The SG-4 is the latest in SBIG’s series of ground-breaking autoguiders for imaging. Indeed, SBIG’s very first product was the renowned ST-4 autoguider, introduced nearly 20 years ago. Many of the venerable stand-alone ST-4s are still being used with great success by film astrophotographers and for digital imaging with DSLRs and CCD cameras.
Now, after two decades of experience making autoguiders and unique self-guiding two-sensor cameras, SBIG has developed the single-button/easy-to-use SG-4 for those who just don't want to use a computer to guide their DSLR or non-self-guiding CCD camera during a long exposure.
The SG-4 connects to a small photoguide refractor (not supplied) that is piggy-backed onto your main scope. While it is possible to use the SG-4 in an off-axis guider, it is much easier to use it with a guide scope. Simply attach the supplied tracking cable between the SG-4 and your telescope drive's autoguider input, plug the SG-4 into a 12VDC power supply (an AC adapter is also supplied), and it’s ready to go. The SG-4 uses opto-isolated mechanical relays with a RJ-11 type 6P6C jack output, so it can be used with any equatorial mount (even the Losmandy G-11), with no external relay box needed to isolate the mount from the guider.
An RS232 port allows an initial setup for your particular scope and drive, which does require the use of a computer, after which SG-4 operation is completely independent and computer-free. SBIG chose an RS-232 interface for a variety of reasons. Since the SG-4 was not intended to be used with a computer at all times, a fast interface was not that important for normal operations like viewing images and focusing. In addition, computer users often have problems with a high speed USB connection when trying to extend the cable beyond the standard 15 feet. With the RS-232 link the SG-4 can communicate over 150 feet cable runs with no special equipment. An optional Bluetooth transmitter may be connected to the RS232 port for wireless setup and download of the image if desired.
The SG-4 is completely stand-alone, with no computer or other external device required for operation after the initial setup. It uses a sensitive KAI-0340S CCD detector that has a 640 x 480 array of 7.4 micron pixels, with low read noise and low dark current. Minimum exposure time for guiding is 0.1 second, while the maximum exposure time is 10 seconds. The SG-4 is a true one-button device. Once it’s set up, push one button once to start it guiding and push the button a second time to turn it off when you’re through. That’s all it takes to autoguide.
To use the SG-4, you will need a user-supplied photoguide refractor at least 100mm in focal length. A 200 to 400mm focal length is optimal, with as fast an f/number as you can find. SBIG estimates that 95% of the time the SG-4 will find a guide star bright enough for a one second guide exposure if your guide scope focal ratio is f/7 or faster. A small, quality 60mm to 80 mm aperture refractor is recommended, and an Astro-Tech ED doublet in that size range would be an ideal choice. With a 60mm guide scope, the SG-4 can guide on stars as faint as 10th magnitude. The SG-4 mounts on your guide scope using either the 1.25” nosepiece provided with the SG-4, or the T- threads on the face of the SG-4 that are exposed when you unscrew its 1.25” nosepiece. A 2” nosepiece is available as an option for maximum connection rigidity with a guide scope having a 2” focuser.
There is a mechanical shutter that makes taking dark frames automatic, with no need to cover the camera or the scope by hand to capture a dark frame. Dark frames are desirable for autoguiding to remove any hot pixels that might otherwise be mistaken for stars by the guider.
There’s an RS232 port for connection to a computer for initial setup and image download, should you want to use the SG-4 for very wide field imaging through the short focal length guide scope. Even though the SG-4 is not cooled, the low read noise and low dark current of its KAI-0340S detector will let you take monochrome images with exposure times from 0.05 to 600 seconds. The full well capacity of the SG-4’s detector is 50,000e- with 16 bit A/D conversion, so the image quality will be quite good. Optional camera lens adapters are also available for imaging with C-mount lenses and Nikon SLR camera lenses.
A (user supplied) Bluetooth transmitter can also be plugged into the RS232 port for wireless setup and image download. The SG-4 will also support SBIG's new differential guiding system (patent pending) by providing power to illuminate an artificial star.
The SG-4 needs a 12VDC power supply (nominally 9-18V) with a 200 milliamp output. One of the many available 12VDC rechargeable batteries available to power telescopes and accessories in the field would be very suitable. The SG-4 comes with an unregulated universal AC power supply that converts the 100-240V 50/60Hz AC power from any household AC outlet in your back yard or observatory dome to the required 12VDC.
The SG-4 measures 4.4” x 3.8” x 2.8” in size and weighs 20 oz. (580g.) It can be used over an operating temperature range of 14 to 114 degrees Fahrenheit (-10 to +45 degrees C).