CCD Astrophotography - Cost Comparisons to DSLR and CMOS Sensors

[admin]

There are several reasons for this, CCD chips are more sensitive (up to 50 times more) generally especially the more expensive ones. You can check QE or quantum efficiency of a sensor here:
Quantum efficiency - Wikipedia, the free encyclopedia

Even though the resolution is the same (pixel array size ie 1000X1000), that does not make them equal in terms of sensitivity, dynamic range, or even noise (noise readout measurements).

DSLRs use CMOS technology which is cheaper, mass produced more (more popular for the last few years as the main camera/video camera sensor technology - and therefore is cheaper) however newer CCD sensors are still used for astronomy as they are more sensitive and may have less noise when compared to CMOS chips.

Most ccd's on astronomy cameras have fewer pixel defects. There are grades or classes of ccd's and not all are equal, some have more defects than others - for research the highest grades are used.

A lot of the cost is assigned to having a low noise system (this is not an issue with exposures with DSLRs where photos of 1/250s or even faster are usually taken - with astronomy cameras 1 hour (3600 second +) shots are common!), or methods to reduce it including cooling, active or passive such as fans, TEC - thermoelectric cooling to reduce noise or even water cooling.

The dynamic range is higher in CCD sensors than DSLR sensors (CMOS) meaning CCD's can capture more faint and bright details in a single exposure than the exact same resolution/pixel array size CMOS/DSLR chip.

A main reason as stated again, is supply and demand. Digital SLRS (DSLRs) are mass produced, so are naturally cheaper as Canon has sold over 1.5 million of its EOS Digital Rebel XT series cameras alone - whereas most ccd cameras sell only a few thousand.

The above is my simple explanation, a great more detailed response is here:
http://www.dalsa.com/corp/markets/ccd_vs_cmos.aspx


see also this thread for additional cost analysis of CCD vs CMOS/DSLR imaging cameras:

Need a CCD camera primer

[admin]

I did not know this but fill factor is a very important aspect of ccd sensitivity and usually is much higher in ccd's

The fill factor defined as the ratio of light-sensitive area to total pixel size, also determines the maximum achievable sensitivity. The fill factor of FF and FT CCDs is close to 100%, whereas this drops to about 30% for most IT-CCD and CMOS APS image sensors.

more here:
www.ifp.uni-stuttgart.de/publications/phowo01/blanc.pdf

see here for Fill Factor:

http://en.wikipedia.org/wiki/Fill_factor

The interline architecture extends this concept one step further and masks every other column of the image sensor for storage. In this device, only one pixel shift has to occur to transfer from image area to storage area; thus, shutter times can be less than a microsecond and smear is essentially eliminated. The advantage is not free, however, as the imaging area is now covered by opaque strips dropping the fill factor to approximately 50 percent and the effective quantum efficiency by an equivalent amount. Modern designs have addressed this deleterious characteristic by adding microlenses on the surface of the device to direct light away from the opaque regions and on the active area. Microlenses can bring the fill factor back up to 90 percent or more depending on pixel size and the overall system's optical design.

http://en.wikipedia.org/wiki/Charge-coupled_device

[fogfire]

in the base technology CCD has lower noise than a CMOS BUT DLSRs sensors have advanced technology to deal with noise that make them better for high mega pixel count sensors.

Canon was the first to to do this..
CCD captures the image and then the image is played off a line at a time into the buffer. The reading of the image data "removes" it from the sensor and there is no way to read an individual pixel etc.

CMOS allows for direct access of pixel locations like a memory device.

What Canon did was allow for each pixel to have its noise measured prior to each exposure. The info is stored at the pixel location, than the image is taken and the noise is subtracted on pixel by pixel basis. Sony has done its own CMOS based noise removal technology.

When dealing with very low light, the concern is making sure the noise removed is not data.

Keep in mind.. you can get some older DSLRs from Nikon and Konica-Minolta (before Sony bought the camera division) that have CCD sensors.. up to 6 MP. Past 6 MP they all went to CMOS

That would give you a CCD sensor.. larger pixel pitch to gather light making for a better SnR. ..

The fascinating thing to remember is that unless you are using a camera that offers the RAW file, the jpg process is tossing out tons of image data that might matter in this kind of photography.

[Leo.A]

The Sony A350 is a 14.9MP DSLR with a CCD censor.
I was just searching for a comparison to decide which type of censor would be better and this search result gives me the perfect answer.
I didn't realise I joined 300+ days ago, now I feel a little embarrassed to consider I've been finding lots of my google search results leading to here and I was already a registered member.

[Dphipps]

All the issues mentioned differentiating CCD and CMOS imagers are quite right. However, one issue I haven't seen yet touched upon is that astronomical (and indeed all professional scientific) CCD imagers and DSLR cameras do not operate at nearly the same digitization levels. DSLR cameras, be they CCD or CMOS, are only sampled at 12-bit/color channel, with a few of the latest products now being sampled at 14-bits/color channel (Pentax's K-5, e.g.). This means that the cameras are only capable of detecting 1/4,096 to 1/16,384 of the chip's dynamic range. Astronomical imagers are, however, sampled at 16-bit, which means they can differentiate 1/65,536 of their CCD's dynamic range. This means that the 16-bit cameras can operate significantly closer to their noise floor without digitization "rounding" up to higher gray levels. Put in terms of apparent sensitivity, for a given exposure the 16-bit camera is going to have a greater sensitivity and S/N ratio down at their low end than any prosumer DSLR out there. Also the amplifiers in astronomical CCD cameras are specifically designed to reduce readout noise, and they are also typically cooled to reduce dark noise - not a problem in DSLRs that spend most of their duty making snapshots. This is why you can buy a DSLR under $1,000, but the same size CCD imager in an astronomical camera will run you into the thousands of $$.

If you are going to use a DSLR camera for astronomical imaging, however, be it CCD or CMOS, you really want to look for one that operates at 14-bit in RAW mode, and make sure you shoot in RAW mode. One tell that you have a 14-bit imager will be that the DSLR camera will be capable of delivering ISOs of 25,600 to 51,200 (again, like the Pentax K-5). These cameras will vastly outperform the older lower bit level cameras, and require far shorter exposures which helps combat the inherent dark noise in their non-cooled imaging chips.