I had not heard that anti-matter had actually been identified in a laboratory environment; I'll have to read up on it.
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A 25 megaton nuclear weapon would produce 1.046x10^17 joules of energy.
Assuming that your warhead detonates, say 10 kilometers off target, inverse square law says that you'll spread that energy over a surface that's 1.25936x10^9 m^2.
Dividing, we find that there'll be 83,058,061 joules of energy per m^2. That's 83 megajoules; enough to vapourise metal faster than the speed of sound.
I would consider a 25 megaton warhead to be a very powerful weapon, and not one widely in use.
Looking at something you might be able to throw at the enemy en masse , like a 10 kt nuclear weapon (the Hiroshima bomb was 13 kt), we find that it produces 4.184x10^13 joules of energy. At that very same distance of 10 kilometers, we find ourselves with 33,223 joules of energy. 33 kilojoules; enough to singe the paint.
So to answer your question about how powerful nuclear weapons are, it all depends on your missile design philosophy and the proliferation of nuclear materials in the proposed future. If you guys are throwing 25 megaton warheads at eachother like they were candy, you better hope to God or whatever else you hold sacred that you have something other than bare steel protecting you. One the opposite extreme, if you guys are tossing a small number of 10 kt warheads around and nothing more powerful, then you can feel pretty safe in the fact that about all that will take you out is a direct hit.
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LIDAR can be rigged to function like Radar, sending out omnidirectional beams. It would render radar-absorbing paint futile; I had no point beyond that.
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There is no point to visual cloaking unless your starships are massive enough to blot out the light of several stars when seen from a distance. Why? Because on interstellar ranges, finding a ship that's even 1,000 meters long with visual scanning alone is completely pointless; you will not find it in any reasonably useful amount of time. Sensor masking and silent running are also difficult. Why?
Temperature of space? About 3 degrees K.
Comfortable temperature for humans? About 285 degrees K.
Operating temperature for a nuclear reactor? About 800 degrees K.
Your heat emissions will betray you even if your drive signature does not. You cannot just magically make that heat disappear; it must be radiated away from the ship. "Well, couldn't I refridgerate my ship and radiate it in a direction that you're not in?" You could, but that assumes you know where all of my sensor arrays are.