History of Long-Range Photography in Astronomy

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For example, a gain of 1.0 would certainly minimize the electronics contribution to noise, but would only allow 65,536/1.0 = 65,536 electrons of the 85,000 to be digitized.

System gains are designed as a balance between digitization counts, digitization noise, and total well depth.

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Digitization

Digitization, also referred to as analog to digital conversion, is the process by which an analog charge from the CCD is translated into a binary form used by the computer.

The term binary refers to the base 2 number system used.

A 12 bit camera system will output 4096 levels.

A 14 bit system will output 16384 levels.

A 16 bit camera will output 65536 levels.

The higher the digital resolution, the slower the system throughput.

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Dynamic Range

The dynamic range is often represented as a log ratio of well depth to the readout noise in decibels.

For example, a system with a well depth of 85,000 electrons and a readout noise of 12 electrons would have a dynamic range = 20 log (85,000/12), or 77dB. The higher the number the better.

This ratio also gives an indication of the number of digitization levels that might be appropriate for a given sensor. A sensor with a well depth of 35,000 electrons and with 15 electrons of read noise would yield a ratio of 35,000/15=2333. Unless there is a desire to resolve this noise with a 16 bit system into 65,536 gray shades, a 12 bit system with 4096 levels would be more than adequate for this sensor.

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Quantum Efficiency

The quantum efficiency (Q.E.) of a sensor describes its response to different wavelengths of light

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Charge Transfer Efficiency

As each charge is transferred, not every electron will be carried along with perfect efficiency. Some get "left behind" or trapped.

Charge Transfer Efficiency of 0.99997 can be thought of as 2.5 electrons out of 85,000 electrons not transferred or "lost in the shuffle".

Since the CTE is sensor dependent, all manufacturers of imaging systems using the same sensor will start with the same efficiency specification. Improper camera system design can cause less than optimum charge transfer efficiency.

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Data Rate and Transfer Rate

The key consideration here is, "How long from the time the shutter closes do I have to wait to see the image?"

The shorter this time is, the easier to focus the system and more enjoyable the CCD imaging experience will be.

Also, dark count continues to build during the delay and readout. The longer the actual charge transfer time from the CCD, the greater the top to bottom dark count gradient.