St 4.Command.Codes

SBIG SANTA BARBARA INSTRUMENT G ROUP Applications Note Interfacing to the ST-4 This note describes the operation of the ST-4 from a remote control standpoint. First of all some general points of interest about the camera are: • The ST-4 camera consists of a 192 (h) by 165 (v) element CCD, with readout electronics, an 8 bit A/D converter, and a microcomputer with image storage buffers for two images (light and dark frames). Although the CCD has more horizontal elements than vertical elements, the size of each pixel is such that the overall CCD array has an aspect ratio of 1:1. • The ST-4 can be run stand-alone as a star tracker, controlling the relays associated with the telescope drive to keep a star positioned at a fixed position in the CCD array. • The ST-4 can be controlled remotely over a three-wire serial interface (Transmit Data, Receive Data and Ground) at various baud rates. Upon power-up the camera starts at 9600 baud and can be switched remotely to higher or lower baud rates. In our software we commonly use 57.6K baud, but 19.2K, 9600 and 1200 are also supported for installations with longer runs between the ST-4 and the computer. To communicate with the camera you need 8 bits of data, even parity, and 1 stop bit. • Instructions are sent to the camera and data is received from the camera in packets, with each packet containing a checksum. Instructions sent to the ST-4 are acknowledged by the ST-4 in different ways depending on the type of instruction sent. • Each packet contains a single checksum byte at the end to allow detection of corrupted packets. The checksum byte is the sum of all the byte values in the packet, excluding the checksum itself. Since it is a single byte, the checksum wraps around to 0 at 256. For users computing the checksum you would want to use an unsigned char type or the least significant byte of an unsigned int. • The majority of the remote control of the camera is achieved by setting bits or writing bytes to memory inside the ST-4. The ST-4 then interprets those bits and bytes and acts accordingly. • Image data and image data alone sent from the ST-4 to the remote computer can be compressed if the remote computer enables data compression in the ST-4. The image data is sent as compressed data only if the compressed data is shorter than the uncompressed data would be. • The ST-4 has a special half-frame mode where an image in the lower half of the CCD (83 lines) can be transferred to the top half at a high speed (avoiding streaking due to integrating while reading out) and then those 83 lines are digitized as a half-frame image. With these items in mind, below is a description of the instructions you can send to the camera. There are only…