Check out the Mars trilogy by Kim Stanley Robinson. It has a pretty detailed description of using orbital mirrors to focus sunlight onto the surface. In that case though, it is being used to cook the martian regolith to release gasses to thicken the atmosphere and speed up the terra-forming.
It would not be fired but simply pointed.
It would also be an easy target to attack.
Solar radiation at mars is about 550 w/m^2 for it to be effective you would need at least a 10,000m^2 mirror (100m * 100M) with incredibly precise positioning and station keeping ability.
@Pons_Aelius@inkican Pointing is aiming. And if the thing is activated, that is "fired". Sure you could attack it but orbits can be temporary and changeable. I'm not saying we shouldn't have it or use it--I'm thinking of using it as a sort of retrofitted weapon in a novel, is all. And it will happen if there's combat in space, which there will be.
Not really as it is always on, this is not a gun. It is a fire-hose that cannot be turned off. While you are "pointing it at the target it is still laying waste to everything it points at on the way to the target.
This is not a precision weapon, this is a destroy a massive area weapon.
And what I am saying is that it would be impossible to defend.
If the opposition had assets in orbit as well, destroying it would require no effort at all.
This is not something that can be retrofitted as it would have to be precisely designed to work at all.
I understand that all that can be hand waved away if you are not writing hard SF but there are plenty of other weapons that would be much easier to design, use and protect.
If you are going to ignore all that, just go with a fusion powered death ray.