[time-nuts] A real world project need for timing accuracy...
jimlux at earthlink.net
Wed Nov 3 14:53:52 UTC 2010
Marshall Eubanks wrote:
> On Nov 3, 2010, at 10:12 AM, jimlux wrote:
>> Bob Camp wrote:
>>> I have indeed seen people try to do this with 18 ~ 24" aperture optics. They don't seem to do a lot better than the smaller stuff spotting holes at distance. They do get a nice bright image though. Bob
>> Yeah, at that size, the atmospheric propagation issues are probably the limiting aspect. For astronomical use, you tend to be looking closer to straight up, which helps (same for looking down).
> Looking down is typically better than looking up; the disturbing parts of the atmosphere are further away.
>> Clearly (a pun) it is possible to get this kind of performance from reasonable sized optics e.g. the proverbial reading license plates from orbit... 10cm resolution at 300 km range.. .1/3E5 -- .3 microrad (I do calculations in my head MUCH better in metric)... even 1 meter resolution (which is widely published) from orbit is 3 microradian.
>> the KH series satellites are rumored to have 2.4 meter optics. Whether they have 10 cm resolution in use is uncertain.
> The HiRise camera on the Mars Reconnaissance Orbiter has a 0.5 meter aperture and routinely does 20 cm / pixel resolution from orbit.
Mars doesn't have a lot of air.. (surface pressure is like 100kft on
earth), and if resolution is proportional to aperture, then the 2+
meters on KH-11 type satellites would get you 5cm.. Not quite reading
license plates (for plates laying on the ground), at least not without
some post processing.
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