STT-8300M
Micron-Precision Filter Wheel / Guider
The filter wheel design for the STT series sets it apart. The carousel holds eight 36mm filters and inserts are available for 1.25″ filters. Two new unique features make this an imager’s delight: First, the design incorporates a self-guiding CCD in the filter wheel cover so that light from the guide star is captured before passing through the filters. An optional filter wheel cover is available for wide angle imaging with Nikon or Canon 35mm camera lenses, or for anyone who does not need or want the built-in self-guiding. Full sets of 36mm LRGB and Narrowband filters are available for unvignetted imaging at any f/ratio. The second unique feature of the STT filter wheel is a positive centering mechanism that precisely re-positions and firmly holds filters in exactly the same position over the CCD every time, regardless of a loss of power or intervening movement of the filter wheel.
In our tests of the new design, using an STT-8300M, the re-positioning of the filters was accurate to better than 5.4 microns after several complete rotations of the filter carousel. This degree of precision means that flat fields taken after the filter has moved and returned are accurate to a single pixel. An example of the improvement in flat fields with this kind of precision is demonstrated in the comparison images below.
The image at left, above, is a large opaque spot on a filter in STT filter wheel. The filter carousel was rotated several times before the filter was moved back into the optical path and a flat field was taken. The image at right shows the resulting flat field image.
Even-Illumination Shutter
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. Since the early days of the original ST-7 camera, SBIG has been providing even-illumination shutter 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 STT-8300M we use a simple and very robust shutter wheel. The STT shutter sweeps over the CCD without leaving any area of the sensor exposed for a different period of time than any other area. 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 15 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 accurate, proven and reliable. Even-illumination shutters are also referred to as photometric shutters because they produce flat frames of photometric quality. This is extremely important when taking twilight flats or any flat field image using short exposure times. The effect is demonstrated in the twilight flat field images shown below:
The left image shows a twilight flat field image taken with a camera using an iris type shutter. The right image is a flat field image taken under the same conditions using our even-illumination type shutter.
Efficient 2-Stage TE cooling
The STT uses two-stage TE cooling, twin variable-speed fans and a highly efficient pin fin array heat sink that has proven itself in the STX design. Pin fin heat sinks are more expensive than typical parallel fin type heat sinks, but offer more efficient heat dissipation in a given space. In our tests of prototype models, the cameras reached an average delta greater than -50C in five minutes and an average maximum delta of -57C in less than 10 minutes using air cooling only. And, while is should not be necessary to use water cooling in most circumstances, if it should be desired, the STT series cameras include water cooling capability as standard equipment. It is not necessary to buy an expensive add-on or separate back. Simply supply water to the adjustable fixtures on the side of the camera body to obtain further cooling or to run the TE cooler without the fans.
Exceptionally Low Dark Current
The KAF-8300 CCD has very low dark current, even at room temperature. Cooling the CCD reduces the dark current by 50% for every drop of 5.8 degrees C of cooling that is applied. Kodak specifies a dark current of less than 200 electrons per pixel per second at +60 degrees C. Extrapolating we calculate that at -15C the dark current is approximately 0.02e-. Our tests of the 8300 CCD in production cameras confirm this extraordinarily low dark current at typical operating temperatures.
High Quantum Efficiency
Another desirable characteristic of the KAF-8300 CCD is its relatively high Quantum Efficiency (QE). Microlens technology is used to focus more light on the sensitive area of each pixel, increasing the effective QE of the 8300. Many nebulae emit a great portion of their light in the red portion of the spectrum, particularly at 656nm, the wavelength of H-alpha light. Other objects such as stars and galaxies emit a great deal of energy in the near IR. According to the KAF-8300 spectral characteristics, this CCD’s sensitivity is spread well across the visible portion of the spectrum and into the near IR with a peak of nearly 60% at 550nm and still 50% at 656nm. It is as sensitive at 850nm as it is at 350nm. This is quite a broad range and explains the CCDs popularity (and success) in astro-imaging.
Standard Autoguider Output Port
The guider port on the STT-8300M is the same “ST-4 style” as on our other cameras, using a modular telephone type 6-pin jack to connect to the user’s mount when using the camera as an autoguider. The internal drivers used in the STT-8300M design are opto-isolated, so that no external relay box is required with any mount.
12VDC Operation
The STT Series cameras come with its own universal AC power supply. This supply will operate from 100-240VAC and provides 12VDC at 3.5A to the camera. The STT-8300M also has a built-in voltage regulator and can be powered directly from any unregulated 12V (10 – 14 volts) source such as a battery for operation in the field.
Optional AO-8T Adaptive Optics
All STT models can operate with the AO-8T Adaptive Optics accessory. The AO-8T uses a tip-tilt window to quickly and precisely adjust the image position in response to autoguider feedback. With Adaptive Optics there is no backlash, stiction, or lag, and the movements are precise and accurate. This allows the AO-8T to not only remove the effects of periodic error, drift, and wind gusts, but also reduce the effects of atmospheric seeing.
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