Talk:Charge-coupled device
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This article may be too technical for a general audience. Please help improve this article by providing more context and better explanations of technical details to make it more accessible, without removing technical details. |
[edit] Remove Corporate References?
This articles seems to have references to amateur astronomy companies such as SBIG, Astrophysics and Losmandy even though they contribute nothing to the content of the article. In the case of SBIG, the claim is also very misleading because the A0-7 isn't even SBIG's only autoguider and other companies make autoguiders too. Additionally, the AO-7 is an adaptive optics system which uses a smal, fast mirror in addition to standard autoguiding methods to keep target stars centered and to compensate for errors which standard autoguiders cannot. Likewise, Astrophysics and Losmandy are not the only companies to make "study mounts" and debatably not even the best. Lastly, these references are only to amateur companies, not professional telescopes where some of the cutting edge work in CCD astro-imaging is going on (ex: The Sloan Digital Sky Survery).
I would make changes, but I thought I'd put it here for comment first.
I agree - take the corporate references out. SBIG is, IMO, misusing this page for commercial purposes.
I don't think it's SBIG, just some user who likes SBIG stuff, because they keep describing the AO-7 incorrectly (or at least imprecisely) as an autoguider when it's really an adaptive optics systems. It's like describing a handheld computer as an electronic appointment book. I've changed it back (again).
In the text it is said that because of the quantum efficiency of the CCD detectors astronomers started using them. It worked the other way around. Astronomers were the ones developing CCD's for beter detection of astronomical sources. When telescopes reached a certain size, it was too expensive to build bigger ones. The only way to detect fainter sources was to build bigger telescopes at the time. This was the drive to design better instruments for detecting fainter sources. This lead to the CCD. Just as the photo sensitive emulsion in film was first developed by astromers and used in photographic plates. -J.P Kotze
[edit] Add circuit diagram?
Alison Chaiken 17:00, 26 September 2005 (UTC): A simple circuit diagram illustrating the topology of the array and the attachment to the D/A and power supply would help this article a lot. The image doesn't have to be fancy, just illustrate how things are connected. I'd volunteer to make a drawing myself if I knew the layout details for certain.
[edit] CCD's in Non-Tracking Telescopes
I was interested in your comment: "An interesting astronomical application is to use a CCD to make a fixed telescope behave like a tracking telescope and follow the motion of the sky." I've not heard of this application before, do you have any further details or links?
The context I've heard about it is: take a bunch of frames of the moving target with relatively fast shutter speeds and align them based on some common bright objects in all of the frames and then use a stacking algorithm. Stacking is not as effective as long exposures though.
Stacking has other advantages. By taking a large number of frames and selecting only the best to stack, one can avoid the image degradation of atmospheric seeing or other transient factors inherent in long exposures.Wschart 01:50, 17 June 2006 (UTC)
I think what the comment actually refers to is not stacking, but drift-scanning. In this technique, the CCD is oriented so that the stars will appear to move (due to the earth's rotation) along a column of the CCD. The CCD is then "clocked out" at the same rate that the stars are moving. Thus, the accumulated charge travels along with the star's image. The main downside is that you have no control over the exposure time; the exposure time will be however long it takes the star to cross the CCD. The upside is that you're continuously reading out the CCD; you don't have to close the shutter and waste a couple of minutes doing a readout. Here is a web page about drift scanning with amateur equipment]; the technique is also used on the Palomar 48-inch Schmidt telescope and the Sloan Digital Sky Survey (they call it TDI, time delay and integrate). -- Coneslayer 19:33, 10 July 2007 (UTC)
[edit] Basis of 70%
PizzaPuzzle, I am a little puzzled by the statement "CCDs commonly respond to 70% of the light to which they are exposed", because I'm not sure what the 70% is a measurement of. Is it the percentage of the CCD surface that is covered by sensing material, as opposed to inert structures such as metal conductors? Or is it the percentage of the incident energy in the visible spectrum that falls on the sensors that is converted to electric charge, instead of being reflected or absorbed as heat? Or is it the product of both of these? I would be grateful for clarification. -- Heron
- The latter, 70% of the photons react with the CCD - the rest are "wasted". As opposed to only 2% usage by a photographic plate. It would probably be useful to divide this page into astronomical CCD use, and more general/technical aspects. Pizza Puzzle
Thanks. I added your 2% figure to the article, and moved the section to "Applications" to avoid repetition. Perhaps only the general/technical aspects should be described here, with the applications (astronomical and otherwise) moved to digital photography. What do you think? -- Heron
Why does light cause an accumulation of charge? Omegatron
You have to know a minimal bit of quantum physics for that. When a photon strikes an atom, it can elevate an electron to a higher energy level. CCDs are commonly made of negatively-doped silicon with many electrons - when light strikes them, it frees electrons to move around and they accumulate in the capacitors (I believe these are also called "wells.") Those electrons are shifted along the CCD and "counted" by a capacitor, giving an effective black & white image of how much light has fallen on each individual pixel. Several things can affect whether a photon releases an electron: polysilicon circuits on the front-illuminated CCD surface can block light from entering, certain depths of the CCD may be penetrated by longer wavelengths without interaction with the atoms, certain screens may reflect off or absorb some wavelengths of the light, etc. (Thus BOTH of Heron's interpretations above are accurate.) Some wavelengths of light can free more or less than one electron per photon. Knowing how many of the photons which fall on the CCD will release an electron is an accurate measurement of the CCD's sensitivity. This figure is called "quantum efficiency." Some CCDs can be made to reach as high as 95% QE. Several Japanese companies have developed many methods of increasing the sensitivity of the CCDs. For example, illuminating the CCD from the side without circuits, increasing the area of the CCD which is receptive to light. Socrates's Universal Disclaimer: I don't know anything about anything. - Nocturnal [edit:] Correction: the doped layers have spare electrons, but if they were free from the start that would of course defeat the purpose of the CCD! Sorry, I'm not a physicist, just an amateur photographer.
Frigoris 12:06, 5 April 2006 (UTC)ithink we need more about *how* a CCD work in the article
Hey can anyone tell me avout how it works in taking X-ray images ?
[edit] Achitecture
We certainly need a few diagrams here. The architecture is also not very clear from the text alone. Those who can, please do. JMK 19:20, 30 April 2006 (UTC)
A closely related device is the analog Bucket Brigade Device, which was (is?) used for audio delay lines. Some typical examples are the EG&G/Reticon SAD-1024 and the Panasonic MN3004. It may be useful to add this to the article.
[edit] External Links
I renamed this section CCD Vendors because that's all it was. "External Links" is an invitation to linkspam. Next, I suggest changing the external links to Wikilinks. If a company is on this list, it should be noteworth and have it's own page. Otherwise, it should be removed from the list. Jehochman (Talk/Contrib) 02:21, 2 May 2006 (UTC)
- I removed the Vendors section. Wikipedia is an encyclopedia, not the Yellow Pages. There are plenty of resources for anyone who wants to buy such a device, starting with Google. This was just spam. --Kbh3rdtalk 04:15, 14 June 2006 (UTC)
[edit] Sensitivity and bandwidth
Can someone please say something about sensitivity (I read something that about 8 photons are necessary to stimulate a CCD while 70 are neccessary to "switch" a grain of photo-sensitive chemical in a film) as well as the bandwidth of sensitivity of a "normal" CCD (in nanometer wave length). THANKS -- Michael Janich 09:23, 6 June 2006 (UTC)
[edit] Camera specifications
Can someone explain the meaning of the measurements given for CCD size by camera manufacturers? For example, what is 1/2.5" vs. 1/1.7"?? Originally, I thought these were shorthand inversions of inches (ie 1/2.5" = 0.4 inch diagonal), but it doesn't seem to work out correctly. Thanks! Slowmover 15:39, 6 June 2006 (UTC) EDIT: As far as I know the inch used for describing CCD-sizes isn't 25.4 mm, it's something around 16 mm. This came from the old vidicons. http://de.wikipedia.org/wiki/Vidicon
[edit] Technical Details
I added the technical template to the top of this page. I've had small electronics courses and I had trouble understanding the article--I can't imagine what it would be like for a layperson. Certainly more context, preferably less jargon, etc. I don't know enough to do it myself, though. Thanks to anyone who can help! 71.57.91.231 08:20, 16 November 2006 (UTC)
[edit] Cooling of CCDs
The article claims that astronomical CCDs are cooled because of their infrared sensitivity to the emission of blackbody radiation from room-temperature sources. This is not true. CCDs are cooled because of the "dark" current produced by stimulation of electrons into the conduction band by thermal excitation from the temperature of the CCD itself. Astronomical CCDs, and perhaps all CCDs, are not sensitive to the wavelengths where the blackbody peaks for room-temperature (around 10 microns or so). AmberRobot 19:08, 31 January 2007 (UTC)
[edit] Physics of operation
The "Confusing" template was added to the "Physics of operation" section in April 2007, but the comments about the confusing nature of the section were inserted into the article rather than being placed on the talk page, so I'm transplanting them here. These aren't my opinions necessarily, but just what was on the page, mildly edited for reference translation and grammar. --Drake Wilson 23:43, 27 May 2007 (UTC)
- "The first two paragraphs of the section are extremely confusing. The paragraphs below, concerning the physics of operation, are confusing, but it seems they are necessarily so. See Janesick's book (2001) Scientific Charge Coupled Devices, for any serious study of astronomical imagers (i.e., imagers like the one described below)."
[edit] Physics of operation, first two paragraphs
I find these two paragraphs (cable and ring electrodes stuff) incredibly confusing and they don't make any sense to me at all, despite reading them ten times and having studied a very fair deal of semiconductor physics as well as electronics. Shouldn't they be deleted or rewritten? The guy who made the edits (Arnero) seems like a real contributing user but as i said this makes absolutely no sense.
Grapetonix 20:50, 4 July 2007 (UTC)
I totally agree with this assessment. These two paragraphs should be removed and replaced.Brews ohare 18:24, 6 November 2007 (UTC)
[edit] Interline transfer LESS sensitive to IR than frame transfer?
Hello, I was just look at "http://www.tpromo2.com/ssmag/cctv/cctv3.htm" where it says that frame transfer CCD's are much less sensitive to infrared light than are interline transfer CCD's. This article says the exact opposite??? 65.183.135.231 (talk) 14:33, 23 May 2008 (UTC)
