Hey James! (Nicoll)

[seawasp]

rosencrantz23 posted this Astronomy Picture of the Day .

I looked at it and I cannot figure out how the hell that can be a real picture rather than a composite. I mean, Titan is miniscule compared to the Rings, so if it's behind the rings I shouldn't be able to see it, let alone have it looming up so it appears to be the size of Neptune.

Is that a straight Cassini shot, and if so, how's that work?

(Why am I asking James? Because he seems to know everything about anything an SF geek would ever want to know)

Comments

[elainegrey.livejournal.com]

Ha! The Encke Gap is 325 km wide. I estimate the photo is 11 Encke gap widths wide: so that's about 3575 km at the depth of field where the ring intersects the field of view at right angles to the point of view. (and since the A ring is 14 km wide, we're not seeing much of the A ring in the image.) Titan is 2,576 in radius, 5152 km in diameter. So the photo seems very plausible.

Measurements from wikipedia.

(The HA be cause i spent too much time first looking at the width of the A ring and trying to estimate how much was in the image from the location of the Encke Gap, when the width of the gap was there to provide a very convenient unit.)

[seawasp]

Yes, that's all well and good, but the radius of the rings from Saturn is on the order of about 100k km, while Titan's out at 1.2 million km. That means it is a *LOT* farther away from the camera and should appear much smaller, or so I would think. I mean, I have a cup right here in front of me that's about 4" across, while I have another that was left out in the yard by the kids, and the 4" cup near me looks like it's about 10 times the size of the one outside.

Or is this an *EXTREME* example of that effect you see with telephoto lenses, where the scientist who's a hundred yards from the volcano looks like he's standing on the edge?

[howardtayler.livejournal.com]

That's it exactly. From where Cassini sits Titan is probably only about 5% further away from the rings, and therefore at almost exactly the same scale. What fools the eye into thinking that can't be the case is the incredibly shallow (relatively) depth-of-field of the photo, with the rings in sharp focus and Titan blurry. Usually we only see the effect this pronounced in macrophotography where the camera is right on top of the subject.

[seawasp]

Okay, that makes sense. So Cassini has a powerful telescope on it and was currently at a distance of like 20 million miles from Titan when it snapped that shot, meaning that it was like looking at a long, thin tree branch with my 4" cup behind it. The shot makes it look like I'm real close to the tree branch and thus makes the cup look huge or a lot closer than it should be.