New Guiding Techniques for the STX
It is well known that the advantage of easier guiding through a separate guide scopes is often limited by differential deflection of the guide scope relative to the main optical axis due to mechanical flexure or shifting of the mirror in the main OTA. This difficulty is addressed with two new guiding techniques developed by SBIG and implemented in the STX series cameras: Simultaneous Guiding and Differential Guiding.
Built-in Guiding CCD with Adjustable Focus: Simple self-guiding and control of our Adaptive Optics accessory using just the internal tracking CCD will be a standard feature. The guiding CCD in the STX cameras will be a new KAI-340 CCD with 640 x 480 pixels at 7.4u. As the imaging CCDs get larger, the guiding CCD gets pushed farther away from the center of the optical axis. Depending on the nature of the optical system, this can cause the image on the guiding CCD to be slightly out of focus when the image on the main CCD is in focus. To address this, the new STX cameras will have a user accessible adjustment for changing the focal point of the on-board guiding CCD.
Optional Remote Guide Head: A Remote Guide Head will be optional for the STX cameras. Self-guiding through a separate guide scope or off-axis guider, and control of the Adaptive Optics accessory, can be done using the Remote Guide Head instead of or in addition to the on-board guiding CCD. The same KAI-340 CCD will be used in the new Remote Guide Head made for the STX series cameras.
Simultaneous Guiding: The external guide head will not only continue to guide while the imaging CCD is downloading an image, but it can be operated simultaneously with the on-board guiding CCD. This allows a unique type of guiding through a separate guide scope that will correct for differential deflection by using the external guider to make fast corrections while the on-board guider makes slow corrections. Differential deflection tends to occur over a longer period of time than drive errors, typically minutes vs. seconds. A long exposure taken with the built-in guider is capable of reaching dim stars without searching, even through narrowband filters, and a long exposure with slow corrections with this on-board guider will correct for the slow differential deflection typically experienced with a separate guide scope. In the mean time, the Remote Guide Head can simultaneously make fast corrections using the brighter stars easily found with a short refractor mounted piggy back to the main OTA. This technique, developed independently, is similar to the technique employed successfully by Italian amateur Filippo Ciferri using two separate cameras for which he deserves full credit. With the STX, however, a single camera and remote head will perform both the fast and slow guiding, as well as the imaging, all at the same time, using a single instance of the control software and only one connection to a computer.
Differential Guiding: SBIG has a patent pending on a new guiding technique using an artificial guide star. Although artificial stars are used in a variety of techniques on professional telescopes, the SBIG technique is somewhat different and easy to implement on amateur scopes. An artificial star is created near the focal plane of the imaging CCD and an image of this star is retro-reflected into a separate guide scope.
STX Series Product Information Sheet