I've seen posts on shifts in parallax range settings with scopes at different temperatures, but not much on temperature-dependent POI shifts, so hopefully this post and the discussion it generates will be interesting and useful.
I found a dramatic POI shift in my Walther LGU/Sightron SIII 10-50x when I was sighting in at a Northeast Ohio Airgunners' FT Match a few weeks ago. They ran a great match with lots of excellent and very friendly shooters, so I highly recommend going to their matches! During sight-in, the POI was about 1 MOA low at most ranges. The sight-in range was in an open field and it was a sunny day. The thermometer tape on my scope was pegged at the max temperature of 94 degrees F. I didn't see much of a shift with rangefinding, but then again my rangefinding at longer ranges is so poor that I probably wouldn't notice a difference. The match was in a wooded area, and on some targets holding about 1 MOA high helped (on others it didn't). The scope temperature was around 86 degrees F during the match, but I didn't monitor it very closely.
To verify this under more controlled conditions, I clamped my rifle to a table, put a heat lamp next to the scope, and looked for POI shifts in my basement. The photo below shows the setup.
I kept track of the temperature reading on the scope tape and took photos through the scope with my cell phone. The results are qualitatively consistent with what I observed at the FT match; as the scope warms up, the reticles moves up, causing the POI to go down. At 50x, the hashmark spacing is 0.96 MOA. The shift that I observed in this test was around 0.5 MOA when going from 71 to 94 degrees F (please see figure below). The series of photos below were taken at progressively later times as one goes to the right and the lamp was turned off after the second reading at 94 degrees F. The reticle returned to the original position once the scope cooled back down.
I didn't measure the scope temperature independently since I'm relying on the scope's thermometer tape in the field and wanted to see how well those temperature readings correlated to POI shifts (and I was a bit lazy and didn't want to make this a full-blown quantitative experiment).
Please let me know if you have this problem and how you deal with it. At FT matches, how much do you rely on the sight-in right before the match to adjust your scope elevation settings? Do you modify your range card after each sight-in before the match? I keep hoping to get my range card to converge on a single set of values, but clearly external variables, like scope temperature, cannot be ignored.
Thanks,
John
I have a very temperature sensitive scope. It's the first generation Schmidt and Bender 10-50X. Newer models are much better. The perfect fix was sold by a UK 3D printer business called Maestro. He sells adjustable pointers is several clever designs. I bought the model made especially for my scope.It works perfect. It allows you to adjust the pointer to the correct yardage for CURRENT conditions. All I do is focus in on a known 55 yard detailed POA and adjust the pointer to the correct 55 yard mark on side wheel. Anytime you think it's too cold or hot and want to verify ,it's quick to recalibrate.
Thanks for your post. It was very interesting.
Thanks for the reply. Does your rifle's POI also change as the scope warms up? I made a DIY adjustable pointer for my parallax wheel, but unfortunately, it and the Maestro only help correct for temperature-dependent parallax range changes, but not POI shifts.
Both the gun itself and the scope will wander with temp changes. Metal expands when warmed contracts when cooled. law of nature you can't get around, dissimilar metals expand/contract at different rates. another point as the sun arches across the sky your poi will change just from that. Also cloud cover vs open sky will cause a poi. Dosen't matter if open sights. peep sight or a scope.
Thanks for the reply. Does your rifle's POI also change as the scope warms up? I made a DIY adjustable pointer for my parallax wheel, but unfortunately, it and the Maestro only help correct for temperature-dependent parallax range changes, but not POI shifts.
Hi John,
My rifle's POI does change only when very cold. I solved that problem by lubricating the hammer and spring with synthetic AMS OIL for compressors. It's viscosity remains the same over a wide temperature range. Many guys prefer a dry type lube. Heat does not seem to affect my POI as much as cold.
Hello John!
Nice to see you coming back to FT.
I have tested a number of scopes and I have found a few interesting things:
1.- ALL scopes are temperature sensitive. In a more or less pronounced aspect, but ALL of them are temp sensitive.
In a conversation about lens passive athermalization with the "experts" at Edmund Optics after some serious experiments had been conducted (IMHO with some scientific method faults), we came to the conclusion that EVERYTHING is a thermometer. Even the measurement devices.
2.- I usually conduct my experiments at different temperatures, but in the shadow, ¿Why? because there is a vectorial gradient to the flow of fields (whether electrical or thermal), and so, doing the same experiment in the sun and in the shade can yield DIAMETRICALLY opposing results. This came to light a few weeks ago when my friend Trevor at Palmyra, was telling me that his scope "pulls" the POI in the sense where the sun is shining from.
To illustrate this, I re-conducted the SAME test with direct sunlight hitting the scope and with no sunlight hitting the scope. Both cases over temp excursions between 73°F and 97°F; the scope is a Tac-Vector Continental 5-30X56 FFP and the test has two points of observation:
a) Where the reticle is at
b) Where the range is at.
So, for the position of the reticle
With direct sunlight hitting the scope, from "Cold" to "Hot" temperatures the POI changed 1 mRad DOWN, meaning that the POI would have climbed.
With JUST ambient temperature taking the scope from "Cold" to "Hot" made the reticle go UP, which would have brought the POI down by 1.4 mRad.
I also know that the biggest changes are located between 93°F and 97°F. From 72°F to 92°F changes are relatively small.
Doing the SAME test in a NightForce NX8:
With direct sunlight, the reticle moved 0.1 mRad up and 0.2mRad left
With just the ambient temperature doing the warming up, the change was just as small but in the opposite directions.
Now, many will be wondering why I am not shooting the NF in FT, well, because of the ranging.
From "Cold" to "Hot", the T-V varied the Rangefinding by 1-1.5 yards at 55 yards.
From "Cold" to "Hot" the NF varied by more than 14 yards.
In a way, it is natural, the NF scope is NOT designed to do rangefinding by focusing. It is designed to focus crisply, and correctly eliminate parallax, but no need to worry about repeatability, or temperature stability in the ERECTOR assembly. AND it does keep the POI where it should even with extreme/forced temp. changes.
BOTH scopes are FFP.
So, how to cope with the two aspects of thermal instability?
A moveable indicator is one way (I also use the Maestro Design "knife edge") in my 34 mm's T-V to help me deal with the parallax/focusing/ranging aspect. I wait for a few minutes (while saying hello to everyone in the range), and when temps are equalized between ambient and scope, I start my sighting-in.
You can also build different marks into your sidewheel tape and use different marks at different temperatures. A friend used a Leupold Comp 35 with a complex curve for the parallax, with four or five points for each distance, one per temp. In the end, he bought a Kahles.I still cannot bring myself to that, LOL!
For the POI "shift", you just need to keep tabs on what is going on. Unless you want to make one trajectory card for every temperature, like John English used to (and he was one of the best FT shooters I've met).
Now, having said that, there are OTHER variations with temperature that need to be taken into account:
Stock shape
MV
BC (not only with the velocity over the trajectory, but also with the humidity and temp of the air where the pellet travels).
Since your SIGHTRON only moved by about ½ MOA (that's about 1/6th of a mRad), I would consider that performance as "solid". Problem with SIGHTRON is that once the "old guard" went out, all the suggestions about better reticles are falling in deaf ears.
In the same way that "Know Thyself" or "Conquer yourself and you will have most of the battles won", knowing YOUR rig is the most important aspect in FT shooting.
Keep well and shoot straight!
HM
Thanks for the info Hector. I agree that the key is to know how (as quantitatively as possible) the rifle/scope behave under different conditions.
This doesn't mean that wind isn't important. I have a very hard time reading and compensating for wind. I'm afraid that I didn't understand your comment about radiant shielding. Please elaborate.
Thanks,
John
Dan, what I meant was that the shape of the stock (therefore the bedding, and therefore the POI, and possibly the dispersion, etc.) can change with temperature and humidity changes.
People think that modern laminates are impervious to temp. & humidity changes, they are not.
Even solid aluminum stocks can and do change their shape, therefore they can affect the POI.
Trying to narrow all the temp shifts to a single element in the system is a bit optimistic (and in a bad way). Best is to get to know your system as a whole.
Keep well and shoot straight!
HM
I just found out that the POI goes the other way when the scope is cold! Last week I shot a match at temperatures in the low 60s F. Initially, the POI was right on (scope and rifle were still warm from the car), but after about 5 minutes I was shooting about 1.5 to 2 MOA HIGH at 55 yards. When the scope was over 94 F, the rifle shot about the same amount low. The range card was made when the scope was in the mid 70s F. I don't think that differences in air density in the compression tube could account for this, although it may make sense qualitatively (when denser cold air is compressed by the piston, it will reach a higher pressure)? On the other hand, colder/denser air between the muzzle and target will increase air resistance, which will cause the pellet to drop more. My first guess is that the scope tube is bending due to differential thermal expansion/contraction of the scope/mounts/rifle receiver and this causes the reticle to move. This is not a small effect and I find it much more disconcerting than the smaller shifts in range finding due to temperature changes (I knew the distances to the targets during sight in, so it definitely was not a temperature-induced range finding error).
It's good to be aware of this and now I'll keep the temperature in mind when I'm sighting in and modify my range card accordingly right before the match.
@johnc Heat is transferred by radiation, conduction, and convection. On a 100 degree day the air conducts/convects heat to all parts of the scope while the sun adds it's two bits from whatever direction it's coming from It affects all of your gear by adding radtiant energy. That means your stock, receiver tube, scope mounts, and scope.
The setup with the heat lamp may re-create one of the environmental conditions youn encounter, but it is predominantly a radiant heat transfer source, Does that make a difference? Who knows.
My point was that your lamp is heating everything, so you're getting expansion effects of the receiver tube and mounts, along with the scope.
I can't tell you it's wrong, because it's too complicated, but I think you're not just isolating the scope.
K
Thanks for the reply. Yes, I agree that it's not just the scope. I have a parallel thread on this topic on Shooting the Breeze, and some very wise and knowledgeable folks there think the shift may be due to thermal expansion of the piston seal, which increase piston/compression cylinder friction. This decreases the MV slightly (not enough to cause a major vertical POI shift), but more importantly, changes " the point in the recoil cycle that the pellet exits the muzzle, and where the rifle is pointing."
Please check out this link for more info:
https://shooting-the-breeze.com/threads/sightron-poi-shift-with-temperature.61030/page-2
To confirm that this is the case, I could mount the scope on a PCP and check how much POI changes with temperature. Even if it does change, I think it's not just the scope, but the fact that it's mounted to a rifle, which may be expanding with temperature differently than the scope.
I just found out that the POI goes the other way when the scope is cold! Last week I shot a match at temperatures in the low 60s F. Initially, the POI was right on (scope and rifle were still warm from the car), but after about 5 minutes I was shooting about 1.5 to 2 MOA HIGH at 55 yards. When the scope was over 94 F, the rifle shot about the same amount low. The range card was made when the scope was in the mid 70s F. I don't think that differences in air density in the compression tube could account for this, although it may make sense qualitatively (when denser cold air is compressed by the piston, it will reach a higher pressure)? On the other hand, colder/denser air between the muzzle and target will increase air resistance, which will cause the pellet to drop more. My first guess is that the scope tube is bending due to differential thermal expansion/contraction of the scope/mounts/rifle receiver and this causes the reticle to move. This is not a small effect and I find it much more disconcerting than the smaller shifts in range finding due to temperature changes (I knew the distances to the targets during sight in, so it definitely was not a temperature-induced range finding error).
It's good to be aware of this and now I'll keep the temperature in mind when I'm sighting in and modify my range card accordingly right before the match.
Hello John;
I've been running tests with different scopes and it has been an education.
I am not going to comment more because I am still getting more info and will comment at a later date in a more extensive blog entry.
What I DID find, that surprised me is that FROM MY SETUP, the biggest change with ambient temperature came from the interfase scope/rifle (the mounts).
Once that is corrected, my changes are 0.33 mrads (about 1 MOA) between 24°C and 32°C.
It is also impressive to see the INSIDE temp of the scope (as registered by the contact "fermometer") to exceed ambient temperature by over 6-7° C.
I will continue experimenting with this and different mounting procedures.
Additional comment is that, for the most part, the other thermal variations pale in comparison. You can do a simulation using MV's that are up to 10 fps apart in any software and you'll see that, for our ranges, and our distances, it is not that relevant.
Rubber (ALL forms of rubber) changes with temperature, yes, that is why the LESS rubber you have in the sealing process, the better.
😉
Keep well and shoot straight!
HM
@johnc Heat is transferred by radiation, conduction, and convection. On a 100 degree day the air conducts/convects heat to all parts of the scope while the sun adds it's two bits from whatever direction it's coming from It affects all of your gear by adding radtiant energy. That means your stock, receiver tube, scope mounts, and scope.
The setup with the heat lamp may re-create one of the environmental conditions youn encounter, but it is predominantly a radiant heat transfer source, Does that make a difference? Who knows.
My point was that your lamp is heating everything, so you're getting expansion effects of the receiver tube and mounts, along with the scope.
I can't tell you it's wrong, because it's too complicated, but I think you're not just isolating the scope.
K
While you are right that John is not isolating the scope, the converse is that in the field, the scope is not isolated.
😉
I am making two sets of readings: Scope on V Blocks (that is the best I can do for isolating the scope for measurement), and MOUNTED scopes, and the differences are radical, and not in a good way.
Keep well and shoot straight!
HM
Hello.
The spacing in the reticle ("I" in the scope manual) is 0,480 at 50x.
Also in the scope manual is noted that "all values in MOA at 50 yards"
So 0,480 at 50x at 50 yards, if i understand it correct.
I read that this test was in your basement. I what distance was your target placed ?
Dear @newbie_airgun,
That's a very good point! Now I see that I read the reticle table wrong. The I hashmark spacing is indeed 0.48 MOA at 50x, not 0.96 MOA. Maybe I was looking at dimension F, which is right below dimension E (the same as I) in the table? I'm not sure why Sightron says "MOA at 50 yards" since MOA is an angle, so it's independent of distance. Sightron also makes this mistake in their online reticle spec descriptions. They're usually very good at their technical descriptions, so this is puzzling. 1 MOA at 100 yards is 1 MOA at 50 yards and all other distances.
The target in my basement was at 20 yards, but since I reported the deflection in MOA, the distance doesn't matter. Of course, thanks to your astute observation, the values I report should be divided by two. This gives additional support to the argument that there are other thermal effects changing the POI besides the scope/base.
Thanks,
John
Hello again.
MOA is independent of distance. But not sizes. What we see in your photos ( a grey square ) has a size. And this size, if we measured it in MOA, changes with distance.
So if for example you measure it 0,5 MOA at 20m, if you measure it again in 10m, it would be 1 MOA.
1 MOA= 1" at 100y, 1 MOA=2'' at 200y, 1MOA=10'' at 1000y.
So the question is, that grey square what MOA value has at your basement lenght?
The lenght of your basement i believe that plays a crucial role for your experiment.
I conduct similar tests with this scope (Sightron SIII 10X50-60 FT) and until now i can confirm that at 22,5 degrees celcius the POA moves up from cold to heat, and down from heat to cold. I also can confirm that the movement is 1/2 of MOA, or 4 clicks at 10m, 10x magnification. I try to find more.
Again all this if i understand this subject correctly.
Sorry about my bad English trying my best.
Thanks.
Dear @newbie_airgun,
Your English is good and clear. This is a complicated problem and I'm not sure if my explanations are making sense. I think that I'm starting to understand the issue. I agree that the target changes size with distance, but I'm only using the top edge of the target square to look at the relative movement of the reticle hashmarks. The top edge of a line on a target square marks a single spot at all distances, regardless of the target square changing size with distance. I could have made a target square twice as big, but the top edge of a line marks a precise single vertical position regardless of the size of the square, or even the thickness of the line, which DOES change with distance. I hope that this makes sense.
It's good to know that you're seeing similar drift with temperature. I think that the scope mounting system plays a significant role in the magnitude of the shift.
Best wishes,
John
Hi John,
I experienced a similar issue with a Sightron years ago. I also put a thermometer on the scope and noted a definitive shift at around 88 degrees F. It was most crucial for the close, smaller kill zones at say 10 - 15 yards. What I noticed was that my scope would under range the target when the temp shifted above 88 degrees. It was enough from what I recall at closer distances to miss almost every time on a small kz. I did not see enough shift in ranging at the longer distances to miss very many out there, but there still needed to be some compensation for the issue, but it was not as crucial as it was for the closer targets. So, I set up a second pointer and tracked the temperature where the shift occurred. Then I would simply change over to the high temperature pointer to correct the issue once the scope reached 88 degrees.
Good luck with it!
Harold
I concur with Harold too.
High mounts allow less clicks at far distances and ALOT more up close. Harold's temperature was 88 degrees before change. My Schmidt and Bender 10-50 is around 90 degrees. The newer model is much better.
Dear Harold and DonC,
Thanks for the helpful comments. I also see shifts in ranging, but my main point here is temperature-induced shifts in POI (not due to ranging error). The crosshairs move with temperature. Do you see T-dependent POI shifts as well?
Cheers,
John
I have a Sightron as well and I actually found a temperature shift at about 48 degrees. I started putting one to two layers of plumbing teflon thread tape between the rear ring and scope, some of the shooters around here have been putting under both rings. Since I started doing this I have not had any temperature shifts. FYI I am in Oklahoma and shoot in all temperatures as well.
Hope this helps.
Mark
Hi Mark,
Thanks for the info. It's great that you were able to solve this problem! Do you think the teflon tape is allowing the scope tube to slide in the rings without binding as the tube expands/contracts with temperature changes? I've switched to Burris Xtreme Tactical Signature Rings with the plastic Posi-Align inserts and have lowered the torque on the ring screws to 15 inch-pounds (as Sightron recommended). Before, the aluminum rings were in direct contact with the scope tube and the rings screws were torqued to around 25 inch-pounds.
Hopefully the plastic inserts and the lower ring screw torque will reduce temperature-induced binding in my scope. I haven't shot on a hot day yet, and will report my findings here.
Cheers,
John
I use Burris Signature rings on most of my collection. I torque the rear tight and leave the front ring with modest torque. Paste wax on the front ring plastic insert. IMO Burris is the best. Like their web site says, they never leave ring marks on the scope tube, theygrip better than alloy rings, they are adjustable with Plus and Minus inserts. I always order the size 20 inserts for severe droop.
@johnc I have seen a lot of people while shooting our match in the woods place there guns in the shade and some people place a towel over there scope so it does not see direct sunlight
@bigbobbybeef Yes, after that experience, I've also been placing a towel over my scope and try to keep it in the shade.
I tried this again with the same scope but different rings (Burris XTR, with plastic shims, instead of Warne QD rings). The results were similar but there were some interesting differences. Please see the new post at:
https://airgunforums.co.uk/threads/scope-poi-shift-with-temperature.105836/
