The limit for focusing light from a star is based on the surface temperature of that star and of the arc of the optics.
It is obvious that the brightest thing in our sky is the sun
however there are distances from the optic and sizes of sail that dictate focusing light from something other than the sun is the best way to push an object.
For ease of calculation in a thought experiment lets pretend the moon is covered in mirrors Since the appearant size of the moon is the apparent size of the sun at our distance unless you are wanting to push your sail towards the sun (an idiotic notion I grant) your force on any light sail is no greater than without the aid of the optic. At that distance it has no more relevance.
At 6 times the angular luminousity of our sun our bright A class neighbor Sirius steps up and can power a sail 12.6 meters across. Since the sun is as big as the moon from our distance. Geometery says It has an advantage for this size of sail over the sun from a distance from the moon where it looks 6 times bigger than it does from earth. That is a little under half the distance. when Sirius becomes more relevant than the sun.
At some point in distance a hotter more distant star will take over. It may take a really small sail and some impossible tracking but technically and mathematically some B or O class star will come into relevance.
Practically speaking This has very little relevance given the tracking requirements and the small sails. If you don't have another reason to put mirrors on the moon I doubt they will get put there for this purpose. The best I can claim is that if you already have such a system and your mirrors are on the dark side it starts to make sence to use it. however The closer your solar system gets to some of these brighter B and O class stars the more relevant it becomes in a solarsystem where you get 1% of your light from such a star it starts to make all the sence in the world with a B class of 7 solar radii and 20000 times the luminosity and O class stars with 15 radii and 1400000 times the luminosity. You don't have to orbit it to use it very effectivly.-- MercuryNotMars, Mar 14 2007 Inverse Square Law http://csep10.phys....ight/intensity.html [nuclear hobo, Mar 15 2007] magnitude scales http://www.astro-to...magnitude_scale.htm [nuclear hobo, Mar 15 2007] Despite the apparent brightness of Sirius (magnitude 1.42), it's distance of 8.6 light years and the inverse square law renders its radiation insignificant. The reflected light of the Moon would be far more effective in driving a solar sail than the light pressure from Sirius.
If you are going to use Moon mounted mirrors, they should reflect the sun's light rather than that of a distant star.-- nuclear hobo, Mar 15 2007 No offense, but I think I like your annos better than your ideas.
For solar sails, the sun will probably work reasonably well for many applications. It's very bright, and by aiming the sail at an oblique angle to the sun's rays, you can either increase, or decrease velocity, which should allow you to easily alter your orbit.
Of course, when behind the shadow of another planet, that may not be so good, but in most such situations, solar sailing will not appreciably alter the course of your craft soon enough to really matter. Either you're too close to the planet anyway, and will crash, or you're too far from the planet anyway, and will soon be back in an illuminated part of space.
For such situations, it may be more efficient to get solar powered lasers set up. Unlike mirrors, solar powered lasers would also be useful for altering the course of asteroids and meteorites to some degree. Plus, they would be able to focus more energy on the sail, which could more than make up for the losses due to energy conversion.-- ye_river_xiv, Mar 15 2007 Thanks for the tip on the inverse square law. That also coorisponds perfectly to the percentage of the sky your light source takes up.
At twice the distance from the sun you get 1/4th the light also the sun looks like it has 1/4th the area. coincidence?
The angular luminosity is constant at any distance. If your light sail is small enough to fit in either focus point and you can factor out tracking then the brightest(angular luminosity) star wins .-- MercuryNotMars, Mar 15 2007 makes perfect sence to me.-- the dog's breakfast, Mar 15 2007 I think that is a good point, but there is more of an unseperable divide between Solar sails and Laser sails. What I am discribing would be used both ways solar and stellar one after another or even concurently, since you can be shooting from the dark side of the whatever moon(certainly less favorable when they are canceling).
I am sort of of the opinion that they should both be called Stellar Sails, all the sudden. The Sun is technically another star. I appreciate the comment. Nomenclature is very important since we need to communicate with other people. In spite of what some people may think I don't just randomly pound on a keyboard.-- MercuryNotMars, Mar 15 2007 Photokinetic sails? That ought to apply whether the light is being provided by the sun, moon, stars, lasers - or any other photon emitter.-- zen_tom, Mar 15 2007 I see a definite difference between solar sails and laser sails. I would say they are very distinct to the point of incompatability. Though they have photons in common they technically start to diverge as much from each other as they are unique from rockets. Combining them is probably about as useful as putting a rocked on one of them. At the moment I am in favor of having the names laser sail and stellar sail
Though I have some affinity for putting mirrors on the Moon I would not list that as a lightsource that can ever be used with respect to the idea at hand. I don't mean to imply that you said it either.-- MercuryNotMars, Mar 15 2007 Angular luminosity doesn't power solar sails, photons do. At 8.6 light years the photon density from Sirius is negligible, at least as far as solar sails are concerned. If Sirius had an apparent brightness that exceeded Sol then you might have a case, but it doesn't, so you don't.
Sol's apparent magnitude is -26.7; Sirius' is -1.44. Sirius is 10^10 (10,000,000,000) times less bright than the sun in apparent magnitude.-- nuclear hobo, Mar 15 2007 and yet the discribed scenario has me getting 6 times the photons from Sirius that I could get from the sun. Did I get some calculation wrong? Did you not notice the limited usefulness that I described? Lets not talk past each other.-- MercuryNotMars, Mar 16 2007 All this talk of sailing...a keel would be useful. Then you could Star Tack.-- Ling, Mar 16 2007 Could you put solar cells on your solar sails and include a couple of small electric motors on them to make them flap and billow in the lack of atmosphere? It would really help me visualize the concept.-- the dog's breakfast, Mar 16 2007 [MNM] I agree with you. This would work as you say, within the limits you have stated.
Imagine a projecting telescope (any optical telescope can be used this way by holding a screen at the correct distance from the eyepiece). The area of an image on the screen is directly proportional to the object's apparent area, and the amount of light recieved is proportional to the object's apparent luminosity, so the light per unit area on the screen is proportional to the object's angular luminosity, as you say (+).-- spidermother, Mar 16 2007 Aye mate! with a whole moon of mirrors and a star at our command we could power a whole ship in a bottle!
Flap Flap. Get your tweezers out and weigh anchor! [+]-- MercuryNotMars, Mar 16 2007 //That also coorisponds perfectly to the percentage of the sky your light source takes up// Err, no. Not really. In fact, not at all. Think about a solar eclipse.-- AbsintheWithoutLeave, Feb 06 2009 I still think the detractors haven't understood this properly. It's not likely to be useful, or even to work at all, but is theoretically interesting. An overlooked reason why this might not work is that an angular resolution finer than the angular size of the star in question is required; this might not be possible even in theory.-- spidermother, Feb 07 2009 Yeah I did kind of include tracking though as a problem to factor out. This may be a bigger deal in other solar systems. Think of it this way. Can you take a long exposure shot of a star and have it appear brighter than the sun in any given spot? yes (that is including the same level of tracking to line up a star on a given peice of film.) Certainly if we could not do this on any peice of film with a short exposure we could not measure angular luminosity in the first place. (in any way I can imagine). I think I put this idea out there as a thought experiment because of people talking about laser's not being practical on the solar level, adding optical error in places that they shouldn't and their not understanding what the inverse square law implies and what it doesn't imply.-- MercuryNotMars, Feb 08 2009 random, halfbakery