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To: Carry_Okie
Carry_Okie says:   "FYI"

Very interesting article. Not to be too skeptical, but I'd like to see that photo first, sil vous plais, before making any judgment.

Overcoming the problems of making an optical system that could perform (as alluded to in the thread article) are daunting, to say the least. Photographing low contrast surface features on an object traveling at 12,500 mph and 200,000 feet away through a turbulent atmosphere with high resolution, is nothing short of spectacular.

Absent atmospheric affects, the absolute bare minimum resolution (the diffraction limit to the so-called "airy disk") is given as (approx.) 120/D (where D is the diameter of the telescope mirror in millimeters). And that limit is only valid when viewing high contrast objects, which definitely would not be the case here.

While it would at first appear that all you need, in order to get the needed resolution, is to increase mirror size, the atmospheric turbulence that causes "speckle", or spatial aberration, increases with mirror diameter. The result is that in mirror diameters greater than about 250 mm, any increase in resolution is offset by an increase in speckle. And thus, your theoretical resolution, under the very best circumstances, of high contrast features, 200,00 feet away, using a telescope with a 250 mm (10 inch) mirror, would be approximately 1/2 foot.

In the last 20-30 years, astronomers (and especially the military) have developed and use adaptive optical systems to over come the effects of speckle. But these systems are quite complex and require either multiple (hundreds of) identical exposures and/or a tracking laser in order to successfully overcome speckle. Whether such optical systems could have worked in the present case is just pure speculation, without seeing the actual photos that this article claims exist.

(Just guessing here, but the actual sensor to keep the tracking system pointed at the orbiter was probably radar or a transponder, rather than the more common optical sensors.)

There are other serious problems that make we wonder about the true resolution of these photos. In order to maintain such spectacular angular resolution, the tracking system (in this case) would have had to be able to track the object smoothly at tracking rates exceeding 5° per second. When trying to imagine such a tracking system, keep in mind that there is no such thing as an electrical motor that does not produce torque ripple. Torque ripple is not a big deal until you are trying to push the limits of diffraction and speckle correction.

Additionally, since the orbiter vehicle is reported to have an angle of attack of approximately 35° during re-entry, the telescope in question would have only a limited angular window during which to view the wing in a manner that would reveal the damage they reported.

Could they really have an optical system capable of such a stupendous feat and overcoming all those problems? Apparently they do, but damn, that has really got to be one hell of a state-of-the-art system and one that only the government could afford to build! I would have given my eye teeth to have been involved in the design and development of that system.

Regards,

Boot Hill

270 posted on 02/07/2003 5:18:54 PM PST by Boot Hill
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To: Carry_Okie
Look at post #269, above. It looks like I was correct about being skeptical both about the limits of resolution as well as whether they would be able to capture the low contrast features like that of the wing. The picture appears, at first glance, to be of the re-entering orbiter, but in reality, all they captured was its silhouette. And the resolution appears to be about 4-5 arc seconds (or about 4-5 feet at that altitude). Still, that is a pretty remarkable accomplishment!

Boot

271 posted on 02/07/2003 6:01:41 PM PST by Boot Hill
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