Tuesday, August 24, 2010


The Cassini spacecraft just keeps sending back splendiferous images of the Saturn system. It's latest gym is below (get big versions here!)

Cassini is on the night side of Saturn. You can see the sunlight scattering through the upper part of Saturn's atmosphere producing that neat effect. The Moon is Enceladus. Look closely and you see a little blurry streak coming out of the bottom of Enceladus. That is a geyser erupting!

Okay, it gets more interesting. Since we see the night side of Saturn, we should be seeing the night side of Enceladus, so why is it so bright and easy to see? Two things are going on here. First, the imaging team brightened Enceladus by a factor of two relative to Saturn. Second, the Moon is being lit by sunlight reflected from the other side of Saturn...aka Saturnshine. You ever see a slender crescent Moon and the dark side is illuminated just a little bit so you can make out the outline? The dark side of the Moon is being lit by sunlight reflected from the Earth and given the name Earthshine. Same thing here with Enceladus.

Love those Cassini images.

Reprinted with permission from the Half-Astrophysicist Blog.


OrbsCorbs said...

Amazing photography.

kkdither said...

The pictures are so spectacular and clear. It worries me though that we are being falsely entertained with the tweaking that occurs before we see it.

hale-bopp said...

Interesting point, but I don't agree with it totally. They are quite open about what they did to the image. If they didn't adjust the image, you wouldn't see all the details. Few professional astronomical images show what you would see with your eyes. Even some of my time exposures show things your eyes could not.

If you really want to, you can donwload raw unprocessed images from Cassini. Most NASA missions and many professional observatories make their raw data available to the public (it is required for publicly funded missions, although they usually have a one year proprietary period).

However, be warned, raw data means raw. You frequently need a special software program to read it (usually available free from the mission's website or you might just need a good fits viewer depending on the data you want...many programs that handle fits files are available free as well). Some of the programs only run in Linux and don't have pretty easy to use interfaces and the instruction manuals are written by astrophysicists for astrophysicists (they assume you know a lot of jargon).

The raw images can look pretty nasty. The raw images have all the camera defects, hot pixels, dead pixels, cosmic ray hits, donuts (rings caused by dust particles on the lens or mirror and yes we call them donuts) and need to have all these effects processed out. Then you have to apply dark frames, flat fields and bias frames to your image, each of which corrects different things introduced by the CCD chip and the electronics. If you want a color image, you have to do this whole process three times (once each for the image you select for the red, green and blue channel) and then align and combine your images and get your color balance correct.

Once you get the data, a really pretty professional quality science image still takes a long time to produce. Few non-professionals have the time and drive to learn all the tricks of the trade (although some do...there are people that have made their own versions of images from Hubble or the Mars probes that are incredible and show off new details).

When I was in college in the mid-80s, we had an observatory with an early CCD camera. One of my projects was writing software to process the data and do differential photometry (measuring the brightness of objects) with the images. Now this software is commercially available, but I had to hack my way through the code back then.

So all in all, its probably best to have someone get a nice looking image and tell you how they did it :)