. Snell's Law explains why an object observed through a glass of water or at the bottom of a pool may appear distorted or a different size than normal. Sunlight can heat air, which can change its density, and therefore its refractive index. We generally view mirage as convection currents, or multiple layers of differing density, moving in response to any wind. That is a different effect than is being discussed here, where visual displacement of the point of focus appears uniform, rather than the wavelike mirage we typically think of. This phenomena has been described by shooters using scoped rifles for a very long time. Conventional shooters have also described an effect on the target/aim as the sun increases its angle during the day, but that is likely a effect caused by the way the light hits the target, how that affects the appearance of the target to the eye, and therefore, the aiming process when using iron sights.
I have personally never observed the sun = up, clouds = down phenomena with any certainty. My POI often rises during the course of a match, sometimes by as much as 0.5 MOA, but I've always attributed that to the general increase in temperature throughout the day. The phenomenon being described here can apparently happen much more quickly, such as when clouds pass by. This suggests to me that it would most likely be a air density/refraction effect. I think it is unlikely that how brightly the sunlight was shining on the target itself would affect its appearance such that an apparent reticle movement was produced, or the aiming process altered, like it can be with iron sights. That leaves an air density effect as the most likely culprit.
I have personally never observed the sun = up, clouds = down phenomena with any certainty. My POI often rises during the course of a match, sometimes by as much as 0.5 MOA, but I've always attributed that to the general increase in temperature throughout the day. The phenomenon being described here can apparently happen much more quickly, such as when clouds pass by. This suggests to me that it would most likely be a air density/refraction effect. I think it is unlikely that how brightly the sunlight was shining on the target itself would affect its appearance such that an apparent reticle movement was produced, or the aiming process altered, like it can be with iron sights. That leaves an air density effect as the most likely culprit.
I first noticed this when I was doing land survey work. Point the crosshairs on an instrument to the back sight and leave it , look away for a bit, wait for a cloud to overcast, or wind to change direction , etc it will be pointing to the side, up down, etc. this was on long shots, over 500 yds , I remember making a mile long shot once and had to wait for the clouds to come over so it would read the distance. Too much mirage would not let it read.
snert
Silver $$ Contributor
My range card for a farm I hunt says "12 noon to 330 in Sun, 4 clicks down all ranges". Lots of woodchucks lived till I figured that one out.
A mile is a huge shot for surveying. I think I used to use my K&E at 300 or so.
jsn
Gold $$ Contributor
Someone explained to me once that on a windy day the bullet's path doesn't move as much as the wind bends the light we see.
I could see the same phenomenon playing with warm air rising.
I wonder if this has anything to do with all of the scope complaints that have been making the rounds lately. How would a scope proving fixture help if this is true?
I could see the same phenomenon playing with warm air rising.
I wonder if this has anything to do with all of the scope complaints that have been making the rounds lately. How would a scope proving fixture help if this is true?
I don't THINK it has anything to do with scope issues. If you set up a scope on a target in the morning and watch as the day goes on, you will be surprised. This is without shooting a shot. So the crosshair in the scope isn't moving. But it will not stay on target as the sun moves. On a day when the clouds are as Shootdots says, and is going in and out, if you look through the scope as it goes from darker to lighter or vice a versa, you can see it happen. It is instant. Matt
This was a top of the line topcon back in the 90's.
it was very finicky but got the shot. We were surveying a bunch of sugarcane fields being sold off.
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