Quite a few of the Cassini narrow-angle shots of Titan that have come down the wire in the past couple of days show a white smear at left of centre, which is in the same location regardless of where the camera is pointing. It's really obvious if you look at two or three of the images.

I was worried about this at first, because I had never noticed it before, and thought it was some recent piece of damage to the lens, caused by an impact with a big dust grain or something like that.

However, after looking back at some images of Titan from a couple of weeks ago (Sept. 8th), it shows up in a few places there, too. It only seems to be visible at certain lighting levels, and the recent photos just happen to showcase it.

It doesn't show up at all in the recent ring photos, so it can't be that serious. Still, it threw me for a scare, so I thought I'd ask about it, reasoning that others might be wondering the same thing.

Why does this thing look different from the "donut-shaped" dust artifacts that are visible elsewhere in the field of view; for example, the small one at 4 o'clock that can be seen in many narrow-field shots? The "splat" has a donut-shaped artifact within it. Are they related to each other?

Yeah, I've seen that thing in almost every raw narrow-angle Titan image myself...it has to be some sort of crud in the optics. Every time I see it, I reflexively think "wow...a crater or a cryovolcano!!!"

Probably the reason that I never noticed it before, is that I don't usually look too closely at the raw images of Titan. They're usually so fuzzy that you can get a good idea of their content just from looking at the thumbnails. I generally only glance at them while sifting through the Raw Images pages, looking for images of the rings and the icy moons. (The radar swaths -- now, that's a different story. But I digress.)

This time around, though, the "splat" was so glaringly obvious that it couldn't be missed even when looking at the thumbnails.

That thing is part of the camera system, not a defect or a recent problem. It only shows up when an auto stretch is applied to a very low contrast image. If I recall correctly...

Phil

Not sure what it is, but it is a part of the optics system, some where along the line. Always looked like a fingerprint to me, but I'll take Phil's word for it that is caused by some part of the optics.

I suspect could be something on the sensor... Jason, do you know if NarrowCam pointed to the Sun sometime in the past?

There's no cause for alarm. The thing was here a long time ago, though some of you haven't noticed it. Once you flatfield it goes away. Phil's right, it only shows in very low contrast targets - such as clear filter frames of Titan due to the greatest auto-stretching there.

I'm a bit curious, though: why the clear filter sequence ot Titan this time around? I see very few CB3 frames. Is the majority of data still pending downlink or this wasn't a dedicated ISS mapping pass? Calibration?

The "bugsplat" was in the T0 Titan flyby images. It is most visible in short-wavelength filtered images and appears to be an area of anomalous, i.e. non-uniform, response of the CCD detector at short wavelengths.

And this match with a surface "damage" of the sensor, eventually from focused Sun; in fact, shorter wavelength correspond to less penetrating radiation through silicon...
Anyway, I agree is not a serious issue because can be easily removed through a flat field subtraction. Is only a bit disturbing looking to the raw, low-contrast images.

The cameras have shutters that prevent light coming in unless the shutter is open for the exposure and the camera is taking images. The chances of the camera being accidentally pointed at the Sun and shutters leaking enough light in to do damage are pretty low. The splat, along with the mottling subtly seen across the images is probably slight contamination on the CCD or the selenium substrate (or whatever it is) deposited on it. The mottling itself is wavelength-dependant so maybe they have a common cause.

Perhaps it is a ring particle.

Rather than consider it a nuisance that one wishes could be wiped off,
perhaps there is a way to examine such a bonus opportunity via the
camera, since Cassini was never meant to collect ring particles.

Has there already been a discussion about sending a Stardust type
mission to Saturn to collect ring particles for return to study? Could
it also be designed to swing by Enceladus to pick up some geyser
debris, too?

No, no, no... If this were a high-speed particle hit, we'd have a much worse situation than a merely darker area on the CCD. The entire pixel area would be destroyed, possibly rendering the entire column of pixels useless as they are shifted through that area during readout. Cassini takes precautions of flying through suspect E ring regions in the HGA to ram direction so the cameras would not be affected. There's also the nontrivial issue of the particle coming through the NAC optics, reflecting of the primary mirror (!) passing TWO filter wheels and shutter assembly and then sticking on the CCD. It would be vaporized the instant it hit the mirror at those energies.

I'll believe pigs fly before I'll believe that!

By the way, Cassini ISS really has a electro-mechanical shutter?

Borek