The ST-4 Interface
ST-4 is an interface you will come across using astronomy equipment with some form of computer control. It might be as an ST-4 socket on a mount, a socket on the back of an astronomy camera, or an interface on some other form of guiding device such as an AstroHutech Hinode Solar Guider. The ST-4 interface dates back to the 1980s and stands for Star Tracker version 4.
ST-4 cables are often supplied with astronomy gear and, if you mention it to fellow astrophotographers, they might say they don’t use it. But what is the ST-4 interface and should you use it?
Astrophotography requires the camera to be able to keep pointing to the same spot in the sky while the shutter is open, so the camera can follow the stars as the Earth rotates and the camera exposures of the sky are not blurred.
Control signals originating from guiding software or devices can be used to nudge the mount back on track so that the camera is kept pointing to the same spot in the sky. An ST-4 interface is one way to provide the guiding signals to the mount.
Various methods can be used to improve the tracking of the sky to eliminate the blurring of images taken by a camera. The following typical methods can be used individually or in combination:
• A quality mount;
• A good polar alignment of an equatorial mount;
• Send correction data to the mount to counteract mechanical variations in gears over time (PEC);
• Create complex computer sky models for the mount that alter the movement of the mount depending on the measured atmosphere and mechanical variations;
• Constantly providing feedback to the mount with correction signals to keep the mount locked onto the stars (guiding).
Guiding is a method of constantly monitoring the position of a star, analysing the change in the star position on an image and then constantly providing correction feedback to the equatorial mount, nudging the RA (Right Ascension) speed or Dec (Declination) angle, to compensate for any drift away from the expected path of the monitored star.
The guiding adjustment signals sent to the mount can be in the form of computer software commands (pulses) via the mount’s computer command interface, or by directional movement signals on specific wires for each required direction via a Guide Port on the mount. Guiding was originally achieved manually by using switches controlled by a person but has evolved to automatic control from a software application and the switches are now computer-controlled switches using an ST-4 interface.
ST-4
The ST-4 port, an RJ12 (6P6C) socket, found in most computer controllable mounts, is often referred to as a Guide Port or ST-4 port. The same ST-4 Guide Port is found on some astro cameras and some stand-alone USB to ST-4 devices. While the ST-4 guiding interface dates back to the 1980s computerised mounts, it didn’t originally use a 6-wire cable with an RJ12 plug/socket on both ends of the cable.
Today, the ST-4 interface is more of a de facto name for a particular 6-wire cable with RJ12 connectors at each end and not a true standard interface. It is also not a standard followed by all mount vendors. The manufacturer’s implementation of this guide port or ST-4 port can vary but they still use the same ST-4 name.
ST-4 is essentially an interface that closes a circuit between a common wire and one of four other wires, to move a mount’s position or change the speed of movement of the mount’s motor.
Modern ST-4 guiding control is done via USB to ST-4 circuitry. This circuitry is often included in planetary cameras (also called guiding cameras) or stand-alone USB to ST-4 converter units.
ST-4 Connector.
The current ST-4 interface uses an RJ12 connector or socket.
RJ stands for Registered Jack, which was defined by Bell Systems in the USA in the 1970s as a telecommunications connector, and later codified by the US authorities.
The 12 in RJ12 refers to RJ type number 12. Some documentation also incorrectly labels the ST-4 connector as RJ11. The RJ11 has the same physical connector but with only 2 pins. The RJ11 connector/plug only has 2 pins and 2 wires, whereas the RJ12 has 6 pins and 6 wires.
The 6P6C sometimes used with an RJ12 cable basically means a 6-wire cable, i.e., 6P or 6 positions on the plug/socket & 6C or 6 conductors. Note the RJ11 is 6P2C.
History of the ST-4 name
The Santa Barbara Astronomy Group (SBAG, https://www.sbscientific.com/about-us/history/) in the USA, in the mid-1980s, needed a way to guide their backyard telescopes. As such, between 1985 and 1989, they designed a Star Tracker (ST). They tested ST-1, ST-2, ST-3 & ST-4 (i.e. Star Tracker versions 1 to 4) with version 4 being ready for other astronomers. To sell the ST-4 to astronomers, they set up a business called the Santa Barbara Instrument Group (SBIG).
In 1990, SBIG built version 4 of the Star Tracker (ST-4).
Note 1: SBIG was later acquired by Diffraction Limited in 2014 and continued selling gear under the SBIG name.
Note 2: After the transition of products to Diffraction Limited, some ex-SBIG developers formed a group called Santa Barbara Scientific (SBS) and started developing products for the Chinese company QHYCCD.
The original SBIG Star Tracker was designed to connect to a mount by simulating activating the movement buttons on the old manually-controlled mounts. The ST-4 unit had a 15-pin “relay port” to control the four different direction buttons for the mount. In this way, it was able to replace the human guider pushing buttons to correct the pointing and automatically nudge the mount in a direction to correct the alignment to the stars.
The original ST-4 used a 15-pin ‘D’ connector exposing relay contacts that used three wires for each movement direction (i.e. common, normally open & normally closed relay contacts).
Some telescopes available when the Star Tracker was released were the Celestron Ultima and the Meade LX200. These mounts had a 6-wire RJ12 (6P6C) socket for interfacing to the controller. A few years later, in 1992, the Losmandy G11 also adopted the same RJ12 socket and wiring in parallel with its hand control.
The six wires of the current ST-4 cable are equivalent to combining the four common wires from the 15-pin D connector of the original SBIG Star Tracker ST-4 to a single common wire as well as using each of the four directions normally open relay contact wire on the 15-pin D connector. A cable was available to connect the 15-pin D connector to the 6-pin RJ12 (i.e., common wire plus four directional wires plus a spare pin equalled the six wires used in the ‘ST-4’ cable).
As this was the first use of autoguiding by Star Tracker at the time, the 15-pin D to RJ12, 6-wire cable was sometimes referred to as the ST-4 cable which plugged into the guide or ST-4 port. It has kept its name over the years, even though later Star Trackers and then Star Guiders moved from 15 to 9 pins and eventually the 6-pin RJ12 socket. The ST-4 cable has become a de facto standard name for a wired guiding connection.
ST-4 wiring.
The original Star Tracker used a 15-pin D connector Guide Port (see Figure 2, p15, for the pin wiring from the SBIG ST-4 manual).
The mounts used an RJ12 socket for the Guide port so the cable from the Star Tracker connector to the Guide port on the mount became known at the time as the ST-4 cable. (The guide cable today is still called the ST-4 guide cable even though it uses an RJ12 at both ends).
Table 1: Pin Wiring for ST-4 RJ12 to ST-4 DB15 port (Northern Hemisphere directions referenced).
The original wiring details for the ST-4 and TIC (telescope interface cable) are provided in Figures 2 and 3
| RJ12 (6P6C) | DB15 | |||||
| No Connection | 1 | |||||
| Common | 2 | 5,8,11,14 | ||||
| RA- (X-) | 3 | 4 | ||||
| Dec- (Y-) | 4 | 7 | ||||
| Dec+ (Y+) | 5 | 13 | ||||
| RA+ (X+) | 6 | 10 | ||||
Note: See Figures 2 & 3 for the details on original ST-4 pin wiring.
Table 2 ST-4 pin allocation
| RJ12 pin number -> | 1 | 2 | 3 | 4 | 5 | 6 |
| Most manufacturers | NC | Common | RA+ | Dec+ | Dec- | RA- |
| Meade, Atik, TIC (Fig 3) | NC | Common | RA- | Dec- | Dec+ | RA+ |
| SBIG (STV), Takahashi, Paramount. | RA+ | Dec+ | Dec- | RA- | Common | NC |
Notes:
- ‘Most manufacturers’ = QHY, Sky-Watcher, Orion, Atlas, Sirus, Celestron, iOptron, Astro Physics, Losmandy, ZWO, Vixen.
- NC = No Connection
- RA & Dec direction labelling is Northern Hemisphere referenced.
- Early Celestron RJ12 sockets were (incorrectly) numbered in reverse. As such, pin numbering from manuals may not align with the above table.
- Some manufacturers incorrectly label the plug as RJ11 rather than the correct RJ12
ST-4 movement direction labelling.
The labels listed for each pin in many ST-4 specifications are Northern Hemisphere based, so are often quoted from the perspective of an equatorial mount, pointing to the North Celestial Pole.
Mount directions in the Northern Hemisphere
RA+ = Left (East or X+)
RA- = Right (West or X-)
Dec+ = Up (North or Y+)
Dec- = Down (South or Y-)
Mount directions in the Southern Hemisphere.
RA+ = Right (West)
RA- = Left (East)
Dec+ = Up (South)
Dec- = Down (North)
It doesn’t matter if a Dec- on a Guide camera is wired to a Dec+ on a mount (the same goes for RA).
As long as the Dec on the Guide Camera goes to the Dec wires on the mount and the RA on the Guide Camera goes to the RA wires on the mount (i.e., you need to have the correct RJ12 to RJ12 cable - straight through or reversed).
Direction summary (Most manufacturers):
Pin 2 is the common pin.
Pins 3 and 6 affect RA movement in each direction.
Pins 4 and 5 affect Dec movement in each direction.
Direction summary (SBIG (STV), Takahashi, Paramount):
Pin 5 is the common pin.
Pins 1 and 4 affect RA movement in each direction.
Pins 2 and 3 affect Dec movement in each direction.
Due to other issues in guide configuration, such as On-Axis & Off-Axis configurations, or Southern versus Northern Hemisphere, guiding applications cannot be sure of the directions of each wire. As such, the guiding application should be configurable or work it out during guiding calibration routines.
An application such as PHD2, during calibration, will attempt to move the mount in each direction and monitor the movement results of a star in the guide camera. The PHD2 configuration is then adjusted based on these tests.
If you needed to correct the Dec and RA directions, then the following cable would be needed:
1->1 NC
2->2 Common
3->6 RA-
4->5 Dec-
5->4 Dec+
6->3 RA+
Should you use ST-4?
Three options exist for the connection of the guide software such as PHD2 to the mount.
(1) Guider to mount using ST-4 cable (guiding).
(2) Guider to mount using the mount’s computer interface (mount commands) and ST-4 cable (guiding).
(3) Guider to mount using the mount’s computer interface (mount commands & pulse guiding).
If the mount is capable of pulse guiding, then PHD2 documentation recommends option (3) as the superior guiding method compared to options (1) or (2) which use ST-4 only or ST-4 and the mount’s command interface.
Using the mount’s command interface (ASCOM or INDI), a guiding application such as PHD2 can also obtain the pointing position of the mount, especially the declination and side-of-pier, which can be used as factors in guider calibration and provide greatly improved ease-of-use. Pulse guiding commands can also use the same interface without the need for the extra ST-4 cable and this is basically why option (3) is superior.
The main advantages of just pulse guiding, i.e., option (3) (see reference PHD2 documentation
https://openphdguiding.org/man/Basic_use.htm#ASCOM_Benefits)/
(1) A drastic reduction in the number of re-calibrations you’ll need to perform. Changing targets will not require another calibration because PHD2 can know where the scope is pointing and automatically make adjustments to the guider calibration. Most users get a good calibration and then re-use it until they make hardware changes of some kind.
(2) Automatic adjustment for meridian flips - no need to remember to manually flip the calibration data.
(3) Automatic adjustment of RA calibration to handle targets in different parts of the sky (declination compensation).
(4) Elimination of the ST-4 guide cable as a point of failure - this is a surprisingly common problem because the cables can be damaged or confused with similar-looking cables (e.g., telephone cables).
(5) Elimination of a moving cable that can snag, drag, or bind as the scope is moved around.
(6) Improved ability for PHD2 to sanity-check calibration results and warn of possible problems before you waste hours of imaging time.
(7) Better diagnostic and troubleshooting information, which is particularly helpful if you need to ask for assistance.
(8) Availability of scope-slewing options during drift alignment which can further speed the process of polar alignment.
If you have an older mount that does not have firmware-level support for pulse guiding, then the ST-4 guiding interface is your only option.
Appendix
Note to Figure 3: The original TIC cable allocation was followed by most mount vendors but reverse numbering was used by SBIG and Takahashi. These mounts need the following crossover cable wiring:
1->6
2->5
3->4
4->3
5->2
6->1