[time-nuts] A real world project need for timing accuracy...
Jean-Louis Oneto
Jean-Louis.Oneto at obs-azur.fr
Wed Nov 3 00:47:26 UTC 2010
In the atmosphere, if your aperture is smaller than the R_0 (Fried
parameter, typically about 1~2" in daylight, can be several feet by a still
night), your resolution is similar to that corresponding at your aperture,
and the whole image will seems to move. If your aperture is greater than
R_0, the image will explode in several "speckles", each with a size
corresponding to the diffraction limit of your aperture, grouped in a
cluster corresponding to the diffranction limit of R_0, and the pattern of
the speckle will change rapidly (several 10's of Hz). You can exploit the
theoritical diffraction limit by using a fast camera, processing each frame
(autocorrelation or FFT) and then averaging the results. (search "Speckle
Interferometry" for details, that will hardly be low-cost or easy...)
HTH,
Regards,
Jean-Louis
----- Original Message -----
From: "Bob Camp" <lists at rtty.us>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Wednesday, November 03, 2010 12:04 AM
Subject: Re: [time-nuts] A real world project need for timing accuracy...
> 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.
>>>
>>>
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at febo.com
>>> To unsubscribe, go to
>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>> and follow the instructions there.
>>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
More information about the time-nuts
mailing list