What actually moves when a scope "shifts"

Discussion in 'Hunter (HFT) & Field Target (FT)' started by Lol Moore, Sep 17, 2013.

  1. Lol Moore

    Lol Moore Banned

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    This may be a silly question but when a scope is said to shift at a particular temperature what actually shifts

    Is it the optics/lenses that move so that when you range in the normal way you actually come up with the wrong range which then means you dial in the wrong number of clicks for the shot :confused:

    or

    Is it the elevation mechanism that moves so that you range in the normal way and get the correct range but when you dial in the correct number of clicks the point of aim moves to the wrong spot :confused:

    or

    both :confused::confused:
     
  2. Conor

    Conor Never been banned from sales Staff Member

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    It has to be a lens carrier of sone description or a part inside the parallax mechanism
     
  3. Lol Moore

    Lol Moore Banned

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    so,,,,,

    so does that mean you actually range wrong and concequently apply the wrong number of clicks and so miss ;)
     
  4. RobF

    RobF Administrator Staff Member

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    Yep. I suspect it's either the parallax tube along which a coil shaped groove runs which moves the lens as the wheel moves. That or what the lens sits in inside the groove. I think different scopes use different materials and slightly different designs. And added to which the way the coil shaped groove is shaped may increase or decrease the problem.

    I'd say there's a fair bit of maths involved in getting the parallax to work how we like it vs lens characteristics. I'd say without chopping one in half there's probably not much to add. I've seen one of my scopes xrayed and chatted to the khales designer and that's as much as I've been able to guess at. Khales guy confirmed my theory of how the parallax worked on a sidewheel scope.

    Reckon the ideal way will be to chop up a make of scope known not to shift, of not shift a lot and compare to one that does and perhaps one that shifts like a flick switch.
     
  5. neilL

    neilL New Member

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    I think it was back last winter when I did a simple experiment. I placed two rifle+scope rigs on a bench outside with a digital thermometer taped to the scope box to display the temp drop, with ambient temps in the sub-zero. I focussed at high mag on a branch at the back of the garden (with the 50ft lens the wheel was in the tricky 50-55 yd range). Then as the temp dropped I checked the focus without touching anything. The image was always near as dammit in focus but when I gave the sidewheel a big movement (end stop to end stop) and re-focussed the wheel indicator was 'shifted'. So for the scopes I tested (Nikko and Leups) the optics don't shift but the mechanism does end up with an offset as Rob described. As others find once the temp drops below some value the shift stops. Slightly more puzzling, I sometimes find there is a residual offset when it all warms back up above the shift temp. Which bugs me no end :)

    Have fun!

    Neil
     
  6. rich

    rich Active Member

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    Think about a front focussing scope for a moment; what is the gap between say 50 and 55 yards on the front ring? Maybe 10mm between them?

    Now, the AO scope works by effectively unscrewing the front element (or group) to focus on closer objects. The movement from 55 to 50 yards represents a very small increase in the length of the scope. Probably around 0.1mm.

    Take that same scope and warm it up from 5 degrees to 25 degrees; the increase is 20. From O level physics we recall that the scope increases in length due to thermal expansion, and if you know the coefficient of expansion for the metal in use you can work it out. For aluminium, it's 0.000022 per degree.

    If the scope is say 400mm long, then the increase in length in this example is 400mm x 20 degrees x 0.000022 which comes to 0.176mm.

    So the scope has grown more in length due to temperature increase than by being refocussed from 55 to 50 yards.

    Frankly it's a real surprise and an achievement that scopes shift so little with temperature.
     
  7. RobF

    RobF Administrator Staff Member

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    Inside the scope the range of movement of the pa assembly is in the order of inches. I think it works in reverse to the ao system. In ao the end lens moves and the rest stay in place. In a sidewheel the end lens stays put and the pa lens set inside moves back and forth. I believe it actually turns following a coiled path. The shape of the path can be linear like the coils in normal spring or progressive so the coils are tighter at one end than the other. What keeps the lens on the coiled path I'm not sure of but I wonder if its this area that sees slack or tightness rather than the tube expanding or contracting.

    I've got a scope that's shot away so may take it to bits.
     
  8. blackscale

    blackscale Member

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    The 'expanding scope' seems a likely theory!
    As Rich say's it's only a tiny movement to give you a false px reading...:D
     
  9. peterh

    peterh Stoopit forriner.

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    If you would suspect that this might be a problem: there's an easy test you can do yourself.

    1. Take a fairly large paper target at short range, say, 8 yards, and fire off three shots. Start at your zero position for 8 yards somewhere at the top left. This will be your aim point for the entire excercise.
    2. Then, dial all the way through your range up to where you come off highest (somewhere in between 20 and 30 yards I suspect), and shoot three times, aiming at the same point.
    3. Now dial a full turn to the right, shoot three times, again aiming at the same point.
    4. Now dial the elevation turret back to 9 yards, leaving the windage turret untouched. Fire three shots, still aming at the aim point mentioned in step 1.
    5. Lastly, dial the windage turret back to the left a full turn, and fire off a shot.
    If your turrets work well, you should now be back to exactly where you started, and the four groups you shot should form a nice square.

    I've done this with all scopes my wife and I have or have had, and there was only one that malperformed - a very crummy 4x20 scope. All others (more than a handful of Swifts, a Falcon, two BSA Platinums, a BSA 8-32x44, and an Optisan El Cheapo 3-9x42) stood the test with flying colours.
     
  10. Lol Moore

    Lol Moore Banned

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    Peter

    Thanks for that will have to give it a try ;) ....however

    Its not actually what i was getting at, I was wondering what actually moves in the scope to cause shift and why is it a step change, if it was simply expansion surely it would be gradual unless something "clicked" out of place as the expansion reached a certain point :confused:

    The comment of Rich, we are lucky its not much worse, rings true for me, the engineering must be exceptional not to get more shift than we do

    From your description of the "square test" and the results that only one scope has failed is it not too easy, should it not be at longer range to magnify any errors - if everything passes not a real test ;)
     
  11. peterh

    peterh Stoopit forriner.

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    Not really, because at 8 yards, you'd be looking at all pellets going through the same hole for each test point.
     
  12. RobF

    RobF Administrator Staff Member

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    Some scopes don't jump in steps. Some like the s&b just move with the temperature in an analogue type fashion.
    It's something to do with the pa assembly, but no one seems to know what, even high end scope manufacturers. What is known is that the characteristics are common to each scope of the same design, and that different scopes have different shift profiles.
     
  13. Lol Moore

    Lol Moore Banned

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    Interesting....

    Rob

    From the answers it sound like it is always that the actual optics that shift so you read a range but that read range is wrong which means you allocate the wrong number of clicks....or....can it be that your read the range correctly but the mechanism that moves the POA has shifted somehow so when you apply the clicks it looks in the wrong spot :confused:

    Just curious ;)
     
  14. RobF

    RobF Administrator Staff Member

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    Oh I see. Yes it's the optics that move and you dial the wrong range.

    That's when it's determined to be the scope.

    When your clicks change for a fixed range then it's the gun. Either mechanical shift or power.
     
  15. Jon

    Jon Member

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    I get that with mine, trust the scope dial accordingly, everything bang on line all year round.
    The only other scope to do that was a 94 Tasco mark1.

    Considering the number of lenses in the scope each of which has to focus at a fixed point, the temp expansion on length will have an effect. Some will vary more than others of same make especially if tops out to focus at a closer range whereby the focus may become cammy. A little expansion in length can be massive at the higher ranges. In other words the focus at top end usually requires less movement than short range and will be in a different place. Not so much with the rack and pinion types.
     

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