Imperial Fleet Display in Endor Bunker

[McC]

I wanted to indepedently verify a few things, and one of them was the discussion about the Imperial fleet movements at Endor just prior to the arrival of the Rebel fleet. The common conclusion reached from the display is that the ships are moving at a relatively constant velocity and the the display is also updating relatively constantly.

My findings on this are below, and differ with that conclusion:

First "Slide"
Frames 000-031 inclusive
Length: 32 frames
Fleet Location Frame 000: 293px X
Fleet Location Frame 031: 293px X

Second "Slide"
Frames 032-063 inclusive
Length: 32 frames
Fleet Location Frame 032: 289px X
Fleet Location Frame 063: 289px X

Third "Slide"
Frames 064-095 inclusive
Length: 32 frames
Fleet Location Frame 064: 280px X
Fleet Location Frame 095: 280px X

Fourth "Slide"
Frames 096-116 inclusive
Length: 21 frames (likely incomplete*)
Fleet Location Frame 096: 268px X
Fleet Location Frame 116: 268px X

Movement between Location 1 & 2: 4px/32 frames
Movement between Location 2 & 3: 9px/32 frames
Movement between Location 3 & 4: 12px/32 frames

(29.97 frames = 1 second)

That pretty clearly shows that the Imperial fleet's velocity was increasing during this entire sequence, rather than staying constant as is apparently commonly assumed.

Unfortunately, there are very few data points to go with, but if we take the above "movement between" figures to be velocities and then take delta-Vs of those three, we are met with a delta-V of 5px/32 frames/T for the first half and a delta-V of 3 px/32 frames/T for the second half. In other words, it appears that the Imperial fleet was accelerating, but also reduce thrust by the end of this sequence.

It should be noted that there is some slight perspective distortion in this scene, but all of the lines on the screen measure six pixels apart, so it is a relatively low amount.

* I noted this is likely incomplete because...

The speed estaimate may also need to be increased because the bunker screen was updated only every second or so; the ship positions seemed ready for a forwards update when the scene of the movie changed.

It was, in fact, 11 frames away from a refresh, given that the screen had been refreshing every 32 frames to that point and that there's no reason to conclude it would suddenly change.

Hope this is useful or interesting in some way.

[hvb]

...we are met with a delta-V of 5px/32 frames/T for the first half and a delta-V of 3 px/32 frames/T for the second half. In other words, it appears that the Imperial fleet was accelerating, but also reduce thrust by the end of this sequence.

It could also be that they accelerated at the same thrust for part of the time in the last interval and then ceased acceleration altogether. If so the next image (+11 frames) would have shown them coasting, but as you say we never get that frame.

Anyway this is an alternate possible interpretation consistent with the data, and sounds more logical to me then for them to reduce acceleration in mid-manouver.

So what we see may be the end of their "orbit initiation/unmasking burn"; there likely also was a counter burn at the point where the order is given to "hold position", which we don't get to see.

[McC]

Right, that's an equally valid interpretation. My thought with the reduced thrust was that we were seeing that their acceleration around the planet was beginning to back off (i.e. they were finishing their maneuver, and gradually reducing thrust, preparing to coast and then decelerate).

[hvb]

Indeed.

I just think that cutting accel mid-update makes more sense then reducing it gradually (simpler math & fewer control imputs etc. ), unless the engines require a gradual decrease in power, which seems not to be the case for any ships observed thus far in SW.

Anyway, as you say, in both interpretations the scene depicts the same (initial burn) part of the orbital manouver.