Hello Leic minded friends:

It is Fri nite and I just got in from work, having taken a bunch of night shots on teh way, waiting for them to wash.

I heard a note on the radio today about satellite cameras being able to resolve license plates on cars from 90 miles up, despite the refractive effects of the atmosphere. If that is so, how come my very good 50'cron can barely resolve a license plate at 250 Meters with Tmax 100 at f8 on a tripod?

I suppose the lenses on the satellites must be rather expensive: maybe almost as much as an f1.0 Noct.:>)>:).

Do any of the ex CIA agents out there, or optical engneers know how it is done? Is it just a case of how much fun you want to have depends on how much money you have, with no different technlogy? My reasonably good astronomical telescope claims to be diffraction-limited to 1/20 wavelength, so there certainly must be some maximum resolving power depending on the index of refrction of the lens medium and the frequency of light.

Any light anyone could shed on this would be appreciated AND enlightening.

Cheers

-- RICHARD ILOMAKI (richardjx@hotmail.com), November 30, 2001

Answers

I work at an electro optical company and we had a discussion about this at lunch one day. The consensus was that based on the satellite altitude and with diffraction limited optics you could not read a license plate. If it were white and placed on a black background you might be able to see that one was there.

I picked up a book on the CIA's Corona satellites from the mid 60's where the authors discussed some of the technology and performance available. They used fairly simple but highly developed lenses such as Tessar and f/2.5 Petzval designs optimized for yellow wavelengths. They used a special high res film, with an ASA of about 6. The cameras didn't have a classical shutter but had more of a travelling design that compensated for satellite motion.

The resolving power of a simple lens is a function of the wavelength and the clear aperture: Angular diameter in radians = 2.44 * wavelength / diameter. Faster, bigger lenses are better but are harder to make.

If I remember correctly the resolution was on the order of >=2 meters so that aircraft could be made out. Later designs no doubt improved on this but I understand it's still classified. Computer analysis probably also helps a lot.

Surprisingly enough it is easier to look down clearly than to look up. This is because the last bit of the optical path has the greatest effect on the image as it subtends a greater angle. In space, the last 80 miles for a 100 mile altitude is pretty much a vacuum. The first few miles where the atmosphere is the most dense has affected the image the greatest but that becomes a proportionately smaller angle as the distance increases. (Does this make sense? I have it clearly in my mind but am not sure if I'm getting it across) So, for what it's worth,

Cheers,

Duane

-- Duane K (dkucheran@creo.com), November 30, 2001.