New Iapetian image series Options

[TritonAntares]

Hi,
CASSINI has transmitted 184 pics (!) over the last days.
Here five takeouts, 3-4x enlargement:



Date: 2009-09-06
Distance: 2.228.548 km
Filters: CL1 and CL2


Date: 2009-09-08
Distance: 3.215.284 km
Filters: P120 and GRN


Date: 2009-09-08
Distance: 3.216.610 km
Filters: P60 and GRN


Date: 2009-09-09
Distance: 3.390.271km
Filters: P60 and GRN


Date: 2009-09-09
Distance: 3.427.313 km
Filters: P120 and GRN

Maybe somebody is able to combine some of those images to show more details.

Bye.

[scalbers]

Ian,

That is a possibility to consider. This might be kind of in mid-stream, as for some of the key images I've switched from a spherical assumption to using a triaxial ellipsoid that has been recently published. Hopefully my software is doing that with enough accuracy. However much of the projection error associated with assuming a perfect sphere ends up being in the north-south direction, at least for images with a sub-spacecraft point near the equator. Most of the offset I'm seeing with the basin is in the east-west direction.

The older image with the shadowed version of the basin (Oct 2004) was projected assuming a sphere, so I am thinking of redoing that one. This was released from CICLOPS with a grid overlay so I was assuming things were fairly accurately done. Another thing is that I see the Voyager image may also need to be switched over to a triaxial ellipsoid and that is a key connecting image. Perhaps this being a polar view will show more of a shift?

[Rob Pinnegar]

This might be kind of in mid-stream, as for some of the key images I've switched from a spherical assumption to using a triaxial ellipsoid that has been recently published. Hopefully my software is doing that with enough accuracy. However much of the projection error associated with assuming a perfect sphere ends up being in the north-south direction, at least for images with a sub-spacecraft point near the equator. Most of the offset I'm seeing with the basin is in the east-west direction.

Ooooh... trusting that triaxial ellipsoid could be part of your problem. Take a look at this image from 2004:



The part of Iapetus that appears on the western limb in the above image is in roughly the same part of the moon as the eastern limb from the images of Roncevaux Terra that came down the pipeline the other day.

Look at the sharp "zenith discontinuity" at the equator. A triaxial ellipsoid can't possibly model that properly. The traxial ellipsoid is probably some sort of best-fit that likely deviates substantially from the true surface in a lot of places. I don't doubt that it's the best we've got, but it won't be anywhere near perfect.

Just eyeballing the new Roncevaux Terra images, it looks as if the true limb is "squashed inwards" near the basin. The triaxial ellipsoid probably lies outside the true surface in this part of Iapetus. This would cause anything lying close to the true limb to appear to be closer to Cassini than it actually is.