Low Noise Scientific CCD Image Sensors

QSI 500 Series cameras employ a broad range of Kodak full-frame and interline transfer CCDs. The default sensors in most models providing excellent quantum efficiency (QE) between 350nm and 1000nm with significant enhancement at the blue end of the spectrum. Low dark current, high pixel charge capacity and a low noise 16-bit analog to digital converter (ADC) all contribute to exceptional dynamic range.

Most sensors used in QSI cameras employ a transparent gate that significantly increases optical response compared to traditional front illuminated sensors.
Micro lenses cover the surface of most standard CCDs to focus the light through the transparent gate to further increase optical response. See the Specifications tab of each camera model for complete details.

  High Performance Design

"The QSI 500 Series scientific camera family was designed from the ground up to attain the highest possible imaging performance from Kodak's KAF and KAI CCD image sensors."

• Sophisticated mixed-signal design practices are utilized throughout the camera. This permits a very compact design while eliminating interference from conducted and radiated noise. All of the many internal clock and bias voltages are set via high precision Digital to Analog Converters (DACs) under computer control, allowing automated optimization during manufacture. Multi-layer circuit boards and surface-mount components are used exclusively. A unique circuit board stacking methodology eliminates interconnecting wires that could reduce reliability.
   
• Arguably, the most important aspect of a CCD camera design is the video processing subsystem. This is where almost all of the camera's performance characteristics are established. The 500 Series video processing begins with a very low noise, precision preamp to accurately amplify the microvolt level pixel signal from the CCD image sensor. This signal is then processed through a Correlated Double Sampler (CDS) to reduce temporal read noise in the pixel signal. Subsequent signal conditioning then feeds the pixel level to a high-speed, precision 16-bit Analog to Digital Converter (ADC) where it is converted to a digital value between 0 and 65535.
  
  The Read Noise contributed by this entire subsystem is exceedingly small. So small in fact, that it is virtually undetectable, contributing less than 1/30 of the combined read noise of a typical KAF image sensor. The low read noise and carefully chosen camera gain yield excellent Dynamic Range. Linearity is also outstanding, limited only by the CCD image sensor itself.
   
• An internal pipelined data architecture with a large memory buffer is employed to achieve a constant 400,000 pixel/second read rate from the CCD. The multi-controller design effectively decouples the camera pixel read operation from the USB transfer process while ensuring maximum throughput.
   
• A carefully designed and isolated switching power supply generates all of the voltages needed to operate the camera from a single 12VDC power source. Low noise design practices and advanced filtering techniques completely eliminate any measurable impact on camera noise performance.
   
• All significant performance characteristics, including Linearity, Read Noise, and Photon Transfer (Gain), are tested and confirmed during manufacture. Each camera's timing and voltages are carefully set during manufacture to ensure maximum Charge Transfer Efficiency, and to minimize charge injection and other secondary noise sources. QSI's exclusive ResearchSpec® profiling ensures optimal performance in every camera.

CCD Cooler Subsystem
 
Key in the compact design of the 500 Series cameras is a very efficient 12 watt thermoelectric CCD cooler subsystem. Intelligent, programmable cooling fans are integrated into the rear of the camera body to remove the heat generated by the cooler. Typically, forced air cooling lowers the regulated CCD temperature by 38°C below ambient utilizing 85% power. Tight +/- 0.1°C temperature regulation is maintained at temperature settings of 10°C below ambient and lower. A slim liquid heat exchanger (shown at right) can be attached to the rear of the camera body to increase the cooling further, up to 45°C below the temperature of the circulating fluid.

The cooled CCD image sensor is positioned in a hermetically sealed environmental chamber covered with an anti-reflection coated precision optical window. The chamber is purged with an ultra dry noble gas to minimize the frost point and heat conduction. To extend the useful period before re-purging is required, a user-rechargeable microsieve desiccant is employed to scavenge water molecules that enter the chamber. It is located behind a sub-micron, permeable membrane to prevent particulate contamination of the CCD chamber.

Cooling and Dark Current
 
Effective cooling of the CCD image sensor is essential for long exposure imaging, especially in astronomy. Thermally generated electrons accumulate in the pixels over time and compete with the electrons that make up the image. This accumulation of thermal electrons is known as 'dark current'. It lowers the dynamic range of the sensor and reduces the signal to noise ratio. Eventually the thermally generated electrons will swamp the image.

Fortunately, dark current can be reduced dramatically by cooling the CCD. Kodak KAF CCD sensors accumulate thermal electrons at a rate of roughly 4 electrons per second per pixel at 25°C. With every 6.3°C decrease in temperature the dark current is reduced by half. Where a 10 minute exposure might generate 2400 thermal electrons at 25°C, it will produce only about 10 at -25°C. This is a very small number when compared to the CCD read noise of 15 electrons and the pixel capacity of 100,000 electrons. At -35°C, a 1 hour exposure will generate less than 20 thermal electrons per pixel!
 

Shutter and Filter Subsystem
 
The 500 Series achieves its compact size and unique configuration flexibility though an innovative shutter/filter subsystem. Both the mechanical shutter and filter wheel can be installed internally while keeping the growth of the camera dimensions at a minimum. This results in minimal loss of backfocus as these features are added. The compact design also reduces vignetting with fast optical systems.  

Shutter

The 500 Series "s" and "si" models incorporate the internal mechanical shutter in the 'mid-size' camera body configuration. The shutter is not only used for timing exposures, but can be left closed to produce 'dark frames' for subsequent image processing. Exposures can be as short as 0.03 seconds or as long as 240 minutes.

Filter Wheel  

The 500 Series "ws" and "wsi" models add an internal five position filter wheel to the camera in a 'full-size' camera body. The filter wheel accepts any standard 1.25" filter and can be purchased with various filter configurations. The filter wheel can easily be removed and replaced to change or clean the glass filters. Additional filter wheels can be purchased allowing quick interchange of different filter set configurations. (Note how the RGB filters reflect the complimentary color in the picture at the right.)

LRGB and Narrowband Filters
 
LRGB and narrowband color filters from Astrodon and Astronomik are available with QSI 500 Series cameras. Filters ordered with a camera will be installed in the QSI clean room during final assembly. See the individual product pages for complete information and pricing.

Integrated Guider Port
 
Selecting the best guiding solution has always required a compromise. The Integrated Guider Port (IGP) of the WSG solves many of the problems associated with existing guiding solutions.

The Right Guiding Solution

Guiding with a separate guide scope provides the most flexibility, but differential flexure can be an issue, especially with long focal length scopes. With an internal guide chip, you're forced to guide with light through your filters and you can't guide at all while the shutter is closed or an image is being downloaded. This is especially problematic for narrowband imagers. To get around those issues, you could add an external off-axis guider, but a traditional OAG can add an inch or more of back focus plus more weight and two new mounting surfaces that need to be held rigidly.



Guide with Light From
in Front of the Filter Wheel

The QSI 500 Series "wsg" models solves the problems with other guiding solutions by integrating a precision off-axis guider directly into the camera body with the pick-off prism positioned in front of the integrated color filter wheel – right where it belongs.

Never Struggle With Finding a Guide Star

One of the main challenges when using a camera with an internal guide chip is to find a star bright enough to guide by within the limited field of view of the intenal guide sensor. When shooting through red, green or blue filters, ⅔ of the available light is blocked by the filter and not transmitted to the internal guider chip, guaranteeing lower signal-to-noise stars for guiding. This problem is compounded with narrowband filters where as little as 1% of the total light from a star reaches the guide chip. By positioning the pick-off prism in front of the filters, you always have all the star's light available for guiding.

WSG Supports Fast Optical Systems

By integrating the Guider Port into the camera, we're able to position the pick-off prism very close to the internal filter wheel adding minimal backfocus and eliminating any possibility of flexure or rotation compared to a traditional OAG. The large ½" square pick-off prism is optimally positioned close to the internal filter wheel, supporting the use of guide cameras with large sensors, while preventing any vignetting of the main sensor even with very fast optical systems.

Flexible Guide Camera Options

The Integrated Guider Port (IGP) built into 500 Series WSG models is designed to support any camera with 12.5mm of back focus or less that can be attached using C-mount or T-mount threads. Many cameras are designed with 12.5mm of back focus to be compatible with CS-mount lenses which use the same thread as C-mount (1" x 32tpi) but with 12.5mm of back focus instead of 17.5mm for C-mount.

Click here for a list of guide cameras known to be compatible >>

Easy, Rigid Guide Camera Focusing

The guide camera attaches to the WSG using a C-mount or T-mount threaded adapter (specified at time of order). The threaded adapter sits on top of the focus ring and allows the guide camera to be rotated to any position. The focus ring threads onto the focus base to allow 3mm of travel when focusing the guide camera. Once focus is achieved, the focus ring is locked with a set screw. The guide camera can still be rotated manually if desired without changing the focus. The end result is a rigid, easily focused guide camera that will not move or flex while your mount tracks the apparent motion of the night sky.

The connector panel, shown above, is thoughtfully recessed into the body of the camera for protection and provides access for all external connections. The two threaded holes on the back of the body are used to attach an optional cable restraint system to support the electrical connections as well as the recirculation hoses if the liquid heat exchanger is employed.

USB Interface

All 500 Series cameras utilize a standard USB 2.0 (USB 1.1 compatible) port for connection to the host computer and imaging application software. Read and transfer time for an entire 2184x1472 image frame is typically less than 8 seconds.

Guider/Control Port

A four channel optically isolated control port is accessible through a standard 6 pin modular connector. The signal pin-out is compatible with most modern telescope mount drive correctors and is intended to be used for telescope guiding under MaxIm/DL and CCDSoft. The outputs can also be used for other control purposes when developing your own applications with the 500 Series ActiveX software development toolkit. The outputs are common emitter, open-collector and can sink up to 50ma. The maximum voltage should not exceed 50v.

Power Requirements

One of the defining features of the 500 Series cameras is power efficiency. A fully configured 500 Series camera operates from a single 12v DC supply and only consumes 18 watts at full cooling, with both fans at maximum and the filter wheel moving. Other camera model configurations have power consumption as low as 3.5 watts. Included with each camera is an approved 12 volt DC power supply with an input voltage range of 90-240V, 50-60HZ.

Notification

Both visual and audible notification is built into all 500 Series cameras. A multi-color LED status display provides visual indication of the various states of camera operation. The behavior of the indicator is configurable and can be disabled by user command. A unique internal beeper provides audible feedback of camera operation and status. Like the visual display, the beeper can be configured and disabled by the user.