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.

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!

Many of the raw Titan images (hence also low-contrast) also have what look uncommonly like the infamous Viking Orbiter 'Cheerios'. I wonder if it *is* dust, or radiation hits on the optics themselves, or impacts etching the outer surface, or...

...whatever. Anyway, let's hope it gets no worse!

Didn't Voyager 2 also have that Problem?

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

Gary:

Sadly, I suspect that the Cassini cameras are always focussed at, or near, 'infinity' - probably at the hyperfocal point of the lens system, as the distance to the discrete elements of the Saturnian system is still astronomical in normal terms even when you're within the system (except in the case of *very* close flybys!). More of a clue might be had in the F-stop of the camera (if such a thing may be said to exist, with the aperture for bright objects stopped down, and dim objects opened up) and it's effects on depth of field and debris. The bottom line is that it looks like something isn't 100% right, some of the time...

Bob Shaw.

Bob,
Your probably correct, I went back and looked at the sequence of pictures before and after that shot and there seems to be a group of four pictures that all show the same anomaly.


http://saturn1.jpl.nasa.gov/multimedia/ima...&storedQ=989318

But all the pictures before and after that group look perfect, even though they were using the same filters. I wonder if stray light coming in from the side could be a factor?

Gary

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)

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.

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.

Ah, of course. The WAC has even shorter hyperfocal distance, about 4.8m.

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

Yes, now question is: how dust entered inside optical system? based on technical drawings reported here:
http://saturn.jpl.nasa.gov/spacecraft/images/iss-wac3.gif
http://saturn.jpl.nasa.gov/spacecraft/images/iss-wac2.jpg
the Wide Angle Camera and it's filter wheel are ermetically closed, so dust should eventually accumulate only on the front lens, contrary to evidence!
The only possible explainations are that most dust is produced in-situ, from camera itself (friction or something else) or was produced and left here during camera building. Last hypothesis, even if incredible, can explain the presence of these features also in images taken almost 1 year ago, as highlighted from Bjorn...

Thanks for those drawings.
My best guess would be that the dust comes mainly from the filter wheel. It is a mechanical moving part and would most likely be the culprit for the source of the dust. The vibrations during launch could have also stirred things up. It only takes a microscopic size dust particle under the right lighting condition to create those rings. If you look at this image

http://saturn1.jpl.nasa.gov/multimedia/ima...1/W00006834.jpg

you can see some light and dark streaks in the image. This would suggest to me that light is entering into the optics system at a peculiar angle and reflecting off internal structures. Akuo is correct that they can use a flat field and subtract some of these aberations from the final image. But that is easier said than done. If these doughnuts are being accentuated because light is entering the lens at a particular angle, you would have to replicate this in the flat field. I'm not sure how they do a flat field in space. Here on earth, the easiest way is to take a defocused image of a clear blue sky. In space you don't have any diffuse lighting source. In the image of the wac, there is a "calibration lamp" , could that be used for the flat field? -- not sure.

Gary

IIRC a low contrast object at close range is used for obtaining flats - meaning Titan. And IIRC some flats were also obtained during one of the Venus flybys.

Unfortunately, calibration lamp cannot reproduce these particular off-axis illumination conditions. So, unless they have developed a special software to remove diffraction patterns (simulator or some kind of deconvolution?) task is impossible...
I agree about possible sources of particles from filter wheel movement or launch vibrations.
Fortunately, most features are almost invisible in most images...
regards. Marco.

I just hope the darn things get no worse!

It is really hard to tell how bad it is since the raws are stretched - A very minor problem looks horrible in a stretched low contrast image. Hopefully the distortions can be modeled to the point that the effects can be removed. We on the outside will be able to tell more when we start getting PDS releases. I wonder how this compares with, for example, MOC over its long life (due to its pushbroom nature, it would be harder to identify durt, as you wouldn't get the obvious diffraction rings.

So what this comes down to, if I am understanding this description correctly, is basically that Cassini just took a picture with the sun in its eyes? Kind of analogous to when you shine a bright light at the side of your eye, and you can see the blood vessels in your retina. They are always there, but you just can't see them until the light is juuuust right.

We are planning to take CB3 filter flats in August. We plan to do this by imaging titan while slewing, this creating a smeared image. Do thing several times, average, and you got a decent flat, hopefully.

Hi, Jason.
For sure, flat fields you are talking should greatly improve narrow cam images like this:
http://saturn.jpl.nasa.gov/multimedia/imag...0/N00032718.jpg
(I noticed that dark spot on the left became clear in MT1 filter:
http://saturn.jpl.nasa.gov/multimedia/imag...eiImageID=38212, so probably is necessary to make flats also in other bands...)

Anyway, first image I posted was taken with clear filter by widecam and, as already noticed by Gary, it would be hard to remove them with a simple flat field:
http://saturn1.jpl.nasa.gov/multimedia/ima...1/W00006834.jpg
Looking carefully to this image, many features are also duplicated toward left/bottom, and shift depends from their diameter (=distance from focal plane). Moreover, comparing it with with other two similar images taken immediately before (W00006832/33), I discovered that diffraction rings change in luminosity and translate in position, probably due to different lighting conditions! see following composition where each image is associated to an RGB channel:

So situation is even more complicated than previously thought, probably due to multiple light reflections!!
Jason, do you know if there was a discussion on this item inside ISS team and if they planned to use some kind of software in order to simulate/remove it???
Thanks.