[ETR3(SS)]

There's so much here I'm going to have to multi-quote it for my reply to make any sense.

I can imagine that the periscope exterior has the same temperature as the interior of the sail, or slightly warmer (some conductive heating from the sub's interior). And that temperature, in turn, reflects the water temperature of the water layers the sub has travelled through in the recent past. That could be both warmer or colder than the surface temperature at the moment the periscope is raised above the surface. If the temperature of the periscope externals is different from that of the water surface (and due to radiative cooling, it likely will be in a short time even if it started out the same), there will be a detectable contrast for thermal imaging systems.

Ok lets start with this. The sail is a free flood area. Therefor the parts of the periscope outside of the hull are the same temp as seawater. Hence you don't go looking for periscopes using thermal imagery.

Now, if you gave the exterior a capillary network of water pipes where surface water was pumped through continuously at all times when the scope is not in its down position, that would virtually eliminate the temperature contrast, making detection much more difficult.

Scopes + water inside = Bad. This pump would have to be inside the hull, wouldn't work very well outside. So that means two additional hull penetrations. The less hull penetrations the better. Also this pump would be another sound source.

Visual stealth could probably be achieved by covering the exterior with some kind of light-emitting material (fiber optics perhaps?) capable of adapting to the surrounding environment. A detector would record the current light level, color temperature and directional differences, and alter the visual signature accordingly.

To the best of my knowledge James Bond is the only one that has adaptive camouflage available. Also weapons and sensors tech has gotten to the point where you don't have to close to within 1000yds of your target to hit it. So if no one knows you're there, who's looking for a scope in the water?

Finally, radar stealth. Extensive use of RAM, non-conductive materials and careful shaping would probably be able to defeat most radars. Newer, LPI radars specialized in detecting such small objects would be harder to fool, but reducing the strength of returns still limits detection range.

I'm curious to know how much you know about the subject of radar? But if you want to find a periscope using radar, this is what you would use. http://www.raytheon.com/capabilities/products/aps137b/