Digital Still cams - CCD quality poll

[LvR]

Tried to use my 2M pixel Olympus to take still pics of a very dark nightsky with just a few stars showing. (longest time exposure on 400 asa setting)

Discovered quite a few "live" pixels causing bright white stars to appear where there are none and also a few colored pixels showing amazingly colorful "distant" stars.

Experimented with a few other more expensive cams - and they all showed the same hassle.

Seems the higher the pixel count on the ccd the worse the results by looks of it.

Is this just as a result of the limitations of the manufacturing process, or is there somebody out there that has a cam with a totally clear ccd?

[VJ]

It has to do with the CCD technology :

http://www.dpreview.com/learn/Glossary/Digital_Imaging/Sensitivity_01.htm

http://www.dpreview.com/learn/key=noise

The cause is that the CCD heats up as it acquires the image, longer exposure rates result in a longer period of time and thus in more errors. Higher ASA settings put more voltage in the CCD, thus contributing more to the heat. The higher the pixel count, the closer the "pixels" are, and thus the greater the effect.

These are not really stuck pixels : in shorter exposures, they work correctly.


The "stuck pixels" at longer exposure rates (often referred to as "hot pixels") can be filtered out as follows :

http://www.dpreview.com/learn/Glossary/Digital_Imaging/Noise_Reduction_01.htm

This method of noise reduction relies on a second frame known as the "dark frame" being taken with the shutter closed for the same amount of time as the main image, this dark frame will be taken either before or after the main image and will be used to identify and subtract "stuck pixels". This has been implemented by Kodak in their consumer grade digital cameras for some time, and more recently in their new DC4800 digital camera. Canon also implemented this method of noise reduction in the Canon EOS-D30 digital SLR.

This method of noise reduction can also be implemented in a software package such as Photoshop. Simply take the original image, then a second image with the lens covered with exactly the same exposure (this becomes the "dark frame"). Open the original image in Photoshop, paste the "dark frame" as a new layer, give it a slight Gaussian Blur (around 0.3 pixels) then change the layer options to "Difference". This will make a fairly good job of removing stuck pixels from the noisy original image. Note: this isn't perfect and will still leave some black "pits" in the image. The examples below were taken with a Nikon Coolpix 990 and the exposure was 8 seconds at F2.9.

It is important to take the dark frame at the same time as the original picture. The reason is that the position of the hot pixels is variable, depending on various circumstances (climate, ...).

Hope this helps...


Jörg

[Dr Mordrid]

The essence of the problem is what's called "dark current".

The dark current is caused by heat induced quantum effects at the SiO2 interface under the CCD's parallel gate causing the random firing of individual CCD elements. It's quantified as electrons/second/pixel.

Dark current can, and often does, cause very annoying false pixels and other artifacts when imaging star fields & other subjects that require time exposures.

Cooling the CCD to -25° to -120°C reduces the dark current below detectable levels. This cooling device can be a single or dual stage Peltiere (most common), liquid nitrogen, etc. etc.

Unfortunately I don't think many photo camera manufacturers are using Peltiers on their CCD's because of their current use, the length of time it takes to cool the CCD and problems with dissipating the extracted heat in a "portable" device.

Peltier or cryogenic CCD coolers are common (actually: normal) in scientific cameras and in astrophotographic rigs.

Dr. Mordrid

[VJ]

Euhm, I was going to say that !

(man, now I feel stupid Now I'm reminded of something Homer Simpson once sang :
"I am so smart, I am so smart, S-M-R-T, ..., I mean, ... )



Jörg

[LvR]

Jörg

Thanks for those links - good reading and techniques there.

Dr Mordrid

The closest I came to -25° C was when I stuck the whole cam in my freezer. I can confirm that the by limiting the "dark current" - ie - freezer lid closed, the number of detectable rogue pixels were indeed reduced after about an hour.

I have however found that a negative result of this technique is the fogging up of lenses and the freezing of certain rather vital bodyparts while one is in the freezer trying to set the manual exposure values on a LCD viewfinder that now refused to switch on at temps below about 3° C.

Come to think of it, all of this may have also been as a result of all the beer I had to consume in order to make space for myself and the cam in the freezer.

[Dr Mordrid]

I don't think the freezer technique is the optimal soution for this problem....

I'd offer you one of my Meade coolers, but I don't think it'll fit into that Olympus

Dr. Mordrid

[xortam]

Great info guys!

[TnT]

Wow, the things you learn when visiting an online message board.

[Greebe]

With heavy modification you could put a pelt on the CCD chip

[Dr Mordrid]

Why did I just KNOW that Greebe would say that?

Dr. Mordrid

[Fat Tone]

"Greebe - designer of the worlds first commercial digital camera dark-current limiting peltier mod with batteries on a trolley"

[LvR]

"...........mod with batteries on a trolley"?

Shouldnt that have been "........mod with solar panel on the roof" ?

[Dr Mordrid]

Nawwww....it's gotta be portable.

How 'bout this: a tethered mini-blimp with solar arrays on top

Dr. Mordrid

[VJ]

Naw, the blimp would give indoor problems...

How about a foldable seat and a set of pedals, just pedal for approx. half an hour prior to taking a picture...?


(and you've got me thinking about Homer Simpson again : "Hey there, blimpy boy...")

--
Is this a new record ?
After merely 1 serious question and 2 serious replies, this "serious" thread ... euhm... shall we say ... collapsed ...

(MURC, the only forums where you can go from photography over quantum effects to camera's with blimps... )


Jörg

[Dr Mordrid]

That's what I love about this place....the variety

Dr. Mordrid