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The STF-8300M (monochrome) and STF-8300C (color) cameras are based on ON Semi’s KAF-8300, 8.3 megapixel, full frame CCD. This CCD array has 3326 x 2504 pixels at 5.4 microns making it ideal for both short fast optics in high resolution (unbinned) mode, and for longer focal lengths it can be binned binned 2×2 or 3×3. The CCD has both antiblooming and microlens technology to improve the effective Quantum Efficiency of the sensor. As a result it can be used in a wide variety of optical configurations. The camera electronics are USB 2.0 for fast download rates and rapid focus mode. The small light body accepts both C-mount and standard 35mm Nikon camera lenses. A built-in mechanical shutter is included for automatic dark frames. In addition to its imaging capabilities, it can also be used as a guider.
The KAF-8300 CCD is approximately 18 x 14mm and has a diagonal measurement of about 22 millimeters. With its relatively small pixels and large image area it is ideal for wide field imaging with short fast telescopes and camera lenses and at the same time is flexible enough to be used on long focal length Schmidt-Cassegrains. The STF-8300C single-shot-color version of this camera can be used in high resolution mode (only) for color images, or binned 2×2 or 3×3 for monochrome imaging. The STF-8300M monochrome camera can be used with a filter wheel in any of several binning and partial frame modes for monochrome, color or narrowband imaging.
The sensor is capable of being operated binned 2×2 (10.8 micron pixels) or 3×3 (16.2 micron pixels), and supports subframing for rapid image download. This versatile camera allows a great deal of flexibility in accommodating different optical systems, imaging conditions, and targets. For these reasons, plus the camera’s small size and breakthrough price point, we feel the STF-8300 is an ideal camera for those stepping up to a cooled low noise astronomical CCD camera from a DSLR, and a superb second camera for advanced users.
A mechanical shutter is included to facilitate dark frames. This is a necessity for anyone operating remotely, even if remotely means a few feet away from the telescope. Adding a shutter to a camera design increases the cost and size of the camera body but we feel these trade-offs are worth the added convenience and functionality. Since the early days of the original ST-7 camera, SBIG has been providing even-illumination shutters in our cameras; something not found in the majority of other cameras made for astrophotography. “Even-illumination” means that the shutter mechanism is designed to open and close in such a manner that it does not change the proportion of light falling on the sensor due to the shape or motion of the shutter itself. This is what one finds for instance with an iris type of shutter that opens-up starting at the center and closes over the center last. In the STF-8300 we use a simple and very robust shutter wheel with a fan-shaped aperture of the same design that we have employed in the ST series cameras for over 20 years. Another benefit of the rotating disk type of shutter is that it has only one moving part – the motor. These motors are extremely reliable and can operate for millions of exposures without failure. In over 20 years with thousands of cameras in the field taking millions of exposures, this shutter design has proven itself better than we can describe. Our design is proven and reliable.
In our tests the STF-8300 reaches a maximum of -40 degrees C below ambient in a warm room in less than 3 minutes after the cooling is turned on. Backing off the temperature setpoint a few degrees allows it to stabilize and regulate; at only 80% TE cooling power it maintains -36 degrees C below ambient. Due to this CCDs extraordinarily low dark current, the dark current becomes insignificant compared to the read noise even at temperatures if -15C to -20C. In one test two 15 minute dark frames taken at -10C were subtracted; the total noise remaining was ~15 electrons, which included both dark current noise and read noise. This is exceptionally low.
According to ON Semi, the dark current of the KAF-8300 is less than 200e- at 60 degrees C with a dark current doubling temperature of 5.8 degrees. This means that for approximately each 5.8 degrees C that one warms the CCD, the dark current is doubled, and for each 5.8 degrees C one cools the CCD the dark current is halved. Extrapolating this to -10C we can calculate the maximum dark current we can expect to find at that temperature. In fact, ON Semi’s specifications are usually very conservative when it comes to dark current, and this value can vary from chip to chip by 50% or more and still remain within the allowed specification. When we measured the 8300 CCDs we found many to be less than half of ON Semi’s allowed specification. This is also typical of what we have seen in other ON Semi CCDs. In our tests we measured the dark current in some samples to be as low as 0.01 electrons at -10C. Again, these are extremely low dark current values for a full frame CCD. Even at zero degrees C the typical dark current is well below 0.2 electrons! At -10C the average of our current production units is around 0.04 electrons making it the lowest dark current of any ON Semi sensor currently sold by SBIG.
In the past, one of the drawbacks in using sensors with small pixels and antiblooming structure has been the trade-off of some quantum efficiency. However, ON Semi’s microlens technology has largely made this trade-off a thing of the past. The KAF-8300 uses microlenses to focus more light on each pixel that would otherwise be lost due to the opaque antiblooming gate structure of the sensor. As a result, the KAF-8300 antiblooming protection can handle up to 1000X the full well saturation and at the same time this CCD has a quantum efficiency comparable to that of a full frame sensor without ABG. Moreover, the QE curve for this full frame sensor has a peak in the middle of the visual spectrum and good sensitivity well into the red and near IR. The QE at the important wavelength of H-alpha is nearly 50%.
Linearity is measure of how flat the CCD response is to light with increasing exposure times. For instance, an ideal CCD will record exactly twice the signal in a 20 second exposure as it does in a 10 second exposure from a constant light source. This is particularly important in photometric studies where the CCD is used as a tool for measuring light at various wavelengths to determine the temperature of stars. The sample we measured is about 10X better than ON Semi’s allowed specification, with a response that is within about 1% of linear (up to approximately 50,000 counts (binned 2×2).
Thanks to the fast USB 2.0 interface, the STF-8300 will download a high resolution, full frame image in less than 1 second, and is much faster still when focusing using subrames or binning. This makes it very easy to find and center objects, as well as to quickly achieve precise focus.
The 9-pin I2C port allows the use of our filter wheels with the same interface. The power and commands for the filter wheel all come from the camera through this port; there are no other cables or power supplies needed to operate the filter wheel. SBIG offers custom 5-position and 8-position filter wheels for the STF-8300 camera. Both are thin enough to accept a Nikon lens adapter for shooting filtered images through any Nikon 35mm lens. In both cases, automatic exposure sequences may be set in advance for hands off acquisition, or the user may control the filter wheel for each exposure if desired.
The guider port on the STF-8300 is the standard “ST-4 style” 6-pin modular telephone connector, which is connected to the user’s mount when using the camera as an autoguider. The drivers used in the STF-8300 design are opto-isolated, so that no external relay box is required with any mount.
Although small and light, this camera is built to last. The front plate, main body, and rear plate are each machined from solid aluminum block for maximum strength and then hard anodized over a special bead blasted finish for durability. Connector labels are laser etched and will never wear off.
The STF-8300 comes with its own universal AC power supply. This supply will operate from 100-240VAC and provides 12VDC at 3.75A to the camera. The STF-8300 also has a built-in voltage regulator and can therefore be powered directly from any unregulated 12V (10 – 14 volts) source such as a battery for operation in the field. The universal AC power supply is relatively small (1.2×2.3×5.2 inches, 30 x 58 x 132mm) and light weight (9 ounces, 254 grams). However in the event that the user needs to locate this supply more than 6 feet (~2m) from the camera, an 8 foot (~2.4 m) low loss extension cable with matching locking connectors is available as an option. SBIG also sells a 12VDC battery cable with alligator clips and a cigarette lighter adapter plug.
Due to the large size of the 8300 CCD, 1.25″ filters can vignette the corners at faster f/ratios. This may be acceptable at f/7 or longer focal lengths, but at shorter focal lengths the effect becomes greater. Also, the STF-8300 cameras are ideally suited for use with camera lenses, and we wanted to make a system available that would accommodate the back focus requirement for typical 35mm camera lenses with a filter wheel attached to the camera. For this reason SBIG designed 5-position and 8-position filter wheels specifically for the STF-8300 cameras that will hold 36mm filters for unvignetted images at any focal ratio, and at the same time accommodate 35mm camera lenses. These filter wheels plug directly into the I2C port on the STF-8300 camera and do not require separate power or control cables. Fully automatic function is controlled by the same software that controls the camera. Full sets of 36 mm LRGB and narrowband filters are available. For those users with slower optical systems, or when used with STF models with smaller imaging CCDs, 1.25 inch filters are acceptable and SBIG offers 1.25″ inserts for these 36mm filters.
The custom made low profile Off-Axis Guider is designed with a 7-element relay lens system that places the position of the guide camera to the side of the main camera and filter wheel. This allows a minimum impact on overall backfocus. The same system also acts as a 0.7X focal reducer that doubles the field of view of the guide camera compared to a typical off-axis system. The OAG adds only ¾ in. (19mm) backfocus to the camera and filter wheel. The guide camera can also be rotated in 90 degree increments about the OAG to orient the guide camera and cables in a convenient position. The OAG can be used on both mono can color cameras (with appropriate adapter) and is 100% backward compatible with all STF-8300 cameras. With the OAG-8300 and ST-I guider, the STF-8300 is a complete self-guiding system with the guider in front of the filters.
Note: If connecting this camera to an OAG-8300 without a filter wheel, you will need a Filter Wheel Spacer.