T66 (January 28, 2010 / Rev 125) Options

[ngunn]

Quote from the latest 'looking ahead':

This is the first of two flybys in a row where ISS will be prime at closest approach.

This is unusual, and in the past ISS images of Titan at close-ish range have been disappointingly blurry. I've never been quite sure why, given the spectacular results obtained from the much closer flybys of Enceladus. Perhaps those successes have fed through to the planning for these two Titan encounters.

[remcook]

Don't forget: there's a thick atmosphere in the way here, with lots of scattering haze particles. I'm afraid you're never going to get as sharp images as on Enceladus.

[ngunn]

Sure, but there seem to be other factors at work too. On earlier observations there seemed to be some (avoidable?) motion smear, and for these planned ones 'looking ahead' states that wide angle shots are expected to be better than narrow angle ones. Why? I'd like to understand all these issues better if anyone can shed further light.

[ugordan]

One reason is because the WAC is a "faster" camera than NAC. It needs much shorter exposure times for an equal signal/noise ratio. Combine that with the fact the CB3 filter is narrowband requiring long exposures by default, rapid spacecraft motion near C/A and the fact the haze-induced photon scattering kind of defeats the purpose of high spatial resolution and one can only get so good images with the NAC.

[Phil Stooke]

Actually I think it's amazing that we do get such good images from the cameras, when we see the processed versions rather than the raw images. The final version of the ISS global map is going to be great.

Phil

[NickF]

Regarding Titan surface imaging, am I correct to assume that we're only seeing far-red (or IR?) reflectance through the continuum band filters? Atmospheric scattering aside, presumably the images are just albedo features, so visible light clues (such as shadows) are absent.

[helvick]

Given the density of Titan's atmosphere and the amount of haze I'd say that there are never any shadows to be seen in any case.

[ngunn]

Thanks for that very useful reply Gordan. That really only leaves one part of my question, I guess for the Cassini team themselves. Are there significant differences (improvements) in the strategy for the close imaging this time around, maybe as a result of having ISS as prime or maybe arising from experience at Enceladus?

I agree it is astonishing what ISS does manage to produce at Titan. I'm glad they're getting the chance of these close imaging campaigns and I look forward to peering through the haze at that dark patch at 40S 280W. BTW which is the best image we have of that so far?

http://ciclops.org/view/6135/Rev125

[Juramike]

I look forward to peering through the haze at that dark patch at 40S 280W. BTW which is the best image we have of that so far?

http://ciclops.org/view/6135/Rev125

My version from T53 ISS images:
http://www.flickr.com/photos/31678681@N07/...57609673814809/

And manually warped and put into context on an ISS map and set up as a blinky version:
http://www.flickr.com/photos/31678681@N07/...57609673814809/

[ngunn]

Thanks Mike. So it's that dark, upside-down smile feature, right? (A sort of anti-Hotei)

[volcanopele]

Sure, but there seem to be other factors at work too. On earlier observations there seemed to be some (avoidable?) motion smear, and for these planned ones 'looking ahead' states that wide angle shots are expected to be better than narrow angle ones. Why? I'd like to understand all these issues better if anyone can shed further light.

As a result of the scattering of sunlight in Titan's atmosphere, there is an effective limit to the scale of features that can be seen at 938 nm in our images of Titan. This limit is around 1-2 km. Obtaining images with resolutions better than that isn't very helpful, which is why when are closer to Titan, we tend to use summation mode. As said in the Looking Ahead article, this may limit the usefulness of our images of this dark feature in NAC images, but we still need them to update our calibration routines. As we have found, new dust motes do show up over time, and we need to correct for these. Close up Titan images are the best for this. The WACs, with their factor of ten less resolution, should be much more useful.

ngunn, yes, it is feature at the center of the graphic in the Looking Ahead article. I still think these might be dry lake beds, but who knows...

[ngunn]

That's great - many thanks. Titan's surface features are blurred by 1-2 km by the time the light reaches the top of the atmosphere, so there's no point in trying to focus more closely than that (unless with a polarising filter that singles out unscattered photons). But I'm still interested in the advantage provided by being 'prime' at closest approach. Is it longer, better targeted exposures?

[volcanopele]

It is a more distant flyby, and we can study the same feature at different lighting and viewing angles, while the incidence angle remains the same. Plus we can build the mosaics to fit our NAC instrument field of view. And like I said, we need more control over the pointing to produce the flats we need.

This is one of only two flybys (along with T67) where we have prime pointing during the XM.

[Jason W Barnes]

I am looking forward to seeing the VIMS ridealong data from this flyby. Though ISS is driving, we have obtained some of our best overall flyby datasets on these higher flybys. The T66 closest approach is 7000 km IIRC. On T9 it was 10,000, and we were able to do giant moderately-high resolution mosaics over wide areas. Super-high-res postage stamps are great, too, but these high flybys' large territory covered have a great niche within the Titan data ecology

- VIMS Jason

[ngunn]

Will the VIMS coverage extend out to overlap with the earlier 'VIMS mountains' images? That area will be obliquely lit from the opposite direction this time so a comparison could be informative.