Over and under ranging in temperature?

Discussion in 'General Airgun Chat' started by RobF, Mar 10, 2013.

  1. RobF

    RobF Administrator Staff Member

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    Just doing a little research to see if the main trend works with a theory of mine.

    Those that see definite scope shift (and not the gun changing power or bending in temp), can I ask what happens to your scope in rising temp and which way the wheel turns.

    Ill make it easy. The march seems to underange in higher temps and over range in lower. Ie in high temps a 55 target will read say 52. In low a 55 may range at 58. The wheel turns so that if 55 yds is on the pointer then 45 yd line will be above it on the wheel. Does that make sense?

    Interested to see if there is a correlation in wheel direction and shift direction. If there is then I might have a theory to look at something in closer detail. Doubt it will be curable but it might be testable if I can persuade a scope maker to try something.
     
  2. rich

    rich Active Member

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    Falcon T35. Wheel turns clockwise for further ranges, viz, 55 is above 50 is above 45 on the perimeter of the wheel.

    A known 55 yard target will be shown as maybe 52 to 53 in very cold weather.

    Once it's above 10 degrees or so it's pretty good UNLESS the scope gets baked in the sun. :mad:
     
  3. NJR 100

    NJR 100 Because I`m AWESIME !!

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    Cold weather, usually below 5 degree and scope will under range, so 50 shows 47m.

    47m above 50 m on wheel, so guess that makes it clockwise for low - high range.

    No problem wth shift in heat.
     
  4. Zatara

    Zatara New Member

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    Mine does the same

    I have a March and it does the same thing. I believe the aluminum body of the scope is the culprit. Aluminum has one of the greater expansion ratios of metals commonly used. Thus the scope is slightly longer when warm and shorter when cold. A math prof friend of mine stated that he calculated that my scope would expand/contract by 1.4mm with a 40 degree fahrenheit change.
    I do not remember who's formula he used to get the numbers, but I trust his math.:)
     
  5. HotShot

    HotShot Active Member

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    The Mk1 and 2 Nikko both under range in very cold conditions. As far as I can remember so does the mk3 but the wheel moves in the other direction.

    Dave.
     
  6. villiers

    villiers Self appointed antipimp

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    My S&B over ranges as the temperature drops 2.5 yards for every drop of 5 deg :eek:o
     
  7. neilL

    neilL New Member

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    Hi Rob
    On my Leupolds it has been the opposite. i.e. the wheel is the same (55yd on pointer, 45yd above) but when cold the actual distance is below the pointer and at some high temp it is above. I range high to low usually and it appears to be the internals rather than the scope tube temperature so I have to wait a while before the offset catches up with the temp strip. On my 20-50 (PR boosted Leup) the "low temp" switch seems to be at 8C-9C and this WL season has been a PITA as courses have warmed up through the session :-(

    Thanks
    Neil
     
  8. Neil-T

    Neil-T Boingers forever.

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    Wheel moves anti-clockwise for longer ranges. In cold weather: IE: temperatures of around 4/6 degrees my 45 will lose around 2 yards max, I would say the 40mag one I have loses less, around 1 to 1- 1 1/2 yds....splitting hairs really. Never tried a 35 so no comment. One of the worst scopes I have owned Was a Leupold 20-50-50 which lost 4 yds in cold temps, but was a good rangefinder under normal temps. Neil.
     
  9. Ceathreamhnan

    Ceathreamhnan WHFTA Champion 2013

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    A scope tube made of 6061-T6 aluminium will have a linear thermal coefficient of expansion of 23.4 ppm/DegC, which is the same as saying 0.00234 %/DegC.
    So a scope that is 300mm long will expand by 0.155mm in a change of 40 F or 22.2 DegC (going from 4 Degc to 26 Degc, say).
    This is aircraft aluminium, so for comparison, a Boeing 747 that's 231 feet long would expand 1.43 inches. This would be 12.9 inches if the math quoted above were correct, I think.
     
  10. maestro

    maestro European Champion 2018

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    S&B 12.5-50x56 FT
    It ranges over in cold weather, continuously (i.e. without jumpings), on a 50 metres target it misranges 4.2 m by every 10 °C difference.
    Some newer examples from the same scope do the same error but with jumps, they misrange with only 0.5 m by 10 °C but then jump 4 metres so the overall result is the same.

    Nikko Diamond 10-50x60 MK1
    It ranges under in cold weather, too, but only 0.5 m by 10 °C at a 50 metres target.
    The error is continuous as the temperature changes.

    Nikko Diamond 10-50x60 MK3
    It ranges under in cold weather, with 3.3 m by 10 °C at a 50 metres target.
    The error is continuous as the temperature changes.

    Tasco Custom Shop 10-50x56
    It ranges over in cold weather, with 2.1 m by 10 °C at a 50 metres target.
    Funny because it is almost the same between -5 and 5 °C and also the same between 15 and 30 °C but changes slowly between 5 and 15 °C.
     
    Last edited: Mar 11, 2013
  11. Ceathreamhnan

    Ceathreamhnan WHFTA Champion 2013

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    For the scopes that over-range in cold weather, that makes sense because, the contraction of the scope body will pull the objective lens further in, which has the effect of re-parallaxing the scope to a distance closer to infinity (as I have to wind out the adjustable objective on my Hawke scope to bring the focus closer).
     
    Last edited: Mar 11, 2013
  12. maestro

    maestro European Champion 2018

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    This question is not so easy. Why moves the S&B 9x more than the old Nikko?
    And why do some scopes underrange while others overrange?
    I think this depends on the inside construction, also the smaller parts move with temperature, there can be parts made from different materials, which move differently with temperature and enlarge or reduce the overall effect.

    This means for me that a scope with negligible temp shift CAN be designed, just they have to check the construction under different temps and play with materials and measurements until the temp effect is almost zero in the -10...+40 °C range.

    Also note that the old Big Nikko has separate parallax adjustment i.e. it moves some lenses before the erector tube while the scopes where the three turrets are in line the parallax adjustment moves some lenses on the end of the erector tube. This means that these lenses can move from the heat expansion of the erector tube, too. And see the difference, Nikko MK1 0.5 m vs. Nikko MK3 3.3 m.
     
    Last edited: Mar 11, 2013
  13. RobF

    RobF Administrator Staff Member

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    I was coming at it from a different angle.

    Agreed about the coefficient and measurements, but i'm wonder if we're not so much concerned with it's overall length as the relative movement inside the scope.

    Parallax error comes from the image focus point not being at the same point as the reticule. The amount a scopes plane of focus moves back and forth relative to the reticule is quite small... i did have a link to the calculation which was able to reveal the order of measurement being (from memory) something around mm's at most.

    What i'm wondering is if the parallax tube assembly material plays a part in this. If it was a less expansive material such as steel or titanium would we see as much shift.

    What's leading me to this is because if we accept that most scopes use ally as a body, then we should see them all ranging the same way in the same temperature direction... but as this thread shows, we don't. So perhaps there is something relative changing.

    I'm not sure how the mechanics of the parallax adjustment works from sidewheel to lens, but perhaps it's something in there that is changing, rather than the overall scope.

    Just was thinking out loud. Still doesn't change the thing that we need to learn our scopes ;)
     
  14. neilL

    neilL New Member

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    Another observation which may help. When the weather was much colder I placed a couple of gun+scope combos on a table just outside and focused them while they were still warm, noted sidewheel, etc. After they dropped down below freezing I first checked the image before touching anything. They were all still in focus near as dammit. Re-focusing gave the switched sidewheel position. So I don't think the tube expansion/contraction was my own particular issue but it is something inside.

    I tried this at 50+yds and also at 16yds with a 50' which is basically the same parallax setting.

    Cheers
    Neil
     
  15. maestro

    maestro European Champion 2018

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    Interesting and useful observation indeed but I think I can explain it.

    You may know that in the scopes parallax wheel rotates a plate with a snail shaped groove in it and a pin is attached onto a lens (or group of lenses) which fits into this groove and moves the lense(s) when the wheel is rotated. I think when you left the scope outside then the inside lenses remained in their position because instead of moving only some tension was built up. And when you rotated the wheel this tension could release and you could set it sharp only with another side wheel position.

    But this all is only my theory, based on my knowledge in mechanics (I'm an architect), it would be so fine to have the possibility to talk with one of the scope designer optic engineers of any big scope manufacturer. I'm sure that they could cure this issue just it's not worthy to bother with it for a dozen of FT shooters while all the other shooters (who don't have to rangefind) don't mind a little parallax shift...
     
  16. Delphinus

    Delphinus New Member

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    I agree that the metal expand and contract, a simple solution will be a scope with a carbon fiber body.
    Where I shoot we don t have subzero temperatures so I don t have your experience but how about a different approach?

    Did any of you made tests to support that thesis?

    My belief is that the biggest factor in the differences that I read here are related with light/contrast and not
    temperature and they are related.

    Rod and all others with March scopes buy a 55mm UW filter attach it to the scope and experience differences off 5 yards at 55yards.
     
  17. sportsmatch

    sportsmatch Member

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    Buy a shite& bender FT model and you will see it move with temp mine moved 7.5yds from 10 to 28 degrees with constant light conditions the scope was set up at 14 degrees the 50yd target was 52.2yds at 10 degrees and 45yds at 28 degrees
    I tested my leupold 20/50 mag at the same time and it read the same range 50 yds

    lucky to get the money back for the shite & bender

    Gary
     
  18. maestro

    maestro European Champion 2018

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    I have tested my S&B and Nikko scopes in Hungary, between -10 and +40 Celsius, so this 50 degrees range tells something already.

    I did tests with light/dark environment, also for myself and for the targets, i.e. I was in shade or the sun shined against me, and I had two targets on the same range, one out in the field the another one in the deepest corner of a big cardboard box of a fridge. Absolutely no differences, only the temperature could change the results.

    Light/dark issues are experienced by those who are too young and/or have good eyes, their eye can accommodate faster if there is more light and contrast and they can see sharp even before it is really sharp. Those who like me are staring at the monitors 16 hours a day and have old and slow eyes they can see it sharp only when it is really sharp.
     
  19. NJR 100

    NJR 100 Because I`m AWESIME !!

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    I think you had a bad one Gary.

    The one Peter was using was much better and El capitano has done ok in the mfta winter series when using his.
     
  20. NJR 100

    NJR 100 Because I`m AWESIME !!

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    I`m moving to Portugal

    I think when the worlds comes to Wales we will hold it in November lol :eek:

    Not done any specific tests, but experience and results show the shift in 5 or below.

    Light can sometimes have an impact, ie i wont range fiond on a white or yellow target in bright sunlight as it can over read by 2.5m past 40, or a black target in a dark wood may under range.

    Know your equipemt as they say.
     

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