APOD: Ringside with Rhea (2013 Mar 29)

APOD Robot · Post by **APOD Robot** » Fri Mar 29, 2013 4:05 am

Ringside with Rhea

Explanation: Orbiting in the plane of Saturn's rings, Saturnian moons have a perpetual ringside view of the gas giant planet. Of course, while passing near the ring plane the Cassini spacecraft also shares their stunning perspective. The thin rings themselves slice across the middle of this Cassini snapshot from April 2011. The scene looks toward the dark night side of Saturn, in the frame at the left, and the still sunlit side of the rings from just above the ringplane. Centered, over 1,500 kilometers across, Rhea is Saturn's second largest moon and is closest to the spacecraft, around 2.2 million kilometers away. To Rhea's right, shiny, 500 kilometer diameter Enceladus is about 3 million kilometers distant. Dione, 1,100 miles wide, is 3.1 million miles from Cassini's camera on the left, partly blocked by Saturn's night side.

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Do_japan · Post by **Do_japan** » Fri Mar 29, 2013 5:57 am

Great shot. Kind of a shame that the quality is so low. A lot of fuzziness around the moons.

instond · Post by **instond** » Fri Mar 29, 2013 7:14 am

Mixing miles and kilometers - that's how Mars got missed wasn't it !?

BikerMike · Post by **BikerMike** » Fri Mar 29, 2013 8:46 am

Ditto instond's remark.

bystander · Post by **bystander** » Fri Mar 29, 2013 8:55 am

http://asterisk.apod.com/viewtopic.php?t=31040

owlice · Post by **owlice** » Fri Mar 29, 2013 9:53 am

This is a spectacular image; it is amazing, amazing!, that we get to see it!

Khono · Post by **Khono** » Fri Mar 29, 2013 10:19 am

instond wrote:Mixing miles and kilometers - that's how Mars got missed wasn't it !?

Lol, also caught that one. It sure would be convenient if the metric system was simply adopted throughout. I don't see why anyone would prefer to use fractions of inches rather than millimeters.

owlice · Post by **owlice** » Fri Mar 29, 2013 10:48 am

It doesn't matter which unit of measure APOD uses; people are still going to complain about whichever it is.

I don't see why anyone would complain about this, but apparently some feel the need to.

saturno2 · Post by **saturno2** » Fri Mar 29, 2013 11:09 am

Rhea, Enceladus and Dione in a interesting image.

instond · Post by **instond** » Fri Mar 29, 2013 11:23 am

No complaint from me owlice - just an observation. Love all the APODs.

K1NS · Post by **K1NS** » Fri Mar 29, 2013 12:25 pm

WOW! This photo is miles better than anything else I've seen.

Tszabeau · Post by **Tszabeau** » Fri Mar 29, 2013 12:31 pm

I'm amazed by the perspective and clarity of focus of such disparate objects in the same field of view. I, almost, feel ashamed to wish for more by wanting to see it in 3D though.

Cal · Post by **Cal** » Fri Mar 29, 2013 12:59 pm

instond wrote:Mixing miles and kilometers - that's how Mars got missed wasn't it !?

Dione's diameter is actually about 1100 km. I guess "miles" was a typo.

I don't know, but I'm guessing the "3.1 million mile" distance is also supposed to be km.

rstevenson · Post by **rstevenson** » Fri Mar 29, 2013 1:20 pm

It's easy to find out for sure, from the links...

Dione (1,123 kilometers, or 698 miles across) ...

The view was obtained at a distance of approximately 3.1 million kilometers (1.9 million miles) from Dione ...

Rob

Chris Peterson · Post by **Chris Peterson** » Fri Mar 29, 2013 3:19 pm

Do_japan wrote:Great shot. Kind of a shame that the quality is so low. A lot of fuzziness around the moons.

Yes, it's rather horrible image quality. Way too much image compression has been used, and the artifacts are painfully obvious. It's not clear why the folks at JPL chose to use such aggressive compression on their released image, as this image would still be suitably downloadable with much less compression applied. Even the fully uncompressed TIFF on the source page isn't very big. Anyway, to properly appreciate the image, I'd recommend going to that page and viewing the TIFF.

LocalColor · Post by **LocalColor** » Fri Mar 29, 2013 3:24 pm

owlice wrote:This is a spectacular image; it is amazing, amazing!, that we get to see it!

QFT!!

geckzilla · Post by **geckzilla** » Fri Mar 29, 2013 4:28 pm

Chris Peterson wrote:
Do_japan wrote:Great shot. Kind of a shame that the quality is so low. A lot of fuzziness around the moons.
Yes, it's rather horrible image quality. Way too much image compression has been used, and the artifacts are painfully obvious. It's not clear why the folks at JPL chose to use such aggressive compression on their released image, as this image would still be suitably downloadable with much less compression applied. Even the fully uncompressed TIFF on the source page isn't very big. Anyway, to properly appreciate the image, I'd recommend going to that page and viewing the TIFF.

I've never been able to find any Cassini raw data that's much higher resolution than this. I think that for a single shot this is as good as it gets, unfortunately. It's also a lot more painful to search through Cassini's data than, say, Hubble, too. I might have given up in frustration before finding better imagery.

Chris Peterson · Post by **Chris Peterson** » Fri Mar 29, 2013 5:16 pm

geckzilla wrote:I've never been able to find any Cassini raw data that's much higher resolution than this. I think that for a single shot this is as good as it gets, unfortunately. It's also a lot more painful to search through Cassini's data than, say, Hubble, too. I might have given up in frustration before finding better imagery.

But the problem here is that the data is higher resolution, it's just a ridiculously and unnecessarily compressed JPEG version. This image to the left is the original TIFF file losslessly compressed as JPEG, and it's still only 40K- and looks identical to the original image (and much better than the JPEG version seen in today's APOD).

geckzilla · Post by **geckzilla** » Fri Mar 29, 2013 6:02 pm

Yes, of course, the APOD jpg version is quite compressed. What I meant by resolution was the pixel dimensions, not the compression. Compression does not influence resolution in that sense and that is what I meant to state is the limitation of Cassini's camera.

Chris Peterson · Post by **Chris Peterson** » Fri Mar 29, 2013 6:24 pm

geckzilla wrote:Yes, of course, the APOD jpg version is quite compressed. What I meant by resolution was the pixel dimensions, not the compression. Compression does not influence resolution in that sense and that is what I meant to state is the limitation of Cassini's camera.

Cassini has two cameras, both of which use 1K x 1K CCDs. The narrow field camera (used for today's image) has an image scale of 1.2 arcsec/pixel, and has diffraction limited optics 190 mm aperture). The wide field camera has an image scale of 12 arcsec/pixel and is also diffraction limited (57 mm aperture).

I can't say that I've found the resolution of Cassini images lacking- some of the ring images, for instance, are amazing. But in an image like today's showing more than one moon, we are necessarily seeing bodies that only use a small part of the field, and therefore don't have all that many pixels covering them.

If larger sensors were used, the spatial resolution would remain the same, but the field of view would increase.

Swashbuckler · Post by **Swashbuckler** » Fri Mar 29, 2013 6:25 pm

Quick question then, as it is one that has plagued me for a while. Why do we not see any background stars or other objects in photos like these? They're photos, and not filtered images for hydrogen or x-rays. Is it just the amount of reflective light pollution from the visible objects?

Chris Peterson · Post by **Chris Peterson** » Fri Mar 29, 2013 6:39 pm

Swashbuckler wrote:Quick question then, as it is one that has plagued me for a while. Why do we not see any background stars or other objects in photos like these? They're photos, and not filtered images for hydrogen or x-rays. Is it just the amount of reflective light pollution from the visible objects?

Yes. These are bright objects (sunlit) and consequently the exposure times are short- less than a second. That's just not long enough for many stars to show up. It is possible to see the occasional bright star in the raw data, but only just above the noise floor. Take your camera out at night and shoot a properly exposed image of the Moon. You won't see any stars in the result.

geckzilla · Post by **geckzilla** » Fri Mar 29, 2013 6:45 pm

Chris Peterson wrote:
geckzilla wrote:Yes, of course, the APOD jpg version is quite compressed. What I meant by resolution was the pixel dimensions, not the compression. Compression does not influence resolution in that sense and that is what I meant to state is the limitation of Cassini's camera.
Cassini has two cameras, both of which use 1K x 1K CCDs. The narrow field camera (used for today's image) has an image scale of 1.2 arcsec/pixel, and has diffraction limited optics 190 mm aperture). The wide field camera has an image scale of 12 arcsec/pixel and is also diffraction limited (57 mm aperture).

I can't say that I've found the resolution of Cassini images lacking- some of the ring images, for instance, are amazing. But in an image like today's showing more than one moon, we are necessarily seeing bodies that only use a small part of the field, and therefore don't have all that many pixels covering them.

If larger sensors were used, the spatial resolution would remain the same, but the field of view would increase.

I misunderstood what you were disappointed in. Never had to consider any types of resolution other than pixel dimensions until now.

Chris Peterson wrote:This image to the left is the original TIFF file losslessly compressed as JPEG, and it's still only 40K- and looks identical to the original image (and much better than the JPEG version seen in today's APOD).

BTW, are you sure that a TIFF file which uses JPEG compression is lossless? I don't think this is possible. Even a TIFF which uses the least amount of JPEG compression is still slightly lossy.

neufer · Post by **neufer** » Fri Mar 29, 2013 6:59 pm

: [b][color=#0000FF]This picture of Rhea was taken on a Hollywood sound-stage.[/color][/b]

Chris Peterson wrote:
Swashbuckler wrote:
Why do we not see any background stars or other objects in photos like these? They're photos, and not filtered images for hydrogen or x-rays. Is it just the amount of reflective light pollution from the visible objects?
Yes. These are bright objects (sunlit) and consequently the exposure times are short- less than a second. That's just not long enough for many stars to show up. It is possible to see the occasional bright star in the raw data, but only just above the noise floor. Take your camera out at night and shoot a properly exposed image of the Moon. You won't see any stars in the result.

Chris Peterson · Post by **Chris Peterson** » Fri Mar 29, 2013 7:30 pm

geckzilla wrote:BTW, are you sure that a TIFF file which uses JPEG compression is lossless? I don't think this is possible. Even a TIFF which uses the least amount of JPEG compression is still slightly lossy.

Standard TIFF files don't use JPEG compression. If they use compression at all, its lossless. The image I posted was made by taking the original TIFF image and recoding it using lossless JPEG, which is a valid form of JPEG compression. As a result, the original TIFF and my recoded JPEG are identical pixel-for-pixel (even thought the TIFF is around 25 times larger in terms of file size).

Because TIFF images are lossless, they are commonly used by imagers to move data between different programs- especially between astronomical image processing apps that typically use FITS format, and Photoshop, which doesn't natively support FITS. Even the popular program FITS Liberator, used by many HST processors, converts FITS to TIFF for use in other programs.

Starship Asterisk*

APOD: Ringside with Rhea (2013 Mar 29)

APOD: Ringside with Rhea (2013 Mar 29)

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