For when both ships are inevitably modded into the game and completely fail to operate on the same scale.

To be perfectly honest, if the intent of this thread is for developing a sense of where things stand to implement a mod where things are at the power levels they either appear to or are stated to have in their own universes, my feeling is that the effort may be well-intentioned but it's seriously misguided. Especially since many of the universes in question simply won't work as they do in their own universes under GCIII mechanics.

Star Wars ships, for example, would need to have at least several thousand actions per turn in order for the hyperdrive speeds to be approximted in game (Star Wars ships can cross a galaxy stated to be ~120000 lightyears across in a period of time on the order of days, possibly less; indications are that the Galactic Civilizations setting spans a region roughly 100 lightyears in diameter, though the size is difficult to determine since the only hard data point that isn't affected by game mechanics that we have is that Toria is 20 lightyears from Drengi, and then we have a bunch of subluminal travel times between a few civlization homeworlds, including Toria and Drengi, which can be used to estimate the distance between the worlds, though doing so requires some assumptions which, of course, may be disputed; the other data points we have, at least the ones that I am aware of, are contaminated by contact with game mechanics; for example, if my understanding of how stars are spawned on the map is correct, an Insane map on the Abundant stars setting should have ~1600 stars on it, which is approximately as many stars as are within 50 lightyears of Earth and a better match to the number of known star systems within the same region, but this is a game map we're talking about and so, even if it is intended to be a reflection of a portion of the real-world Milky Way galaxy, it is a reflection distorted by the needs of the game, and those distortions can allow you to justify the map spanning a larger region of space even with the assumption that the stars and star systems on the map must represent real-world stars and star systems).

For another example, Mass Effect ships are required to make stops at some unknonwn and variable interval to discharge their drive cores into planets and special discharging facilities or be destroyed by their own drives, and the discharge process may be protracted. This isn't really something that translates nicely to GCIII game mechanics, unless you feel like adding a negative repair value to the ship (and if that works to damage the ship over time) and then giving planets and stations a large enough flat repair bonus to counteract this (and even so, it's not really the same thing). Mass Effect also effectively has a jump node system, with very high travel speeds when moving from one node to another but much lower speeds elsewhere; the closest GCIII can come to this is the speed-boosting effect you can put on starbases, but that's fast travel from a specific region to anywhere else rather than to another node in the network.

Beyond that, if you want the mods to work with other mods, they all need to be on roughly the same power level. Whether or not it's true to the source material for Star Destroyers to have 1e6 beam attack while Reapers have 1e5 kinetic attack, if they're in the same mod, they both need to play nicely with one another, and if they're intended to be playable with base-game factions or with other mods, they also need to have similar power levels to what the base game factions or those other mods are capable of. It may not be true to the source material for the USS Enterprise-E and the Executor to be comparably powerful warships (and, in my opinion, Enterprise-E isn't comparable in power to the Executor, if we're being true to the source material), but if both of these are translated to GCIII as huge vessels and the mods are intended to be playable with one another, then both the USS Enterprise-E and the Executor need to have comparable power. Even if the mods are not specifically intended to play with one another but are intended to play with any arbitrarily chosen other mod or the base game factions, the ships will need to have comparable power.

Money's on Batman.  Hands down.

Joeball, I wish you wouldn't take this so personally.  So, yes, 30 tj is to melt an asteroid and 250 is vaporize it- point is, as the movie stands, the effects are completely inconsistent with a beam or bolt coming out of those cannons.  Either those asteroids would react very differently, heavy TL batteries have never been shown and actually have barrels that are 30 to 40 meters wide, or- low and behold- Lucas took a shortcut based on the limits of the media used.  I admitted as much when I cited the engagement range from the video game- which was Lucas gold standard canon at the time.  Which is why I threw that figure out the window as well, and made a range estimate on planetary bombardment ability.  So simmer down and put your pants back on.  You could try responding to the actual points I made, however.

Of course there's no direct evidence that a Reaper could dodge a TL blast.  There have been less Reapers in SW movies than there are people who remember what Billy D Williams' real name is.  My point was that practically everything in SW seems to be able to dodge TL fire, even the point defense and portable stuff, and that's not even only named characters.  Given the maneuvering ability Reapers are shown to have, combined with their explicitly stated ability to not be totally overconfident in estimating their odds, I'd say a Reaper would not remain stationary and tank a barrage it didn't have to.  And 

Oh, and Star Trek is so much better in the scientific accuracy and internal self-consistency fields.

Scientific accuracy: Yes, I'm sure that that inoculation against radiation poisoning will do me so much good, doctor, now if only radiation poisoning were actually a disease. Oh, and since when did event horizons contain 'cracks'? And then there's the whole "we broke the Warp 10 (i.e. infinite speed) barrier and then suffered from 'hyperevolution' and turned into lizards for a week" thing in Voyager. The only thing Star Trek does that makes it appear scientifically accurate is that it hides its lackluster science behind a mixture of technical-sounding nonsense and a small amount of marginally accurate science. On the whole, I consider the approach taken in Star Wars to be more honest; Star Wars at least doesn't lie or make up nonsense to explain things.

Self-consistency: Warp 10 is an infinite speed in TNG, DS9, and Voyager. Time travel is illegal or at least highly restricted in Federation space. There is also an attempt to create a drive which can 'break' the Warp 10 barrier, which presumably means going faster than an infinitely fast speed. Okay, so we want to go faster than infinity, so we must have negative travel times. Err, wait, no, that's illegal because we'd arrive at our destination before we left our departure point. Oh, and the times we see things that supposedly break the Warp 10 barrier? They have positive travel times, i.e. their speeds are less than infinite. Warp 10 is an infinite speed and the speed of these things is greater than or equal to Warp 10 so therefore the speed of these things as evidenced by the travel time required is less than infinity. Makes perfect sense; entirely internally consistent. (Alternatively, we can have an imaginary time. So much better, but it at least doesn't hit the time travel issue.) Oh, and let's talk about the warp factor scale. Nice and internally consistent, demonstrating such a wonderfully rational scale as making Warp 3 both 39 and 487 times the speed of light, warp 5 about 200 times the speed of light, warp 8.4 765000 times the speed of light, warp 9 about 834 times the speed of light, warp 9.9 about 21473 times the speed of light, and warp 9.975 1500 to 3000 times the speed of light. Such wonderful self-consistency. Oh, and since Voyager is expected to take 70 years to cross 70,000 lightyears, the long-term average speed of the vessel is about 1000 times the speed of light on a ship which is supposed to have a maximum sustainable cruising speed of warp 9.975 (i.e. 1500 to 3000 times the speed of light), implying that fully one third of those 70,000 years are expected to be consumed by downtime for ship maintenance. It isn't difficult to create an internally-consistent speed scale; all you need to do is come up with a table (or, better yet, a formula) once and consult it when you need to come up with a speed for the ship to be moving at in this or that episode. What do we have? A bunch of random numbers. Those, by the way, are only points listed on the chart on Memory Alpha's warp factor page; there are likely other points you could add in to further demonstrate the wonderfully consistent warp speed scale from shows not referenced in Memory Alpha's chart. And let's not forget that we keep around all the modifications we made to the ship in last week's show ... oh, wait, we have a ship that looks like it's just out of the shipyard again ... and we apparently have no knowledge of the solutions we've used in the past to solve similar problems ...

Oh, and this is a setting where poorly-designed swords and knives are practical infantry weapons, in a setting where infantry weapons at least equal in killing power to present-day real-world firearms exist and where infantry weapons at least equal in range to present-day real-world weapons ought to exist.

And of course, if having one ship be one ship is good, then having one ship be two ships is better and having one ship be three ships is even better, because of course it's completely sensible to invest in that much redundancy on a single vessel instead of, I don't know, getting two or three full-fledged ships for roughly the same cost.

As far as the wizards go, yeah, sure, Star Trek and Mass Effect are so much better. It's not like Mass Effect's biotics are Jedi in all but name, what with their complete lack of telekinetic powers and the ability to protect themselves against bullets with the power of their mind and an explanation for these abilities that holds about as much water as the prequel trilogy's midichlorians. And it's not like Star Trek has actual god-like beings wandering around; no, Q's just a perfectly normal alien capable of teleporting ships tens of thousands of lightyears with a snap of his fingers and doing all sorts of other weird but totally not magical stuff. Oh, and let's not forget the wide variety of telepaths, who run the gamut from touch telepaths who can read minds and transfer their consciousness to the bodies of others and thereby preserve themselves after their deaths for several hundred years (Vulcans), telepaths who can commit mind-rape from an entirely separate ship (the Reman helping Shinzon in Nemesis), technomages who can assimilate every piece of technology and living tissue they come across except when they can't (the Borg; the only known exceptions are Data and Species 8472) and in doing so apparently gain access to technology that neither they nor the assimilated beings or technology had (seriously, where did One get the ability to transport himself around from? A simple portable holoprojector has no need of that capability, has no reason to have information providing that capability stored within it, and isn't likely to provide the kinds of advances needed to develop that kind of technology just by being assimilated).

Let's not forget the perfectly rational technology, such as transporters that apparently don't need to follow conservation of mass given that they can create identical clones and no one notices the error (and furthermore, these clones apparently have the knowledge and personality of the person cloned; why exactly are any of the known states in Star Trek willing to allow their military personnel to be teleported by a foreign power's transporters?), nanoprobes that can assimilate anything except when the plot says that they cannot and then can create new technology and artificial biological life as a result of assimilating technology that has no relation to any of the stuff that the resulting thing can do, inoculations against radiation poisoning, warp cores that can get you out of a black hole's gravity well more effectively by acting as a bomb than by actually serving in the intended role as the power source of a drive system capable of significantly distorting space, tons of things that can do all kinds of crap justified by 'changing the harmonics of the deflector/phasers/shields/long-range sensors/etc.' Oh, and a ship that can erase things from history, eventually including itself.

I like both Star Trek and Star Wars more or less equally. Neither one is particularly self-consistent, especially if you bring the books into the discussion (for either series).

Where exactly do the Reapers demonstrate particularly impressive maneuvering capabilities?

Your evidence for this is what, exactly? Yes, the Star Destroyers are not armed in a way that makes them particularly capable of destroying small craft, but on the other hand there aren't any small craft in Star Wars that demonstrate any particular ability to be a real threat to a Star Destroyer. There is no in-universe evidence that fighter-mounted weaponry is a real threat to Star Destroyers or other similarly large warships. There is no in-universe evidence that it was anything but luck that resulted in the Executor's loss after an A-Wing crashed into the command tower - Executor was only lost at that point because something caused it to go into a sharp turn and drive itself into the Death Star before the ship's crew could reassert control over the ship, and the Death Star's gravity is very unlikely to have been the cause of this maneuver; more like someone on the bridge accidentally threw the ship into the turn during the A-Wing's explosion, or the controls themselves malfunctioned as a result of the A-Wing's explosion.

Lacking the evidence for an in-universe threat, you then go ahead and say that Oculi would be a threat to Star Destroyers. Until and unless you can present evidence to the contrary, there is no reason whatsoever to assume that Oculi would be any more of a threat to Star Destroyers than TIE Fighters are, even without addressing the question of how the Oculi would fare against TIE Fighters.

Which was 450 km, yes, I saw. Movie dialogue, essentially the standard of evidence accepted by many proponents of Star Trek's technological superiority (except that for the less reasonable of those, the requirement is that it's Star Trek dialogue), suggests ranges on the order of a system radius. Even if we don't accept the ~1 system radius figure for bombardment range, though, we know that the Star Destroyers came out of hyperspace too close to Hoth, cancelled the bombardment, and then closed with the planet to take up blockade stations. It's rather unlikely that coming out of hyperspace further away from the planet, closing with the planet at sublight speeds, and then initiating the bombardment would have been capable of catching the base with its shields down when the ships were incapable of catching the base with its shields down even by dropping out of hyperspace "too close" to Hoth; the evidence provided by the movie therefore suggests that whatever the bombardment range is, it's at least equal to and most likely greater than the range to the planet apparent the first time we see Hoth from the Imperial fleet, and that it's greater than the range to the planet as seen when the ships are engaged by the planetary ion cannon. As for why the ship seen didn't fire on the transport earlier? The shield was likely dropped at roughly the same time that the ion cannon fired, and so there likely wasn't much of an opportunity for the Star Destroyer to engage the transport before it was temporarily disabled by the planetary ion cannon. Why the Star Destroyers are not standing off at longer range and using their weapons' range to engage the transports as they come out from beneath the shield from a safer distance is open to debate, however (my opinion is that it's too likely for the transports to be able to endure or evade long-range fire for a sufficient amount of time to engage the hyperdrive, and the range limit on the tractor beams is likely more constraining than the range limit on the turbolasers). 450km is therefore an extremely short estimate for the bombardment range of Star Destroyer turbolasers.

1. We know next to nothing about Nkllon or its star. Using this to establish limits on Star Destroyer shield capacity is exceptionally questionable; all we know is that the planet is sufficiently close to a sufficiently powerful star that inadequately protected objects will begin to melt relatively soon after coming close to Nkllon.

2. That's an expanded universe source. As with most EU sources, it uses the highly accurate and consistent measuring stick of "what does the plot demand today?" For some strange reason, I tend to feel that this reference may not be entirely reliable. It's almost like, I don't know, the author made something up because it's essentially only a bit of interesting scenery in the story.

And how big is this asteroid which caused the issue for the captain who disappeared? How quickly is it moving? How long have the ships been exposed to this barrage of asteroids? How many times have the ships been hit? How big were the asteroids that hit them? How fast were those asteroids moving? In short, there are too many unknowns to use the Hoth asteroid field to determine an upper bound to the strength of Star Destroyer shields when subjected to attack from kinetic impactors. You can, of course, assume that the asteroid we see impact a Star Destroyer's command tower just before we see the conference is the asteroid which caused the interruption or termination of communications (not terribly likely; the impact was shown before we get to see the conference, but it's the only major impact we see and it looks like the impact might have destroyed the command tower), which would allow you to establish a lower bound for the upper limit on the ability of a Star Destroyer's shields to absorb kinetic impact. This, however, would involve a lot of assumptions and estimated values - average density and size of the asteroid, the speed at which it was moving relative to the Star Destroyer (which is not an easy estimate; the asteroid appears to have been moving on a course which was a large angle off the image plane), you're going to have to make a guess at how much energy the command tower can absorb before being destroyed by a kinetic impactor (more simply, you could assume that the command tower was essentially undamaged and only appears to have been destroyed because it's obscured behind the debris from the asteroid and the scene ends before we can get a clear view), and you're assuming that this was the only asteroid to hit that particular Star Destroyer, or at least that no other asteroids hit that Star Destroyer recently enough to affect its shield strength. Of course, if you're willing to accept video game figures as evidence, might I refer you to Anakin Skywalker: The Story of Darth Vader, which, as far as I know, is at least as canonical and 'accurate' as the video game you referenced earlier and states that the ships were enduring asteroid strikes which were similar in power to multi-megaton bomb impacts - multiple impacts, mind you, without the shields failing.

Or we could go with your previous examples and throw all available evidence out the window simply because it doesn't fit anything that seems reasonable to us, because hey, why not remove any possible basis for rational discussion of the power level of a science fiction setting, in a thread where we're supposed to be comparing the power levels of science fiction settings. The asteroid appears to have been vaporized. How and why did it get vaporized? I have no idea. Unless you can provide an alternative explanation and bolt power estimate that makes sense for the event displayed, however, I see no reason to throw out the melting or vaporization figures for turbolaser power just because a beam more narrow than the asteroid should not have been capable of vaporizing the asteroid. The movies are the primary source material; throwing the movies out simply because we don't like events displayed in the movies.

Regardless, we can do this another way. Power production capacity is roughly proportional to volume. The first Death Star had a diameter, according to Wookieepedia, of 160 km. Assuming that Alderaan was approximately equal in size and mass to Earth (a relatively reasonable assumption since it's human-habitable; Wookieepedia claims its diameter to be ~12500km, roughly equal to Earth's diameter), then the minimum energy required to overcome the gravitational binding energy was about 2.24e32 J (and, given the violence of the planet's destruction, likely much greater). Yavin IV is also an Earth-like body, appearing to have Earth-like gravity (in order to maintain a human-breathable atmosphere and have an apparent surface gravity of about 1g; Wookieepedia claims Yavin IV to have a diameter of ~10200km), and so will likely have a similar gravitational binding energy. The first Death Star expected to destroy Yavin IV within about 1 day of destroying Alderaan. Therefore, the minimum power production capacity of the Death Star is roughly 2.6e27 W, or about 1.21e12 W/m^3. Star Destroyers are 1600m long, making them roughly 898m wide at the widest point and roughly 211m tall if you ignore the superstructure. They are approximately pyramidal, so their volume is about 1e8 cubic meters. Given the power production capacity of the Death Star, a Star Destroyer's reactor might be expected to be capable of an output on the order of 1e20 W. This is of course only a rough estimate of the power output a Star Destroyer's reactor can manage; the power output is unlikely to scale perfectly linearly with volume and we used what is likely a gross underestimate of the energy required to destroy Alderaan. It is still more than sufficient to justify turbolaser bolt energy on the order of 30 TJ per bolt or higher; at 1 30 TJ bolt every 2 seconds from each of the 91 turbolaser cannons listed on Wookieepedia as being carried by an Imperial-I Star Destroyer, the Star Destroyer's armament should require about 1.4e15 W from the reactor, well within the approximate reactor capacity based on the volumes of Imperial Star Destroyers and the first Death Star in combination with the demonstrated destructive power of the Death Star. Therefore, a figure of at least 30 TJ per bolt does not appear to be unreasonable for turbolasers. And if you would care to dispute the power output of a Star Destroyer reactor, well, if we're allowing secondary material such as, oh, I don't know, video games, then the reactor is said to be like a small sun; a power output on the order of 1e20 W would certainly qualify as similar to a star.

LOLOLOLOL ... omg .. scientific???? 

I just spewed my popcorn all over my monitor.

Because there's nothing quite as entertaining as observing joeball's combination of relentless logic and complete lack of sense of humour

We have an example of Star Destroyer acceleration capabilities: the Battle of Endor. Based upon dialogue, the Imperial Fleet is on the 'far side' of the moon, relative to the Death Star. When the Rebel Fleet arrives, the Imperial Fleet is not visible and clearly wasn't detected; it is therefore likely, at this point, hidden somewhere behind the moon. One minute later, the Imperial Fleet has closed so far within visual range of the Rebel Fleet that the ships are visibly quite large and appear to remain in the position where they are first seen until the shield protecting the Death Star is lost (i.e. the ships are approximately stationary relative to the Death Star at the time we are first shown the fleet). Being a bit loose with the numbers, we'll say that the Imperial Fleet traversed a distance equal to merely a third of the moon's circumference in that one minute. According to Wookieepedia, the moon's diameter is 4900km; this indicates that the distance crossed was about 5000km, an average speed of about 83 km/s. If we assume that the Imperial Fleet was stationary relative to the Death Star at the start of this maneuver (necessary to remain over the horizon from the Death Star) and assuming a constant acceleration model, the Star Destroyers displayed an average acceleration of ~5.6 kilometers per second per second relative to the Death Star. If you assume that the Imperial Fleet had already attained its maximum velocity by the time the Rebel Fleet comes out of hyperspace and is instead decelerating at a constant rate for the entire minute remaining until it is revealed, then you're looking at an acceleration of ~2.8 kilometers per second.

For some strange reason, I'm just not seeing an acceleration rate of hundreds of meters per second per second against a gravitational pull on the order of 10 meters per second per second as particularly impressive. It's almost like the acceleration rates displayed differ by an order of magnitude or so in the favor of the Star Destroyers. And Mass Effect's drives apparently don't need to deal with the actual mass of the ship, either, because Mass Effect's drives are stated to function at least in part by reducing the apparent mass of the vessels in order to allow them to attain higher accelerations and be able to land on planetary surfaces. This is not a form of magic that Star Wars is known to employ, but even if Star Wars does employ this kind of magic to the same degree that Mass Effect does, having displayed accelerations about an order of magnitude greater implies superior sublight drive capabilities. If Star Wars does not use magic to reduce the apparent mass of its ships, then Star Wars sublight drives are significantly superior to Mass Effect sublight drives.

A further note on the maneuvering capabilities of large Star Wars ships: the Rebel flagship Home One, a ship approximately the size of a Star Destroyer (the EU gives a figure of 1.3km or so; scaling from the movies suggests something around 2.5km to 4km), appears to have completed a roughly 180 degree turn in 20 seconds or so (between the time Admiral Ackbar gives the order to 'take evasive action,' which is when the Death Star begins to swing out of view from the perspective of his command center, to the next time we see a Rebel capital ship which looks like it could be Home One). Executor, an even larger warship on the order of 11 times longer than a Star Destroyer (Wookieepedia says 19.6km), completes an approximately 90 degree turn in the space of about 20 seconds (after being struck by an A-Wing; I rather suspect you know the scene). For such large warships to do this is quite impressive.

The problem with taking the ability of the Oculi to melt through an armored starship hull in seconds as evidence of its ability to destroy TIE Fighters is that we do not know the material properties of either the starship hulls in Mass Effect or the material that TIE Fighters are made out of.

Once again, the problem is that there is simply insufficient evidence to make any conclusive statement about Star Destroyer shield capabilities based upon this event. You say that it must have been a small asteroid which was not detected and justify the small size based upon the captain's surprise to see it; I offer you the counter that the only asteroid which was shown to strike and probably damage a Star Destroyer (the strike immediately prior to our being shown the conference; that strike, in addition to occurring before we see the conference, does not appear to match the strike which caused the termination or interruption of that captain's conference with Vader, as that captain appears to have been responding to something he saw approaching from above him whereas the asteroid seen to strike a star destroyer appears to have approached from below the plane of the command deck) was not abnormally small, appears to have not been targeted by turbolasers, and came in to the bridge at an oblique angle that would allow it to approach from the periphery of the vision of the captain of that ship. You claim that the captain would only have flinched in Vader's presence (or rather, during a conversation with Vader) due to something which frightened him more than Vader did; I suggest you underestimate the effects of a sudden shock, especially on someone who is already nervous.

There is quite simply very little to work with for establishing an upper bound on the upper limit of an Imperial Star Destroyer's shield capacity based upon that scene. You can maybe establish a lower bound on the upper limit of the Star Destroyer's shield capacity based upon the asteroid we do see strike a Star Destroyer's command tower, but without knowledge of how much the shields were subjected to this really isn't that helpful in determining the effectiveness of Reaper mass accelerators against Star Wars shields.

I will further add that indications are that there are at least five Star Destroyers accompanying Executor in the scene following Vader's "acceptance" of Captain Needa's apology. We see the Millenium Falcon on the back side of the Avenger's command tower. We see two Star Destroyers in positions ahead of Avenger and moving in directions that make it unlikely for them to be visible from the Millenium Falcon's cockpit later in the same scene. We see the Executor somewhat below these. And then, when we switch to the Millenium Falcon's cockpit, we can see one Star Destroyer below and behind the Avenger, and another beginning to pass below and off to the side of Avenger. We have the opportunity to see the command towers of all of these vessels, though not for much time or close enough for a detailed inspection; none of the command towers, however, appear to be seriously damaged. Executor is only known to have been accompanied to Hoth by six Star Destroyers and Vader is not known to have summoned additional Imperial warships to assist in the search or replace Star Destroyers damaged by the asteroid field. Circumstantial evidence therefore suggests that no more than one Star Destroyer took serious damage during the search of the asteroid field. If the Star Destroyer shown being struck by an asteroid immediately before we see the conference really did suffer the loss of its command tower or at least significant damage to it, then the evidence available suggests that no other Star Destroyer suffered any significant damage (i.e. damage visible at the range we subsequently see the Star Destroyers from) to its command tower. There additionally is no indication that Han, Leia, or Chewie believe any of the Star Destroyers to be significantly damaged, and no evidence that any of the Star Destroyers are being dispatched for repairs (which might be expected if the command tower was hit hard enough to kill the captain or otherwise seriously damage the tower) rather than employment elsewhere. The evidence, while circumstantial, thus does not support the conclusion that the flinching captain's Star Destroyer suffered serious damage from an asteroid impact. Additionally, the timing of the flinching captain's departure from the conference suggests that the flinching captain's ship was not the Star Destroyer we see hit by an asteroid impacting against the command tower immediately before we see the conference. I admit that the evidence is circumstantial, but it does not support the conclusion of serious damage to the flinching captain's ship.

True, most of the engagements we see in the movies occur at short ranges. This brings up a question of why. One reason would be if the weapons involved are not capable of engaging targets at greater ranges; this would be questionable given that Star Destroyers are supposed to be capable of bombarding a planet even if they're not capable of bombarding a planet from ranges approaching a system radius, as the engagement ranges seen in the movies are typically short enough that the Star Destroyers would probably need to be in the atmosphere or close to it if the engagement ranges represent the limit of the weapon ranges. Another reason could be significant countermeasures which force low engagement ranges even though the weapons can theoretically be employed at much greater ranges; unfortunately, such can only be inferred as the one time when jamming was mentioned was at Endor, when it prevented the Rebel fleet from being able to determine the status of the shield protecting the Death Star (and unfortunately, we cannot really get a sense of how much jamming at this level would impact long-range weapon accuracy because we simply do not know how difficult it is to detect the status of a shield of this type on Star Wars sensors). A further possibility is the objective of the engagement; the attack on the Tantive IV in the opening scene of A New Hope, the pursuit of the Millennium Falcon as it left Tatooine, the blockade over Hoth, and the pursuit of the Millennium Falcon at Hoth are all scenarios where the objective appears to have been the capture of the vessel(s) in question rather than the destruction of the vessel(s) in question. After the Death Star revealed that its main weapon was operational at Endor, the Rebel fleet had reason to keep close to the Imperial fleet to try to prevent the Death Star from having a clean shot, though before that it isn't clear that the Star Destroyers were involved to any significant degree; this, however, may at least in part be due to the Emperor's desire to show off his new toy and rub it in the faces of the rebels, and Star Destroyers blowing up Rebel warships before the Death Star could might not have aligned with the Emperor's plans for the event. The only major space battle I can recall in the prequel trilogy is the one over Coruscant in the opening scene of Revenge of the Sith, and in that one we know that the Confederate forces have raided the planet's surface (so they have reason to be in close proximity to the planet); the Republic forces may have closed in to pin them against the planet to prevent them from escaping, at least long enough for the 'rescue the Chancellor' mission to go off.

Then, of course, there's the out of universe reason for why the engagements take place at short ranges: it's (generally considered) more visually interesting when you can see both sides shooting one another than a more 'realistic' portrayal which involves much greater engagement ranges would be. Same reason that Star Trek engagements typically occur at close range despite the occasional statement that the weapons (or at least the torpedoes) have ranges on the order of a hundred thousand kilometers.

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