[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?

[garybeau]

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?

It does indeed look like dust on the CCD. I get the same looking rings when taking ccd images with a telescope. It doesn't seem very probable that dust could migrate that close to the ccd. If the dust was on the lens, I would expect it to be more diffuse.

[dilo]

It does indeed look like dust on the CCD. I get the same looking rings when taking ccd images with a telescope. It doesn't seem very probable that dust could migrate that close to the ccd. If the dust was on the lens, I would expect it to be more diffuse.

Are you sure, garybeau?
I always thinked that dust on the sensor should produce almost poiny-like signature, especially if aperture ratio is high... I have impression they are completely defocused images reproducing aperture profile, so they should be far from focal plane... pls, correct me if I'm wrong!

[garybeau]

Are you sure, garybeau?
I always thinked that dust on the sensor should produce almost poiny-like signature, especially if aperture ratio is high... I have impression they are completely defocused images reproducing aperture profile, so they should be far from focal plane... pls, correct me if I'm wrong!

Hi dilo,
I should clarify my statement. The dust probably isn't sitting directly on the silicon CCD sensor, but rather the protective covering if there is one. Most ccd's have a thin protective covering that may sit anywhere from .5 to 1 mm away from the actual image sensor. This would be far enough away from the ccd to cause rings in the final image. Alternatively, the dust could be on the filters if the filter wheel is positioned real close to the ccd. If the dust were on the lens/mirror you most likely would only see a reduced contrast image and not any individual marks.
Here is a link to a flat field image I took with my telescope. You can see numerous little rings that on it that are from dust on the ccd.

http://i6.photobucket.com/albums/y207/gary...ield/flat_1.bmp

However, I highly doubt there is that much dust on the Cassini camera system. And comparing the flat field I took against the Cassini image the rings are definitely not the same. The rings on the Cassine image look more like de-focused stars rather than dust marks. The camera is probably focused up close on the rings and the background stars are out of focus.

Here is an image of de-focused stars. The concentric rings look more like the Cassini image.

http://www.skywatchertelescope.net/EducationST.html

Gary

[dilo]

Hi dilo,
...
Here is a link to a flat field image I took with my telescope. You can see numerous little rings that on it that are from dust on the ccd.

http://i6.photobucket.com/albums/y207/gary...ield/flat_1.bmp

However, I highly doubt there is that much dust on the Cassini camera system. And comparing the flat field I took against the Cassini image the rings are definitely not the same. The rings on the Cassine image look more like de-focused stars rather than dust marks. The camera is probably focused up close on the rings and the background stars are out of focus.

Here is an image of de-focused stars. The concentric rings look more like the Cassini image.

http://www.skywatchertelescope.net/EducationST.html

Gary

Thanks Gary for clarification. I didn't consider protective layer on CCD, dust on it could explain part of these diffraction images, at least smaller ones (the similitude with your image is impressive). However, looking better to Cassini frame I initially posted, they appear different and I'm now convinced they must arise from obstructing dust in different depth position (lens, filter wheel and CCD, for example)...
I tend to exclude your hypothesis of de-focused stars because camera focus should be fixed at infinity and depth-of-field of both Narrow and Wide angle cameras must widely include rings and stars (both are perfectly focused, as already stated by Bob and Akuo).
Finally, I suspect that visibility of these features strongly depends on illumination conditions, especially light from very bright objects slightly outside field of view, wich easily enters in the optical system and directly illuminate dust...

Regards.
Marco.

[garybeau]

However, looking better to Cassini frame I initially posted, they appear different and I'm now convinced they must arise from obstructing dust in different depth position (lens, filter wheel and CCD, for example)...
.

Marco,

I agree, I should have stuck with my original hypotheses. If you look closely at the Cassini image, the ring sizes don't seem to be random, you can see about four or five distinct groups of circles. This would imply dust on four or five different planes in front of the ccd.

I don't see any stars in the preceding or following images, so I guess we can rule that out.

Gary