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Old 19-02-2008, 10:40 PM
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Kal (Andrew)
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Join Date: Nov 2006
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Quote:
Originally Posted by Peter Ward View Post
As to the point, well there are several.
CCD's
Higher QE, lower noise, less noise structure, better colour saturation, higher dynamic range.
CMOS.
Low H-alpha sensitivity unless modified, Easier use, requires external guider.
I just read this page and most of the linked articles, and I am wondering how the astronomy market will fare down the track with regards to image sensors. Will it be like the current LCD V plasma battle with tv's?

Neither of the technologies are new, dating back to the late 60's / early 70's, but it wasn't until recently that CMOS sensors became viable because of reduced lithography sizes (the same reason why computer chips are smaller/more powerful/faster).

Some interesting facts:

* CCD will always have the signal to noise battle won simply by the fact that CMOS chips have more digital circuitry on the chip, which adds noise.

* For responsivity (the amount of signal the sensor delivers per unit of optical energy) CMOS chips have the advantage because it is easier to place the gain elements on the chip. I don't think this is the same as QE, and theoretically I don't see any difference in potential QE between the two technologies.

* For Dynamic range, CCD's win again. I guess integrating the circuitry to do a charge to voltage conversion for every single pixel takes up space where CCD's can utilise this elsewhere?

* Windowing - this is an interesting one, and I wonder of it is patented yet? For a CMOS chip it wouldn't be too much effort to designate different framerates and functions to different parts of the sensor. For example, I was thinking you could define your own guidecamera from any part of the chip, and you could do this every time you slew to an object to image it. Take an exposure, use a 25x25 pixel array centered on a star towards one of the edges, and that can operate at 2 fps for guiding while the other pixels operate to capture yout image. I don't think you can do this with CCD because you have to transfer a line at a time - with a CCD you need a seperate chip to guide with.

*Antiblooming - CMOS wins here as it can drain a pixel without affecting anything around it. It can be implemented onto CCD though, so I don't know if this is really an advantage.

The biggest impact in my mind, however, is market impact. Astronomy is a tiny market - the big market is from security cameras/phones/digital cameras etc. With the ability to integrate more functions onto the chip itself, I see CMOS winning more market space in the future, which could potentially isolate CCD to the specialist market, making it a more expensive technology.

Anyway, I don't know if this analysis is too off topic, but the research was interesting for me at least
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