Beautiful view of three Saturn's moons

reply to post by wildespace


I was looking at the 2012 images, which is when the last images you posted were taken.
Just found this page, which is, I think, what your nice colour images are based on:

Cassini Discovers Titan’s Glowing Atmosphere

www.universetoday.com...

Stars are visible, and a lot of other noise, but it is a 560 sec. exposure.
Another thing I just noted was that the CCDs are treated with something called Lumogen, which converts UV above the response limits of the CCD to visible wavelengths. Doesn't that make things even more complicated?
I'm working through some of the info I have found on the 'net, such as " Cassini Imaging Science Subsystem (ISS) Data User's Guide". Interesting stuff, for some of us geeks anyway.

Originally posted by GaryN
reply to post by wildespace

 

I was looking at the 2012 images, which is when the last images you posted were taken.
Just found this page, which is, I think, what your nice colour images are based on:

Cassini Discovers Titan’s Glowing Atmosphere

www.universetoday.com...

Stars are visible, and a lot of other noise, but it is a 560 sec. exposure.

Nope, that was one extreme case of exposure. If you read carefully, Titan was in Saturn's nightside and was only lit by "Saturnshine", and the image in question is at the top of the article (with all the stars), not the one further down.

Exposures in the data sets are expressed in milliseconds (thousands of a second), so if for example you come across the exposure value of 50, it means 50 ms, or 0.05s. You can find many images taken through red, green, or blue filters (meaning the UV or IF wavelengths were cut off), that used exposures less than 1 sec.

Thanks for pointing to the Data User Guide, there's some good information. Here's a relevant quote:

The medium and broad - band filters allow for color imaging across a wide range of targets and imaging situations. They are BL1, GRN, RED, IR1, IR2, IR3, and IR4 (common to both cameras) , UV1, UV2 and UV3 (NAC only), and VIO and IR5 (WAC only). Only the NAC has UV sensitivity due to its reflective optics, and a special Lumogen coating on the CCD, which was unavailable at the time of the Voyager and Galileo missions. The NAC takes advantag e of this new capability by splitting up its UV coverage over three filters, which together provide visibility of targets such as stratospheric aerosols, auroral phenomena, and ring and satellite materials of special interest. However, solar irradiance at Saturn is extremely low in the UV, so very long exposure times are generally necessary, especially for UV1.

Note the last sentence. It seems like it's easier for Cassini to see in visible light than in UV. Further down the document also states that in both cameras "spectral response falls off steeply towards the infrared," and that "the WAC optics are opaque to UV light."

I was trying to match some of the longer exposures listed in the instrument data files with the tiff images, and the results seem to be all over the place. Some of the 30 second (not miilisecond) images are totally blank, in the raw state anyway, but they are 16 bit images, and my software only runs 8 bit, so maybe I'm losing a lot of info? They sure as heck don't make it easy to view them anyway. I tried messing with the images using just the 8 bits, but can't get anything to show up on most of them by playing with the settings. Now surely there should be stars visible at least, with even a 4 second exposure and clear filters?
The very long exposures seem to be for when they use the methane filters, they apparently have a very narrow bandpass, so let through few photons.
Anyway, taking pictures in space doesn't seem like such a simple job, and a lot of manipulation is required to make them look anything like what our eyes enjoy looking at. I still say you would see next to nothing if you were out there with Cassini, and space travel will be extremely boring, unless you could take a couple of playmates along maybe!

Here's one I looked at. 2 minute exposure, but there aren't many stars for such a long exposure, and some of the specks may be hot pixels, which is what they are testing for with starfield images.

^IMAGE = ("N1611412084_1.IMG",8)
INSTRUMENT_NAME = "IMAGING SCIENCE SUBSYSTEM - NARROW ANGLE"
EXPOSURE_DURATION = 120000.000000
FILTER_NAME = ("CL1","CL2")

Image:
pds-imaging.jpl.nasa.gov...

reply to post by GaryN


Here's an image of Titan, taken through GRN filter (which would cut out any UV or IF wavelengths), with exposure of 150ms (0.15s). The "product ID" is 1_N1717700777.123



I see a fairly well-exposed image, and I don't see how you can say that it required some extensive manipulation to make it visible. There must have been enough photons in the green part of spectrum to get to the sensor and create an image with short exposure and practically no noise.