The QSI model 683wsg-8 CCD Camera, with Mechanical Shutter, 8 Position Filter Wheel, and Integrated Guider Port features the KAF-8300, an 8.3 megapixel Kodak Enhanced Response full-frame CCD image sensor with microlens technology. The high quantum efficiency, wide dynamic range and low noise performance make the QSI 683 ideally suited to a broad range of demanding imaging applications.
The Benefits of Scientific Imaging Performance
QSI's scientific-grade CCD cameras deliver image quality equal to or better than cameras costing thousands more. The lower noise and other benefits of a QSI camera allow you to capture better data in less time. What does scientific imaging performance really mean? There are four key traits that distinguish a scientific camera:
- High sensitivity
- Lower Noise
- Linear response to light
- Precisely regulated cooling
How sensitive a CCD is to light is expressed as its Quantum Efficiency (QE). Higher QE means you can collect more photons in any given amount of time giving you better looking images with a higher Signal to Noise Ratio (SNR). High sensitivity is almost purely a function of the CCD. Just about any camera using the same CCD will achieve a similar sensitivity to light. However, even with the same CCD, the Signal to Noise Ratio between two different cameras can be dramatically different especially in the fainter, lower signal areas of an image. These lower signal areas, such as the fringes of nebulae or dust lanes in a galaxy, often show some of the most interesting parts of astronomical images. Maximizing the SNR in these dim areas is the key to revealing otherwise hidden details.
Lower Noise = Higher Dynamic Range
Noise in a CCD camera is uncertainty in the signal you’re trying to measure. The lower the noise, the greater the dynamic range possible with any given CCD. There are several types of noise that contribute to the overall noise in a CCD image.
CCD images are extremely sensitive to any noise introduced by the camera’s electronics. Mitigating the electronic interference from the digital electronics and switching power supplies is one of the many challenges in designing and building a scientific grade CCD camera. Any noise introduced by the camera is relatively easy to see by analyzing bias frames. A Bias Frame is a zero-length (dark) exposure which captures the slightly varying “floor value” from pixel to pixel (the bias) plus any noise added during the process of reading the image from the CCD. By subtracting two bias frames, you can remove the underlying differences in the pixel bias leaving just the noise added to the image when it was read from the CCD. This subtraction of two bias frames is called a Read Noise Frame. Averaging together multiple bias frames can tell you more about the distribution of pixel values and make it easier to visually identify any non-Gaussian noise in the images.
The Fourier Transform
A Fourier Transform is a mathematical function that converts a signal (any signal) into a collection of sine waves which together exactly represent the original signal. For images, an astrophotographer utiizes a 2-dimensional Fourier Transform which converts an image from the Spatial Domain, rows and columns in a digital picture, into an equivalent Frequency Domain image. A Fast Fourier Transform (FFT) is a special algorithm that runs much faster on computers when starting with square images where each side has a length equal to a power of 2 (e.g 256, 512, 1024).
To analyze the noise from a CCD camera, you create a Read Noise Frame as described above. By subtracting two bias frames, you end up with just the noise introduced by reading the image from the CCD. An FFT of that Read Noise Frame will reveal any pattern noise introduced by the camera.
An FFT of random noise displaying a Gaussian distribution will have a smooth dark background with a single bright pixel in the center. To the left is an FFT of a Read Noise Frame from a QSI 600 Series camera. The FFT of the QSI Read noise frame reveals no pattern noise and is, in fact, virtually indistinguishable from an FFT of mathematically generated Gaussian noise.
Periodic noise will not be reduced when combining multiple images; instead it is reinforced, just like signal. The net effect of such noise is to raise the noise floor, reducing the signal to noise ratio and hiding the faint details in your images. When many such images are combined, periodic noise may show up as visible patterns, such as faint horizontal or vertical lines, running through your images. Lower noise displaying a Gaussian distribution allows you to reveal subtle details in your targets that otherwise would be lost.
Accurately recording the relative amount of light between different objects requires a camera that delivers a linear response to varying levels of light. A CCD camera should record twice the photon flux from an object that is twice as bright. Cameras that use CCDs with anti-blooming protection will start to show a drop in linearity as the pixels begin to approach a high percentage of full well capacity, but should be nearly linear at lower signal levels.
The log scale graph below shows the response to light of a QSI 683wsg which uses an Full Frame CCD with anti-blooming protection. the log scale of the graph amplifies any discrepencies from linear response especially at the lowest signal levels. The data points are linear to less then 1% all the way down to the lowest signal level, well within the specification of the CCD. Lower noise is especially critical in maintaining linearity at these lower signal levels.
High linearity is obviously critical for scientific applications such as accurate photometry, but it's just as important for producing beautiful pictures of deep sky objects. A linear response to light and lower noise allows you to accurately capture the subtle details of your targets in less time. To achieve the same signal to noise ratio in the faintest areas of your images with a higher noise camera can require hours more exposure time. Turning that around, a lower noise camera will yield great looking images with a higher signal to noise ratio in much less time.
Precisely Regulated Cooling
Every image taken with a CCD camera has two distinct types of “signal”. A CCD records light striking it from the objects in your field of view, and it records heat from the sensor itself. Cooling the CCD reduces the contribution of this “thermal” current. With most Kodak CCDs, the amount of thermal current is cut in half for every roughly 6°C drop in the temperature of the CCD. So, dropping the temperature of the CCD by 36°C reduces the rate that thermal current builds by over 98%.
The rate that thermal current builds at any given temperature is very predictable and repeatable. That’s why dark frame subtraction is so effective. Subtracting an average or median combine of multiple dark frames taken at the same temperature as your light frames will remove the thermal current component (and bias) from your light frames. However, if the temperature varies either during the capturing of your light frames or dark frames by even a few tenths of a degree, dark frame subtraction becomes much less effective.
Linearity, Read Noise, and Photon Transfer (Gain),
High sensitivity, low noise, a linear response to light and precisely regulated cooling are four key traits of a scientific camera. All are required in order to maximize the dynamic range of your images. Given a constant signal, lower noise increases the Signal to Noise Ratio and dynamic range, allowing you to produce better images in less time. Capturing good looking images of bright targets is relatively easy, but if you want great results in the dimmer areas of your targets, achieving the same signal to noise ratio with a higher noise camera can require hours of additional dark time.
All significant performance characteristics of QSI cameras, including Linearity, Read Noise, and Photon Transfer (Gain), are tested and documented as the final manufacturing step. Each camera's timing and voltages are carefully set to ensure maximum Charge Transfer Efficiency, and to minimize charge injection and other secondary noise sources.
The QSI 683wsg-8 includes the following:
- 8.3mp Kodak KAF-8300 sensor
- USB 2.0 High-Speed, 16-bit output
- Dual read rates of 800kHz and 8MHz
- Air cooling to 45C below ambient
- Mechanical even-illumination shutter with shutter speeds down to 30ms
- Internal 8-position Color Filter Wheel
- Integrated Guider Port
|Name||Quantum Scientific Imaging - QSI 683wsg-8 8.3mp Cooled CCD Camera w/8-pos filter wheel and Integrated Guider Port|
|Custom Stock Status||This item is special order. Delivery times will vary. Place your order today and your card will not be charged until the item ships.|
|CCD Sensor||Kodak - KAF-8300|
|CCD||8.3mp Kodak KAF-8300 sensor|
|Computer Interface||USB 2.0 High-Speed, 16-bit output|
|Cooling||Air cooling to 45C below ambient|
|Filter Wheel||Internal 8-position Color Filter Wheel|