[X] My Assistant

[Toma B]

26 Nov. Close Flyby (500 km.) of Rhea...(Size: 1538 km.)
27 Nov. Distant Flyby (108,000 km.) of Enceladus...(Size: 512 x 494 x 490 km.)
27 Nov. Distant Flyby (86,000 km.) of Helene...(Size: 36 x 32 x 30 km.)

This should be interesting...

Can anybody give a bit more information on the upcoming flybys???

Is this the best image of Helene that we have???
Vojager 2 - distance :341000 km)

[edstrick]

Tedstryk: "...Of course, the lack of a scan platform has been a real blessing in terms of things like nightside imagery, as Cassini is so stable. ..."

Wrong. In the "good old days", Mariner and Viking and Voyager, spacecraft were attitude controlled by gasjets, wobbling back and forth in deadbands, and outside of occassional maneuvers reorienting the entire spacecaft, used scan platforms for remote sensing instrument pointing.

This had advantages and disadvantages. Mariner 10's attitude control deadband was about the same as the field of view of the narrow angle camera, which wobbled about so badly it could wander entirely off target and put large gaps in what were supposed to be regular raster-scanned mosaics. Voyager had similar problems, but reprogramming the attitude control system reduces image smear at Uranus and further at Neptune, and actually used attitude control drift to perform image motion compensation and prevent image smear, which was significant at Saturn and much less (despite low light levels) at Neptune/Triton.

Viking orbiters had scan platforms with (I think) 1 and 1/4 degree-per-second slew rates. Designed to take 50 meter/pixel mapping images from 1500 km (I think) periapsis, they dropped periapsis down to 500 km, and were able to take images with resolution down to 7 or 8 meters per pixel by slewing the cameras in image motion compensation and just shuttering pictures as the spacecraft's varying ground-motion (as it passed a certain attitude) reduced to nearly zero, reversed, and picked up again. They actually took some color data, stepping through the filter wheel at the predicted "stationary-field-of-view" time, but I never saw it successfully turned into high rez color pic. I tried to do one 16 meter/pixel data SW of Schiaparelly, but had problems with image registration and never carried it through to completion.

If Cassini had a platform, IN ADDITION TO IT'S REACTION WHELL ATTITUDE CONTROL, the platform could have been down-scoped to a one-degree-of-freedom motion: fore-and aft, and vastly improve Cassini's ability to combine imaging and radar remote sensing, as well as take and transmit images at the same time. The standard technique would be move the platform to a desired angle from the antenna boresight, roll the spacecraft to the desired azimuth, and start shooting, using reaction wheel control as it's used now for fine pointing control, only occassionally tweeking the scan platform position. But we can't and we've still got a great mission, but with more handicaps than we'd like.

What I really, REALLY ***R.E.A.L.L.Y.*** don't understand is the horribly slow frame-rate. Viking Orbiters took images every couple seconds alternately from the two cameras, adn that was with 1971 technology. They needed to, to do landing site mapping-mosaics, but Cassini's frame rate limit is apalling.

[JRehling]

What I really, REALLY ***R.E.A.L.L.Y.*** don't understand is the horribly slow frame-rate.  Viking Orbiters took images every couple seconds alternately from the two cameras, adn that was with 1971 technology.  They needed to, to do landing site mapping-mosaics, but Cassini's frame rate limit is apalling.

Certainly the fact that there is only 2.5% as much sunlight at Saturn is a factor.

I suspect that the density of the CCDs, and the corresponding increase in spatial resolution, introduces inherent limits in *time* resolution. But I can't argue specific to the materials in question.