[time-nuts] A real world project need for timing accuracy...

Bob Camp lists at rtty.us
Wed Nov 3 00:04:22 UTC 2010


Hi

The Wikipedia numbers would all work out just fine in a vacuum or in *very* still air. I have yet to find a real world situation (daylight) where you are anywhere near those conditions.

Bob

 
On Nov 2, 2010, at 1:16 PM, Robert Darlington wrote:

> Hi Jim,
> 
> This doesnt' look right to me.  I'm getting roughly 2.3 inches at 2400 feet
> is 0.08 miliradians.    0.01  miliradians (1*10^-5 radians) at 2400 feet is
> 0.288 inches (roughly 30 caliber).  Wikipedia says that to resolve 0.01
> miliradians you need:
> 
> R (in radians) = lambda / diameter (of scope)  (aka, Dawes Limit if you use
> 562nm light)
> 
> 1 * 10^-5 radians = 562nm (green) / X
> 
> X= 5.62cm aperture or 2.2".    This is what it comes to on paper, in
> practice you'd probably need something bigger because of atmospheric
> effects, lens quality, and the like.
> 
> That being said, I can't see my holes at 300 yards with my Leupold scope
> with an opening greater than an inch.  I can just barely make them out at
> 200 yards.  See http://en.wikipedia.org/wiki/Angular_resolution  - Also,
> somebody please double check my math.
> 
> -Bob
> 
> On Tue, Nov 2, 2010 at 7:28 AM, jimlux <jimlux at earthlink.net> wrote:
> 
>> Bob Camp wrote:
>> 
>>> Hi
>>> 
>>> Ok, I mis-understood the question.
>>> 
>>> In my experience, you can have big buck (as in many thousands of dollars)
>>> optics and not see .2" holes at 800 yards. The bull's eye is a *lot* bigger
>>> than the hole the bullet made.
>>> 
>>> 0.2" at 2400 ft is about 0.08 milliradian.. or 0.3 minutes of arc.  Your
>> eye can resolve about 1 minute of arc... I'm not questioning your
>> experience, but it seem that even a moderate power scope should allow you to
>> see the holes.  As I recall, the Rayleigh limit for resolution is something
>> like 0.7 milliradian/mm of aperture, so 10-15 mm aperture would be in the
>> right ballpark..
>> 
>> I can imagine needing more aperture than 3", though.. you're not interested
>> in resolving a star, but something more akin to separating dots.
>> 
>> 
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