[X] My Assistant

[dilo]

I do not know is only my impression, but in the most recent wide and narrow angle images I noticed a lot of diffraction features, probably caused by increased amount of particles on the front lens... they are more noticeable in uniform, low contrast images; see for example:
http://saturn.jpl.nasa.gov/multimedia/imag...1/W00006832.jpg
Do someone has an explaination to this?

[akuo]

They definetely are dust particles on the filters. We've all seen the donuts visible in about every narrow field camera image of Titan, and these were confirmed as dust by the tem. The low contrast conditions bring out the dust much more clearly since the raw images are auto-stretched. This is not a sign of the camera or optics getting dirtier, but rather of the extreme stretching perfomed on the images.

These effects can be corrected by taking flat fields. Flat fields are images taken of flat grey surfaces for all the filter combinations of the camera. By reducing the flat field from other images, the dust spots and other optical imperfections are removed. For raw images this processing hasn't yet been performed.

These definetely are not unfocused stars. Even though the focal length of the NAC is similar to amateur telescopes (2000mm at f/10.5), both the rings and stars are at "infinity" for the camera. In fact I just calculated the hyperfocal (beyond which anything is at same focus with infinity) distance for this combination to be just under 16 metres!

(F (focal length) = 2000mm, f (f stop) = 10.5, C (circle of confusion) = 0.012mm (the pixel size of the CCD sensor)
H (hyperfocal distance) = F / f * C
H = 2000/10.5*0.012
= 15873 mm = 15.9m)

[Bjorn Jonsson]

These definetely are not unfocused stars. Even though the focal length of the NAC

Actually the images in question are WAC images. I suspect this is a high-phase image so scattered sunlight may be the cause of this. IIRC I also saw something similar in raw images last summer so this probably isn't new behavior.