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Author Topic: CCD and CMOS  (Read 18341 times)
larkis
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« on: September 25, 2011, 02:46:11 AM »
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Could someone give me a technical explanation why CMOS can shoot at such high ISO's (nikon D700, etc) compared to CCD's despite CCD's being more sensitive to light and being a more expansive technology ? A friend of mine is a scientist and tells me that in astronomy and industrial applications the reverse of what is going on in the camera business is true, CCD's are more light sensitive and have less noise compared to CMOS. I noticed with most cameras that use CCD's (like the phase backs) anything over 800iso is not really considered to be great. The sensors that go to space are also CCD's.
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ondebanks
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« Reply #1 on: September 25, 2011, 03:22:47 AM »
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Your friend is wrong - or at least, quoting out of date information. CMOS has overtaken CCD in noise performance and is now beginning to displace CCDs in research instrumentation. I'm an astronomer too; my collaborators have just bought some new cameras for use in a high speed photometer on one of the world's largest telescopes - and guess what, they are CMOS (Andor S-CMOS), not CCD. Future space sensors will be CMOS too; just remember that space missions often take 10 years to develop, and usually deploy very old, reliable, tested-to-death, radiation-hardened technology, so it takes a while for new tech to work through to launch.

Excellent High ISO performance requires 3 things: (1) Efficient light capture, (2) Low internal noise, (3) Large sensitive surface area per pixel. Since we can make pixels as big as we like with both CCD and CMOS, that third factor cancels out when comparing them. So let's look at (1) and (2).

(1) Sensitivity/Quantum Efficiency: by using microlenses to direct the light into the active areas of each pixel, CMOS are now ahead of front-illuminated CCDs and gaining on back-illuminated ones. Silicon is silicon in both cases; once photons are on target for an active pixel well, photons will be absorbed in depth distributions proportional to their wavelength in both cases.

(2) Internal noise: At normal (photographic) readout speeds, modern CMOS cameras are several times lower in readout noise than CCDs. Many are in the 2-electron area. S-CMOS is at 1 electron. CCD-based photographic cameras are at best around  6 or 7 electrons, and medium format digital's current best is 12 electrons.
Then there's dark noise. It is almost negligible in good CMOS sensors at everyday temperatures, but still woeful in uncooled CCDs (cooled means taking it to several 10's of degrees C below ambient temperature, as in an astronomical camera). A 300 second exposure with a modern Canon CMOS, without dark frame subtraction, looks as good as a 3 second exposure with many CCD cameras, also without dark frame subtraction. All medium format digital backs/cameras require long dark frame subtractions to alleviate this noise, but of course you cannot subtract its random (poisson) fluctuations, so that adds to the internal noise.

Ray
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jduncan
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« Reply #2 on: September 27, 2011, 06:58:13 AM »
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Your friend is wrong - or at least, quoting out of date information. CMOS has overtaken CCD in noise performance and is now beginning to displace CCDs in research instrumentation. I'm an astronomer too; my collaborators have just bought some new cameras for use in a high speed photometer on one of the world's largest telescopes - and guess what, they are CMOS (Andor S-CMOS), not CCD. Future space sensors will be CMOS too; just remember that space missions often take 10 years to develop, and usually deploy very old, reliable, tested-to-death, radiation-hardened technology, so it takes a while for new tech to work through to launch.

Excellent High ISO performance requires 3 things: (1) Efficient light capture, (2) Low internal noise, (3) Large sensitive surface area per pixel. Since we can make pixels as big as we like with both CCD and CMOS, that third factor cancels out when comparing them. So let's look at (1) and (2).

(1) Sensitivity/Quantum Efficiency: by using microlenses to direct the light into the active areas of each pixel, CMOS are now ahead of front-illuminated CCDs and gaining on back-illuminated ones. Silicon is silicon in both cases; once photons are on target for an active pixel well, photons will be absorbed in depth distributions proportional to their wavelength in both cases.

(2) Internal noise: At normal (photographic) readout speeds, modern CMOS cameras are several times lower in readout noise than CCDs. Many are in the 2-electron area. S-CMOS is at 1 electron. CCD-based photographic cameras are at best around  6 or 7 electrons, and medium format digital's current best is 12 electrons.
Then there's dark noise. It is almost negligible in good CMOS sensors at everyday temperatures, but still woeful in uncooled CCDs (cooled means taking it to several 10's of degrees C below ambient temperature, as in an astronomical camera). A 300 second exposure with a modern Canon CMOS, without dark frame subtraction, looks as good as a 3 second exposure with many CCD cameras, also without dark frame subtraction. All medium format digital backs/cameras require long dark frame subtractions to alleviate this noise, but of course you cannot subtract its random (poisson) fluctuations, so that adds to the internal noise.

Ray
Hi what about Infra RED? I am asking because I have this CCD DSLR and I was thinking of removing the IR filter.  From your argument the old adventages of CCD for visible spectrum no longer holds. Could you elaborate abut IR (for astro photo)

Thanks for your time,

J. Duncan
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fotometria gr
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« Reply #3 on: September 27, 2011, 06:55:46 PM »
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Could someone give me a technical explanation why CMOS can shoot at such high ISO's (nikon D700, etc) compared to CCD's despite CCD's being more sensitive to light and being a more expansive technology ? A friend of mine is a scientist and tells me that in astronomy and industrial applications the reverse of what is going on in the camera business is true, CCD's are more light sensitive and have less noise compared to CMOS. I noticed with most cameras that use CCD's (like the phase backs) anything over 800iso is not really considered to be great. The sensors that go to space are also CCD's.
All MFDBs are CCD (up to now), the P1 P25+ and P45+ are the world leaders in low light because of their ability to keep the sensor cool for long exposures, this is due to the great cooling system not the type of sensor used, the MFDBs are worst in their ISO performance because they lack any kind of noise reduction filter or circuit, this is a choice of the manufacturers due to the demand of the MF market for purity so that low ISO performance is maximum! The IQ of DBs is well beyond any modern DSLR even if they are 5 or 6 years old! The reason for CMOS sensors is mainly the low cost of production, but they do cost a lot more than CCDs to develop. I guess that the production volume of DSLRs compensates for the high development cost of CMOS. CCDs are considered sharper and with greater color than CMOS, I'm not aware which technology is better for high Iso, to compare that we should have a comparison of cameras with both technologies but the same NR system. Regards, Theodoros. www.fotometria.gr
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bjanes
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« Reply #4 on: September 27, 2011, 07:26:36 PM »
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Excellent High ISO performance requires 3 things: (1) Efficient light capture, (2) Low internal noise, (3) Large sensitive surface area per pixel. Since we can make pixels as big as we like with both CCD and CMOS, that third factor cancels out when comparing them. So let's look at (1) and (2).

(1) Sensitivity/Quantum Efficiency: by using microlenses to direct the light into the active areas of each pixel, CMOS are now ahead of front-illuminated CCDs and gaining on back-illuminated ones. Silicon is silicon in both cases; once photons are on target for an active pixel well, photons will be absorbed in depth distributions proportional to their wavelength in both cases.

Microlenses will collect light and focus it on the active sensing area of the sensel, but what about charge density, which limits the full well capacity of a sensel? The transistors in a CMOS will take up space and reduce the active sensing area of the sensel, leading to increased charge density in the CMOS as compared to CCD. I don't quite agree with point 3. For a given sensor size, pixel size is inversely proportional to the megapixel count. You can make the sensor larger, but at considerable expense.

Regards,

Bill
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ErikKaffehr
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« Reply #5 on: September 27, 2011, 11:32:11 PM »
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Hi Bill,

I sort of agree. I'm not really aware how large the physical fill factor is on CCD vs. CMOS, but the transistors definitively seem to take significant silicon area on CMOS. I presume that with narrower design rules the fill factor of CMOS is increasing. I'd also assume that FWC is proportional to physical fill factor.

Best regards
Erik


Microlenses will collect light and focus it on the active sensing area of the sensel, but what about charge density, which limits the full well capacity of a sensel? The transistors in a CMOS will take up space and reduce the active sensing area of the sensel, leading to increased charge density in the CMOS as compared to CCD. I don't quite agree with point 3. For a given sensor size, pixel size is inversely proportional to the megapixel count. You can make the sensor larger, but at considerable expense.

Regards,

Bill
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deejjjaaaa
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« Reply #6 on: September 28, 2011, 01:14:17 AM »
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CCDs are considered sharper and with greater color than CMOS

did you try to compare CCD-based P&S  w/ CMOS based FF dSLR  Wink ? so is the difference because of CCD vs CMOS or because of other factors  Roll Eyes
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deejjjaaaa
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« Reply #7 on: September 28, 2011, 01:17:07 AM »
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CCD-based photographic cameras are at best around  6 or 7 electrons, and medium format digital's current best is 12 electrons.

small CCD sensor based P&S like S95 are <= 4 electrons, are they not ?
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fotometria gr
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« Reply #8 on: September 28, 2011, 03:32:48 AM »
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did you try to compare CCD-based P&S  w/ CMOS based FF dSLR  Wink ? so is the difference because of CCD vs CMOS or because of other factors  Roll Eyes
1. No, neither I will.  Smiley  2. Don't know, is it? You seem to ignore "is considered" from my quote!  Wink Who cares anyway?  Cool Regards, Theodoros. www.fotometria.gr
P.S. I prefer to judge products with what they do for my photography, don't care much to analyze the technical origin of the solution given from manufacturers nor the scientific theory behind it for that matter.
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ondebanks
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« Reply #9 on: September 28, 2011, 06:21:05 AM »
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Microlenses will collect light and focus it on the active sensing area of the sensel, but what about charge density, which limits the full well capacity of a sensel? The transistors in a CMOS will take up space and reduce the active sensing area of the sensel, leading to increased charge density in the CMOS as compared to CCD.

But Bill, we are talking about high-ISO performance - that was the original question. If you are forced into shooting in the high ISO regime, by definition you are not capturing much light/charge per pixel. So charge density and the full well capacity of the sensel are unconnected to this issue. These things matter when you have plenty of light per exposure, so you can shoot at low ISO, and can approach filling the FWC in the scene highlights.

I  don't quite agree with point 3. For a given sensor size, pixel size is inversely proportional to the megapixel count. You can make the sensor larger, but at considerable expense.

Point 3 was simply saying that if I can increase the pixel size/area while holding the readout noise constant, I gain more signal-to-noise per pixel, and get better high ISO. That, for example, is one of the factors which gave the Kodak DCS 620/720x DSLRs the edge in high ISO sports shooting 10 years ago - while maintaining the same 15 electrons readout noise as the 9 micron CCDs of the time, their CCDs had bigger 13 micron pixels, with twice the surface area, and twice the flux accumulated per pixel per unit exposure time. So bigger CCD pixels are better for high ISO. But, I can also make CMOS pixels bigger, and they gain S/N in the same way. And making pixels bigger reduces the megapixel count for both CCD and CMOS, also in the same way. So pixel size matters in absolute terms, but in this CCD vs. CMOS comparative question, this factor cancels out.

small CCD sensor based P&S like S95 are <= 4 electrons, are they not ?

Yes. I think this may be because there is a secondary dependence of readout noise on readout speed. P&S cameras like the S95 are not designed for fast full-resolution frame rates (the S95 reaches 0.9 fps), their users have no great expectations in that area, so the readout can be a little more "leisurely" in order to keep the readout noise down a bit. Astronomical CCD cameras often exploit this by taking the readout speed way down, sometimes to the kHz regime rather than the MHz regime. We are always working at the limit of the S/N, so when you've waited 1 hour for an exposure to complete, you don't really mind waiting another minute for it to be read out, if it keeps the noise down.

I wish that MFDB manufacturers would introduce this as a selectable feature on their backs - a slower, lower noise readout option, for those who can wait a few seconds longer between shots.

did you try to compare CCD-based P&S  w/ CMOS based FF dSLR  Wink ? so is the difference because of CCD vs CMOS or because of other factors  Roll Eyes

That comparison would be unfair, because of the vast differences in pixel size.

Hi what about Infra RED? I am asking because I have this CCD DSLR and I was thinking of removing the IR filter.  From your argument the old adventages of CCD for visible spectrum no longer holds. Could you elaborate abut IR (for astro photo)

Good news for you J.: for IR, in principle neither CMOS nor CCD should have any advantage over the other. But, even with the IR-block filter removed, it does depend an awful lot on the transmission curves of the Bayer RGB filters over the pixels. In fact, the differences between various CCD Bayer transmissions can be larger than the differences between popular CCD and CMOS Bayer transmissions. This is clear from manufacturer datasheets. In MF CCD sensors, for example, the Dalsa CCDs I've seen transmit IR predominantly through the Red-filtered pixels (that's 1 in 4 pixels). In Kodak MF CCDs, the Green- and Blue-filtered pixels tend to recover in transmission as the IR band begins beyond 700nm, and beyond about 820nm, every pixel is transmitting equally; allowing shorter IR exposures, and giving monochromatic B&W IR images which in principle require no de-Bayering interpolation, which would allow perfect per-pixel sharpness.

All MFDBs are CCD (up to now), the P1 P25+ and P45+ are the world leaders in low light because of their ability to keep the sensor cool for long exposures, this is due to the great cooling system not the type of sensor used, the MFDBs are worst in their ISO performance because they lack any kind of noise reduction filter or circuit, this is a choice of the manufacturers due to the demand of the MF market for purity so that low ISO performance is maximum! The IQ of DBs is well beyond any modern DSLR even if they are 5 or 6 years old! The reason for CMOS sensors is mainly the low cost of production, but they do cost a lot more than CCDs to develop. I guess that the production volume of DSLRs compensates for the high development cost of CMOS. CCDs are considered sharper and with greater color than CMOS, I'm not aware which technology is better for high Iso, to compare that we should have a comparison of cameras with both technologies but the same NR system. Regards, Theodoros. www.fotometria.gr

Theodoros, there's quite a bit of confusion and mythology in your response. I'll take each in turn:

"the P1 P25+ and P45+ are the world leaders in low light because of their ability to keep the sensor cool for long exposures" - Here you are confusing long-exposure capability with high-ISO capability. They depend on different things. That Kodak DCS 720x I mentioned above, for example (I got one for a song recently) - excellent at high ISO, dreadful at long exposures. Nice interchangeable viewfinders though!

the MFDBs are worst in their ISO performance because they lack any kind of noise reduction filter or circuit - It's vital that people understand that noise reduction trickery is not the reason why MFDBs cannot compete favourably with CMOS DSLRs on high ISO per-pixel performance. You can turn off the noise reduction completely, on say a Canon DSLR, and save a RAW file which has equal "purity" (to use your term). Apples with apples, the Canon still wins.

The IQ of DBs is well beyond any modern DSLR even if they are 5 or 6 years old! - At low ISO, yes. At high ISO, which is what we are talking about, no.

CCDs are considered sharper and with greater color than CMOS - Take the AA filter off a CMOS sensor and it is equally sharp as a CCD. Really, really. For that matter, put an AA filter over a CCD (I can, with my Kodak), and it is equally unsharp as a typical CMOS. As for color, that depends on the Bayer filters and the IR-blocking filter; there is no reason in principle, other than manufacturer design decisions on these filters, why the colour response of a CMOS unit cannot be the same as that of any given CCD.

What I'm saying is that you have to separate out what is intrinsic to the sensor (things like readout and dark noise) and things which are extrinsic (things like AA, IR-blocking, & Bayer filters layered on top of the sensor). Don't judge "CCD vs CMOS" on the basis of extrinsic things, which can be chopped and changed or easily redesigned. Kodak, for example, often offers the same intrinsic CCD sensor with different configurations of Bayer or no Bayer, IR-blocking coverglass or no coverglass.

I'm not aware which technology is better for high Iso, to compare that we should have a comparison of cameras with both technologies but the same NR system. - Or even better, with all NR turned off, and a RAW comparison with complete "purity". Well, that comparison has been done, over and over: it's an entire website called DxOMark! And I've done my own little version, comparing my MFDB to my Canon 5DII. Surprise, surprise, the Canon trounced the MFDB.  Wink I still prefer the MFDB at low ISO and short exposures, and the handling of a MF camera.

P.S. I prefer to judge products with what they do for my photography, don't care much to analyze the technical origin of the solution given from manufacturers nor the scientific theory behind it for that matter.

That's a perfectly valid position to take, but then why did you depart from it, and feel you had to get involved in this thread?  Huh

Ray
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wildlightphoto
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« Reply #10 on: September 28, 2011, 07:26:16 AM »
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I wish that MFDB manufacturers would introduce this as a selectable feature on their backs - a slower, lower noise readout option, for those who can wait a few seconds longer between shots.

Leica is looking into this as a firmware update for the S2.  It would be a user-selectable menu option.
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TH_Alpa
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« Reply #11 on: September 28, 2011, 09:36:06 AM »
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Thanks Ray, for this detailled and interesting information.

Thierry

But Bill, we are talking about high-ISO performance ....
.....
Ray
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bjanes
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« Reply #12 on: September 28, 2011, 10:51:09 AM »
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small CCD sensor based P&S like S95 are <= 4 electrons, are they not ?

That is true, but a read noise of 4 electrons for a small pixel has a quite different effect on measured noise than for a large pixel because of camera gain, which is the number of electrons collected per data number (raw value). Since the P&S sensors use 12 bits per pixel for their output, I will use this bit depth for comparison.

A small pixel camera such as the Canon S70 may collect 8200 electrons and has a read noise of 3.2 electrons. Assuming that the raw pixel value at full well is near the maximum 12 bit pixel value of 4095, the gain is 2.0 electrons / data number. A larger pixel, such as with the Nikon D3, can collect about 66,500 electrons with a read noise of 4.9 electrons and a gain of 16 electrons /data number. The data are from Roger Clark.

When we look at measured noise in the image, we are concerned with data numbers, not electrons. A read noise of 3.2 electrons for the S70 would translate to 3.2/2.0 or 1.6 data numbers. The read noise in terms of data numbers for the D3 would be 4.4/16 or 0.27 data numbers.

Regards,

Bill
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bjanes
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« Reply #13 on: September 28, 2011, 10:54:58 AM »
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Thanks Ray, for this detailled and interesting information.

Thierry


+1

Bill Janes
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theguywitha645d
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« Reply #14 on: September 28, 2011, 12:10:15 PM »
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In Kodak MF CCDs, the Green- and Blue-filtered pixels tend to recover in transmission as the IR band begins beyond 700nm, and beyond about 820nm, every pixel is transmitting equally; allowing shorter IR exposures, and giving monochromatic B&W IR images which in principle require no de-Bayering interpolation, which would allow perfect per-pixel sharpness.

Ray, this is very interesting. I believe my Pentax 645D has a Kodak sensor and I was thinking of trying some IR (but not remove the IR cut filter). Pentax has really worked on the performance of this sensor with ISOs of 1600 and no limit on integration time--I have done some wide-field astro images with exposures up to 5 minutes and known folks to shoot up to half an hour. You wouldn't know where I could get the spectral response of a Kodak 44x33mm 40MP Bayer sensor?
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hjulenissen
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« Reply #15 on: September 28, 2011, 01:56:12 PM »
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Where is the "cutting point" in exposure time when film generally give better results than digital?
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Radu Arama
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« Reply #16 on: September 28, 2011, 02:04:06 PM »
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Ray, this is very interesting. I believe my Pentax 645D has a Kodak sensor and I was thinking of trying some IR (but not remove the IR cut filter). Pentax has really worked on the performance of this sensor with ISOs of 1600 and no limit on integration time--I have done some wide-field astro images with exposures up to 5 minutes and known folks to shoot up to half an hour. You wouldn't know where I could get the spectral response of a Kodak 44x33mm 40MP Bayer sensor?

Hello,

I believe this is what you were looking for: http://www.kodak.com/ek/uploadedFiles/Content/Small_Business/Images_Sensor_Solutions/Datasheets(pdfs)/KAF-40000LongSpec.pdf the spectral response is on page 15.

Best regards,
Radu
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PierreVandevenne
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« Reply #17 on: September 28, 2011, 03:22:11 PM »
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Your friend is wrong - or at least, quoting out of date information. CMOS has overtaken CCD in noise performance and is now beginning to displace CCDs in research instrumentation. I'm an astronomer too; my collaborators have just bought some new cameras for use in a high speed photometer on one of the world's largest telescopes - and guess what, they are CMOS (Andor S-CMOS), not CCD.

I feel that answer is a bit misleading. Aren't the keywords "high speed" here? You can find all kinds of instruments in professional telescopes, including some based on relatively exotic designs (we aren't getting Aladdin III In:Sb sensors in our cameras any time soon, I think). CMOS based architectures are of course widely used in fields where high speed is possible or desirable (photometry of occultations for example, solar observation etc...). But CCD still reigns in imaging applications. I was so surprised by the above statement that I double checked what current major observatories use as imagers

ESO Paranal - http://www.eso.org/sci/facilities/paranal/instruments/index.html - have a look at the detailed description of the instruments, too many to list here

Gran Telescopio Canarias - http://www.gtc.iac.es/en/pages/instrumentation/osiris.php#Detector

Subaru - http://www.naoj.org/Observing/Instruments/SCam/

If the purpose is going deep and long exposures, everyone seems to still be using CCDs

Not that I disagree with the increased usefulness of CMOS based sensors in many fields in general - but do you have examples of CMOS sensors used for image acquisition in fairly long exposures?



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theguywitha645d
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« Reply #18 on: September 28, 2011, 03:59:12 PM »
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Hello,

I believe this is what you were looking for: http://www.kodak.com/ek/uploadedFiles/Content/Small_Business/Images_Sensor_Solutions/Datasheets(pdfs)/KAF-40000LongSpec.pdf the spectral response is on page 15.

Best regards,
Radu

Radu, thank you. That is very interesting...
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fotometria gr
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« Reply #19 on: September 28, 2011, 04:36:12 PM »
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  Ray,
1. On your first quote on me, I suggest that perhaps there was ....low oxygen in the place where you read my post!  Huh  Shocked What are you talking about? It clearly has nothing to do with my statement!  Lips sealed  Cool
2. On your second quote on me there was certainly much CO2 present in your room!  Grin  Wink Its just an answer to the quoter not a post on the OP!  Cry  Kiss
3. Neither yours nor any other Canon turn their NR off when you instruct them to do so from menu, it stays on to some extend by default!  Tongue Cheers, Theodoros www.fotometria.gr
« Last Edit: September 28, 2011, 04:49:13 PM by fotometria gr » Logged
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