Strange Artifacts

[Alien 13]

I was wondering if anyone can help i have been using a Canon 650D with a WO ZS71 recently and noticed some strange artifacts during image processing.

The artifacts appear as perfect circles and are almost regularly spaced so to eliminate something i tried some sky shots with the canon 75-300mm zoom lens and got similar results.

Ahh i thought got to be the sensor but its spotless so im baffled atm.

I have attached a pic that ive over processed to show the effect and arrowed a few but theres more ...

[JB80]

They are dust bunnies, small specs of dust that are on the sensor of the camera.

Even if it looks spotless it really doesn't take much.

Taking flats will eliminate them from the processing though.

[JamesF]

Indeed so.  They can be exceptionally small bits of dust, too -- enough that they're impossible to see without a magnifying glass.  It took me about two hours to clean the sensor of my SPC900 so I couldn't see any dust bunnies.

James

[jambouk]

I'd have thought those dust bunnies were on the lens on the telescope (either inside or out) and appearing massively out of focus.

If they are exactly the same position when imaging without the scope, then are you using any filter between the sensor and the light path? Otherwise it is the cameras sensor, but i think sensor is unlikely to be the principle cause.

I believe removing flats from your stacked images will help reduce them.

James

[JamesF]

An easy way to work out whether they're on the camera sensor or not is to take two frames and rotate the camera between the two.

James

[JB80]

Well he said he gets similar results with a different lens so I'd say it's the sensor.

But yeah rotating it is an easy way to tell before you go attacking your sensor with cleaning stuffs.

[jambouk]

I'm just surprised that the dust bunnies would appear so large and be on the sensor yet he says it looks spotless.

Have you looked at the sensor, or were you looking at the flip mirror in the camera?

Having a dusty eos clip filter in the camera would probably give rise to these too, and feature when both at prime focus on a scope, and with a telephoto lens.

It will be interesting to see how this is resolved.

James

[jambouk]

And the author says "similar results" not necessarily the same results. It might be there is dust on the scope AND the telephoto lens.

James

[Alien 13]

Thanks for the replies the artifacts are within the camera body but cant see them on the sensor, what i have noticed though is that the pattern of circles looks a bit like the camera viewfinder reticule. Its not noticible until an image requires a boost in contrast during post processing so i will experiment a bit more maybe try some shots with no lens on a flat field light source to get an idea of the full pattern.

[JamesF]

I'm honestly not surprised you can't see them, at least not without a magnifying glass and perhaps not even then.  As I said, it took me two hours to get the sensor of my SPC900 clean (and I've never taken the IR filter and nosepiece off since as a result  That was mostly because I could see dust bunnies on the video frames, but nothing at all on the sensor.  There are loads that I need to clean off on my ASI120MM sensor and that looks spotless too

Remember that a single pixel on the 650D is probably only about four thousandths of a millimetre across.  A speck of dust twenty pixels in diameter is going to be considerably less than 0.1mm across, and quite easily lost on the surface of the sensor coverglass. 

James

[StuartJPP]

I'd say 110% dust on the sensor. You won't be able to see the dust unless they are large specks. Dust anywhere else in the optical train is not going to be visible on the image or will cast a very large shadow which most probably wouldn't be visible under any conditions (even an EOS type clip filter is "miles" away from the focus point).

Use the camera with a lens in AV mode. Stop the lens down to the smallest aperture, like f/16 or f/22 (smallest aperture is largest f/number) and take a picture of an even grey sky or a white card. Those fuzzy blobs should become darker and more in focus although they won't be in focus because they are on top of the sensor glass and not on the the sensor itself.

There are two options if flats don't cure it:

1) Get a "Rocket Blower", which is always useful and should be part of everybody's optics cleaning kit.

2) Get some sensor cleaning swabs and sensor cleaning fluid.

I'd try no. 1 first, repeatedly and only if you really need to, use number 2. I have had a 7D for 3 years and have never cleaned the sensor with a swab before...only had to do it once on my 1100D on what looked like a tiny drop of oil. To those people who say that a rocket blower introduces more dust, I say prove it...never happened to me.

[jambouk]

Just ordered a rocket blower; my old bulb blower thing cracked today, so is useless. Thanks for the tip off.

What is the best way to inspect the sensor in a dslr? Look at it during a bulb exposure whilst the mirror is up?

James

[Alien 13]

I'd say 110% dust on the sensor. You won't be able to see the dust unless they are large specks. Dust anywhere else in the optical train is not going to be visible on the image or will cast a very large shadow which most probably wouldn't be visible under any conditions (even an EOS type clip filter is "miles" away from the focus point).

Use the camera with a lens in AV mode. Stop the lens down to the smallest aperture, like f/16 or f/22 (smallest aperture is largest f/number) and take a picture of an even grey sky or a white card. Those fuzzy blobs should become darker and more in focus although they won't be in focus because they are on top of the sensor glass and not on the the sensor itself.

There are two options if flats don't cure it:

1) Get a "Rocket Blower", which is always useful and should be part of everybody's optics cleaning kit.

2) Get some sensor cleaning swabs and sensor cleaning fluid.

I'd try no. 1 first, repeatedly and only if you really need to, use number 2. I have had a 7D for 3 years and have never cleaned the sensor with a swab before...only had to do it once on my 1100D on what looked like a tiny drop of oil. To those people who say that a rocket blower introduces more dust, I say prove it...never happened to me.

I will give your suggestions a go but i am suprised that the built in ultrasonic sensor cleaning cant shift it.

[StuartJPP]

What is the best way to inspect the sensor in a dslr? Look at it during a bulb exposure whilst the mirror is up?

James

You would need a some sort of magnifier to do this or a "sensor loupe". The problem is that how are you going to remove a single speck of dust? You can see the dust by performing the test above, I can't quite see the point of a sensor loupe other that to be able to say "oh look there's the speck of dust". If they were cheap then fine, but they are quite expensive for what they are.

[jambouk]

I mean if the test suggests there is dust on the sensor, and one wants to blow it off with an air puffer, what is the recommended way to expose the sensor?

[Alien 13]

The Canon 650D has a sensor cleaning menu including Ultrasonic and Manual if you select Manual it locks up the mirror,opens the shutter and disconects the sensor to prevent electrostatic charge. I have read that the sensor is re-mapped during power on and is a good way of eliminating dead or stuck on pixels.

[jambouk]

Any update on getting rid of these yet?

James

[Alien 13]

Any update on getting rid of these yet?

James

 Hi i got rid of the problem i used uv to examine the sensor and found what looked like watermarks, anyway it cleaned off ok.

[jambouk]

Water marks! Odd. But glad it's all cleaned up nicely. Rewarding to get to the bottom of these kind of things of resolve them.

Good work.

James

[perfrej]

Me thinks somebody blew at the sensor with their mouth Always leaves that kind of stains no matter how hard you try to blow dryly...

/per

[ollypenrice]

It's always best to take flats, though, however clean your gear.

Olly

[Alien 13]

It's always best to take flats, though, however clean your gear.

Olly

 I agree with that I do use my camera a lot for video recording so the sensor is exposed to all sorts of stuff like barbeques etc. The lesson learnt is that artifacts like this dont show up on normal pictures, the contrast and levels enhancement for astrophotograpy also enhances defects.

[ollypenrice]

Very much so. We stretch the faint stuff so hard in processing...

Olly

[teleskopjo]

So what are they then when there are specks and dirt in the view which do not rotate upon rotating the camera/?

[todd8137]

Just ordered a rocket blower; my old bulb blower thing cracked today, so is useless. Thanks for the tip off.

What is the best way to inspect the sensor in a dslr? Look at it during a bulb exposure whilst the mirror is up?

James

Rocket ermmm these will blow all the dust around the place I thought the same after advice from other about these actually these make the problem 20 times worse and will put more dust on the sensor