Dirty spots with my web cam. What are they?

[John K]

I have in the last few weeks developed a bit of dirt/mould? problem when I use my 5 x Powermate and NexImage web cam. Essentially I get these "specs" which have now grown into considerable "mould like" specs/spots.

See the attached file for what I mean! Its on an out of focus moon with the Powermate.

In a process of elimination I have tried:

- cleaning the 5 x Powermate - no difference
- when I rotate the Powermate and keep the Web cam still the spots stay still - ie. I suspect that they are caused by the web cam not the Powermate
- I use an Baader UV/IR Filter. If I remove this no difference
- only happens when I use the Powermate, not the web cam alone. Dont have a Barlow or another Powermate to try it.

What can this be? Anyone experienced a similar thing?

Mould? Dirt? Dust? What the H@ll?

Your thoughts appreciated!

[Dennis]

John - you have a mild case of dust bunnies. These are tiny specs of dust on the optical glass that protects the ccd/cmos chip in your imager.

No matter how hard you try, they get in. Your case is very mild indeed and can easily be compensated for by taking a flat field and subtracting the flat field from the light frame (image).

Often, people make it worse by trying to clean the surface, as once you open the glass surface to the atmosphere, dust is attracted like a magnet (electro-static charge effect).

The extreme magnification of the x5 PowerMate makes the out-of-focus specs appear as diffuse blobs. If it makes you feel any better, here is the Flat Field used in the Antennae image I posted earlier. It does need a clean!

Cheers

Dennis

[asimov (John)]

looks like dust on the chip to me, the higher the mag the more noticeable they become.

[iceman (Mike)]

lol Dennis, nice one!

yeh it's dust on your chip. I had some on mine, and tried to clean it at Lostock. I hadn't used it since then until this morning, turns out I removed the ones that were there, but they've since been replaced with new/bigger/worse positioned ones.

So tonight it gets cleaned again.

No matter how many times I clean it, the ones I'm cleaning off always get replaced by new ones. I just have to hope the new ones are on the edge of the field of view.

[John K]

Good info, thanks for that.

Dennis, being new at this, do you mind doing a favour and explaining how you can do this: "...compensated for by taking a flat field and subtracting the flat field from the light frame (image)" using Registax? Would be very useful.

[Robert_T]

John, you just need to learn the dust bunny Hop... I spend a lot of time tweak guiding planetary images around the chip through a maze of nasty bunnies, like one of those old plastic tilt games where you guide a ball through the obstacles..

[John K]

Robert, you are so right, that's what I have been doing, it's like playing Nintendo!

Anyone also have any good ideas on cleaning?

Mike, how did you clean your web cam?

[Dennis]

Hi John

Here is a quick explanation. There are 3 images you can take as follows:

1. Your "light" or sky image which is the object you want to capture.
2. A "dark frame" which is an image with the lens cap on, at the same exposure of the sky image, so you are effectively taking an image of the thermal noise of the ccd chip.
3. A "flat field" which is essentially an image of a flat, evenly illuminated surface. A crude example would be a white wall, although I have built a “light box”. If you had a perfect optical system, the flat field would look like a white rectangle. But, due to vignetting, light gradients and dust motes, you will see variations in the white rectangle.

I have included 4 examples from the previous evening.
Original 180 sec image (includes sky image, thermal noise (exaggerated to show effects) and dust bunnies)
Dark frame (thermal noise).
Flat field (produced by my evenly illuminated light box believe it or not)
The sky image dark frame subtracted and flat fielded to just leave the image details.

Also, take a look here for a more in depth explanation.

http://www.atscope.com.au/techhelp.html

Cheers

Dennis

[John K]

Thanks Dennis, great info.