Heat, light and gun and scope...

Discussion in 'Hunter (HFT) & Field Target (FT)' started by RobF, Jun 16, 2010.

  1. RobF

    RobF Administrator Staff Member

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    Naff all happening this afternoon at work, but clear blue skies, so decided to sneak out and see if I could unravel the shift seen on the March. Slapped some temp strips on the scope, and grabbed a digital temp probe that an engineer had left behind one day.

    First test was to see if both read the same, and indoors, in the shade they both had everything at 19 degrees. Interestingly the strips moved pretty fast dropping from the 25 degrees of the car to the 19 of the scope within a minute or so of being attached to the scope body. The temp probe moves very fast, and often picked up just the effect of a cool breeze.

    The thing to remember in this is that heat transfers. It does this by 3 methods, conduction, convection and radiation. Whilst heat is linked to the radiation that makes you taste your fillings and hair loss, we're not concerned with that aspect.

    Using the illustration of a fire, radiation is what you can feel on your face as you get near it, convection is what you can see as the hot air and smoke rises and draws in air, and conduction is what happens to a metal bar placed in a fire, with some exposed out... when you grab it it's hot. Metal is a very good conductor, bad conductors are known as insulators... a wooden stick in the same scenario is not likely to burn your hand, as the heat hasn't transfered all the way through it and finds it difficult to do so. Organic substances tend to be good insulators.

    Ok, so that's the basics out of the way.

    The primary source of heat outdoors, is the sun... this is radiation. Radiated heat has the ability to pass through things, and different sunstances absorb different parts of the heat spectrum. That's the reason why an object left in the sun may be hotter than the air around it. You can burn your feet walking barefoot on the beach, yet you don't burn all over in the air you're walking through. Different parts of the spectrum pass through different things. Infra red for instance passes through air well, but not you, which is why it's used for heat lamps. Microwaves pass through a lot of stuff, but not water, so they heat food rather than the air...

    Black things are black because they absorb a lot of the colours/wavelengths of light (which is on the same scale as heat) and silver or white things reflect most of it. Things have a colour because they absorb most wavelengths, but reflect one particular band of wavelength of light.

    Anyway, onto the scope.

    Taking the rig down to South Dorset allowed me to put it in light and shade with ease. First test was to see what she measured in the car after an hour of airconned journey. Temp strips read 19, and a probe into the bag read 19, but the air in the back of the car was 25. That suggests the zipped up foam lined bag was doing a very good job of insulating the rig against the air in the car heated by the sun. Something to remember if your akin to getting a shifting rig out and trying to shoot with it in different conditions from home.

    Taking her out of the bag and onto the heavily shaded range, which was also at 19, i did a quick rangefind test, and she measured 55 bang on the money. Letting her settle for 10 mins, no change. The plinking range has now got a heavy canopy and is much darker than when I first set the ranges up, but they all came in true.

    Next test was to take her out in the sun, but I wanted to see what insulating her from the sun would do. So I left the scope on the ground, covered by my coat, which has a silvery grey inside, which i turned inside out and covered just the scope with. I left it for 10 mins. Being an outdoors walking coat, it's a good insulator as well.

    After 10 mins, measuring the air with the probe here had the temp near the scope at 23 degrees. The scope was reading 19 under the coat, but the exposed black muzzle brake inside was reading 25.

    After 20 mins, the air was 22, the scope still 21, the muzzle was 30. The temp strips read 24.

    Testing the rangefinding again in the shade and she was still bang on 55.

    Next test was leaving her naked in the sun, no protection.

    After 10 mins, the air was 24, the scope was 28, the muzzle was 30. Under the objective scope cap, which had been left closed was 33, the strips read 32. The reg, which is silver coloured, read 25 on the probe.

    After 20 mins, the air was 26, the scope was 32, the muzzle was 30. Under the cap was reading 35. The strips were reading 37. The reg had risen to 30.

    After 30 mins, the air was 26, the scope was 32, the muzzle was 30. Under the cap was reading 37. The strips were reading 40. The reg held at 30.

    Taking her straight away onto the shaded plinking range, and the scope was under ranging by 4 yds at 55.

    I had set up a camera to meter a the target, and this showed little change between the start and finish, and the difference was only probably attributable to the sun moving around... even though it was in shade, it was probably hitting now at more of a steeper angle.

    Within 10 minutes of being in the shade, the scope had cooled to under 25 degrees. It was difficult to determine how long it took to get where, but within 15 minutes it was all at 20 degrees, save the reg, which was just under 30 still. As quick as I could, I looked at the shift, which diminished by the second, until it was gone.

    So, conclusions...

    1) The scope reacts quickly to radiation from the sun, and radiates and cools quickly when left in the shade. 20 mins seems to be around the time to see it at almost it's maximum. This goes for the rest of the gun as well, apart from the reg.

    2) The silver of the reg, or perhaps it's different material, means it doesn't heat up as fast, however it holds it longer. Whether this is because it's silver, and traps the heat in, or because it could be a different material, i don't know. What I do know is that the reg heating up, and the scope, did not impact POI at all.

    3) Shielding the scope from the heat with something that reflects signficantly slowed the scope from absorbing radiation from the sun. I suspect that it would also slow the radiation of the scope, thus making it cool slowly as well. Things that insulate tend to do so both ways, resisting the heat transfer both in and out.

    4) The stock was 3-4 degrees different on one side than the other, however the carbon shrouded barrel was only about 1-2 degree different. A more severe exposure of the sun may yeild different results, and i guess it also depends on what's underneath the gun.

    Pondering further... coating with white or silver may slow down absorbtion of the sun's heat. However, as it is in direct contact with the surface of the object, if it conducts, then any heat that gets through will be conducted into the object quickly, especially if that is metal. Shiny aluminium foil is supposed to reflect in the order of 90% of radiated heat, but it is also a good conductor, meaning the 10% could be transfered easily to whatever it touches,again especially if that's metal. That might suggest that coating alone could be improved upon with an insulating layer between it and the object you want to protect, or that coating alone might still have it's weaknesses.

    Also to ponder, is that eventually, the scope will absorb heat by conduction from the ambient air temperature... but considering that was only 20 degrees, (the air was measured near the ground where the air was, so may well be picking up some reflected heat off the ground, or radiated heat from it) today objects got up to 35-40 just from absorbing radiation... so reducing this right down could mean temp shift could be heavily mitigated, if it's slowed to the point to where it stays within a more comfortable bracket.

    I guess the only way of testing that up is to work out what you want to shield and really bake and monitor it.

    Also, removing whatever you use to shield from the sun even for a few minutes, may over time, quite reduce it's effect as it receives hit after hit of heat, and if you then shield it, you may trap the heat in that it's absorbed while exposed.

    Interesting to know if the reg is alloy, because if it is, it might suggest that silver paint may have it's tipping point, and that you may need to spend more time acclimatising a scope then... but once there it should hold for longer if it finds somewhere cool.

    So really it's up to you if you want a steep or shallow temperature curve against time, and whether you can work out how to keep that within a more acceptable bracket.

    You might not even have a scope that shifts... or maybe it does it by a small amount. Or may be your gun does. But at least I have now much more of an idea of what's going on on mine :)
     
  2. Conor

    Conor Never been banned from sales Staff Member

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    tin regulator

    Interesting stuff Rob, the good thing is that you have it pretty much sorted:). Thanks for the science lesson too, AFAIK the reg material is tin:p
    HTH Conor
     
    Last edited: Jun 16, 2010
  3. RobF

    RobF Administrator Staff Member

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    Not yet I don't, but at least I know what the problem is... just need to build a cryogenic cooling system... :D
     
  4. FGYT

    FGYT New Member

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    i would thought a shading system to block sun rays but allow air flow would be the most effective but a littl silly looking if its a samll tent above the scope ;)

    any chance of a look at the scope if your at CGC thursday again ..always interested at looking at new Glass

    i always left my leups to aclimatise for 1hr before hitting the zero range at a shoot ... never had the problem with the Deben or Hakkos tho

    maybe these high quality scopes have metal internals which will react more to heat on expansion than cheaper plastic ones ?????
    ATB
    Duncan
     
  5. chrispro97

    chrispro97 New Member

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    interesting stuff rob,as you no i shoot a leup comp and mine moves 2 yards difference in very hot conditions like down sywell(it over ranges 2 yards ie its 55yards yet reads 57 yards:D:D
     
  6. Artfull-Bodger

    Artfull-Bodger Member

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    I was wondering about this the other day, it cant be that difficult to make up a thermal jacked that can be fitted to a scope, if you think of the aluminised cloth used in cool bags for instance, it might be worth an experiment?

    Mike.
     
  7. RobF

    RobF Administrator Staff Member

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    No, it can't ;) ;)

    Experiment no.2 if the postman arrives in time for Euskadi :)
     
  8. Artfull-Bodger

    Artfull-Bodger Member

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    Great minds think alike :)








    Mind you so do idiots :D
     
  9. Tesla

    Tesla Member

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    Hi mate.
    Let's hope the weather change drastically, so today it's raining cats and dogs, 120 l/m2 more or less in 24hours and big floods
    Very interesting post Rob.
    I've tested my Big Nikko ( alone) in the lab from -20 ºC and up to 50 ºC and I didn't found sensible variations in the POI.
    But if you test the assembly airgun , mounts and scope, probably the different expansion coefficient of each one of the materials can introduce strenghts into the system and to disturb the initial balance.
    I guess this phenomenon should be stronger in the airguns with the mechanism block in two halfs, like HW100, and lighter in the Walther Dommy........but this is only a theory . It remains only to make the experiment!...or Is there soemone who made it before?
    Anyway...if you wants to make this experiment directly , you'll have the chance to do it......IN THE NEXT EUSKADI OPEN ..so you take the right decision coming to this 5th. FT International Open.

    I'll see you soon
     
    Last edited: Jun 16, 2010
  10. RobF

    RobF Administrator Staff Member

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    The POI doesn't shift, but the range does... so where it read 55yds, it reads 51. So it's just the scope.

    But the guns is so consistent, I can tell what 55 is by firing a shot ;)
     
  11. Ali-C

    Ali-C New Member

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    Want to borrow some liquid nitrogen? :D
     
  12. greyskullnz

    greyskullnz Non member

    Rob,
    I see you have found the same as I did......and I suggest stripping away the black anodizing on any sunshade you have on the scope for a start. I did that and got a much more subtle shift in temp when exposed to the sun. I left the main body of the scope black to "normalise" when sitting in ambient temperatures.

    The sunshade tube seems to act as a big heat generator holding and transferring heat back into the scope body, where it absorbs into the largest mass of the whole assembly......the objective lens. Glass (irrespective of quality) expands under heating, and it is common sense that its dimensions will alter minutely, which may explain small shifts in ranging performance in larger diamater objective lenses. "may explain" is the operative term as I just cannot quantify it. I have found the same results as you, and have calmed and soothed my scope into a modicom of predictable performance using cultural methods rather than radical surgery.

    As for a cheap heat shield, try cutting up an ironing board cloth cover,......the silver ones that reflect heat back up off the board into the fabric. Really cheap, and has a foam barrier between the cloth and the scope. Can be kept on in direct sun, or removed in different conditions. Buy a new one tho, don't pinch the missus' one.

    I think the main way to deal with this problem is to test in conditions that you expect will prevail at a given competition shoot, and set up a variable range pointer on your sidewheel accordingly ahead of time, prioritising the longer ranges of course.

    BTW I am not posting on the SA forum any longer, too many T**ts being negative fir the hell of it......sad as it used to be a great forum. Just waiting for the same said T**t to do the same here.



    cheers, glad to see our little side topic bearing fruit. I still reckon there is some light/dark effect on ranging now I have sorted the heat issue.....we get much more UV radiation down here due to the hole in the ozone layer,....so am experimenting with some old camera UV filters now. Getting a marked increase in contrast detail.

    Atmospherics sorted
    Heat sorted
    Light n dark sorted
    pellets sorted

    just gotta sort the damn loose nut behind the trigger!

    GS
     
    Last edited by a moderator: Jun 17, 2010
  13. Gibbs

    Gibbs New Member

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    After cooking my Steyr in my van at Newbury, I have been researching temp shift on gun and scope and have fitted a temp strip to the scope and a digital thermometer under my cheek piece to measure air temp.

    The scope (old big nikko) begins to over range by upto 7 yards! from 28 degrees and upwards. So I double checked this by cooking the gun in the heat (34 degrees) and checking the range finding as it cooled.
    I found the scope continued to over range right down to 22 degrees, which I guess means that the scope internals were still hot and continued to cause the problem.

    If scopes were designed to range find (which they are not) this problem could be engineered out, but until someone builds a real FT scope we will have to chase the temp along with the wind!
     
    Last edited: Jun 17, 2010
  14. Jon

    Jon Member

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    I would just note where a change happens and cater for it, job done.

    Gibbs there is a purely made FT scope its called the S&B FT:D
    Also end of July possibly, the new Falcon T50B FT designed from ground up with significant testing in UK and US. Its designed to rangefind and user friendly unlike the BSA/Nikos plus the turrets correspond to Leupolds with no anomalies again unlike the BSA.
     
  15. sportsmatch

    sportsmatch Member

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    Who is going to be the first with a thermos jacket around a scope :D

    Gary
     
  16. RobF

    RobF Administrator Staff Member

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    ooh i wonder ;)
     
  17. Anon

    Anon Non member

    What we need to do is get one of those tyre warmers that they use in F1 and wrap that round the scope.

    Hhhmmm thinking about it, the Daystate already have the battery, so run a wire from this upto a jacket wrapped round the scope.

    regards,

    Matt
     
  18. Gibbs

    Gibbs New Member

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    I don't believe for one minute that the S&B FT scope has been designed to range find ! I have heard it suffers from temp problems just like all the others and I heard some of them now don't range find beyond 40 yards at all !!
    I'm very pleased to hear T50 has been designed to range find, I can't wait........
     
  19. maestro

    maestro European Champion 2018

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    I'm afraid that this so called "purely made FT scope" has never been brought out to the courtyard from the air-conditioned laboratory during the years of its development... :(

    Anyway, it's a good scope: very wide field-of-view (about 1.5 times wider than the Big Nikko), 32 MOA in one single turn (no more overturning), 1/4 MOA clicks, very clear image etc. If it had a 75-80 mm front lens and wouldn't be so sensitive to temperature then it would really be an FT scope, but not yet.
     
  20. RobF

    RobF Administrator Staff Member

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    Took a couple of indentical scopes down the range today, but the sun wasn't as hot... they're identical all bar one thing, one is painted white, the other left black.

    The scope temps only got up to 26-28 degrees in the sun, and although there was a couple of degrees, perhaps 3 at best between them at worst, the sun wasn't strong enough to really cook them... they cooled down too fast in the shaded plinking range to ascertain if they ranged differently, and i've gotten so used to the march it took me a while to get used to these again, so it was inconclusive.

    Really need a scorcher to test if the white paint keeps the heat off any more than a few degrees, or if the black one really starts to heat up exponentially past the white one when it really gets cooking... certainly seems to indicate though so far that shielding a scope is better than painting it... having 3 scopes of the same spec, I can test all 3 methods when it shines hard next time.
     

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