< Back to FAQ PageIntroduction
Some users report instant success and others have a big problem with STAR 2000. The difficulties are invariably caused by some kind of mechanical/electrical problem with the mount, as STAR is very simple in its operation and is unlikely to give erroneous commands to the drive. If S2K won't work with your mount, nothing else will either :-)
Most common are mechanical backlash and fast periodic errors. The lower-end LX and Celestron mounts suffer from this and cause most of the problems (bias weights can help). When you have a high grade mount, but still get problems, you can be certain that there is some kind of communication or parameter error in the system.
The difficulty with autoguiders is that they are critically dependent on the telescope set-up and drive quality, all of which is outside our control. If your drive cannot operate smoothly, then the guider will be unable to maintain control to the close limits required, so you must do everything possible to make certain that there are no rapid drive errors and no serious backlash in the system. Often these problems can be greatly reduced by bias weights or springs, arranged to keep the drive gears under some pressure to the East and South. A few hundred grams on the East fork tine and the weight of a dewcap are usually enough.
Connecting STAR2000
The STAR module contains a processor which is initialised by the appearance of a voltage on certain pins of the serial port. This will not happen reliably if the module is plugged into the PC after the computer has booted, so be sure to plug it into your computer's serial port before you turn the computer on.
LX-200 GPS Note: you can only connect the ST-4 output of the STAR2000 module to the autoguider port on the LX-200 GPS scope -- connecting the LX-200 output of the STAR2000 to the GPS RS-232 port will not work for guiding! (It will in the "classic" LX-200's though.)If you want to connect STAR2000 to a Vixen SkySensor 2000 unit, you can either make a cable using the information from the pinout diagram below, or you can buy an adaptor from us.
Checking Your Telescope Drive's Performance
It is very informative to set the polar axis a degree or so off the pole and take a few minutes unguided CCD exposure of a star field. You will get a set of zig-zag lines as the RA errors affect the N-S star drift and you will be able to tell if the errors are fast, slow, or both, from the gradients of the direction changes. It will also provide a check on whether you have improved the performance of the mount when adjusting parameters and bias weights. There is little that can be done with a mount which shows significant errors within a few seconds, other than trying to train them out with PEC or replacing the drive gears.
LX200 vs. ST-4 Output
When using LX200 guiding mode, the module does not modify the data sent by the computer to the mount. As others have said, in this mode it behaves as a 'dongle' and so the guiding is entirely dependant on the software and mount. The lamps do not behave in a logical way in this mode, so ignore the LEDs as they are not involved with the guiding at all. The red and amber LEDs tend to operate at random when using the LX200's RS232 input because the data required by the LX is not compatible with the LED driving software (not important for guiding). When using the LX200's CCD input, the LEDs operate normally and usually show some bias in the direction of any drift in the mount. This might be such that the amber LED (West) and red LED (South) tend to stay on longer than the blue or green ones, so nothing significant can be deduced from this, other than that the mount may need some adjustment.
In ST-4 mode, the STAR2000 module converts commands from the port into switched outputs and these are applied to the opto-isolator output devices. The indicator LEDs are in series with the isolator inputs, so if your LEDs light up correctly when keying the software buttons, the outputs are switching in the proper manner. Not only does using the 'ST-4' output of the S2K box give you a visual indication of the status of the box outputs (via the LEDs), it has the additional advantage of not relying on the software using the right control codes for your mount.
The ST-4 mode relies on the mount responding to the isolator switching and this relies on the small output current sink being enough to switch the mount electronics. You can test this by using the software in 'manual' mode and confirming that the motors operate when buttons are pressed on the software control panel (set the mount for 'fast slewing' rate to make it obvious). If the manual control is not correct, despite the lamps lighting properly, it may be that the sink current of the S2K box is insufficient for the mount electronics. This can be completely solved by adding a relay box between the STAR interface and the mount.
Troubleshooting STAR2000
If you think that the scope should guide quite well, but cannot get STAR 2000 to work correctly, then it is essential to follow a logical sequence of tests.
- Firstly, check that you can definitely control the mount from the software control panel. If you set up the camera on a star in focus mode, then it should be possible to move the star around by pressing the 'manual' control buttons in the STAR software. Check that all axes work correctly and do not show significant backlash when reversing. If backlash shows, try the bias weight trick above. You can also make a rough determination of the 'pixels per second' rate by hand driving for a known period on a full image and measuring the trail length in pixels with the photometry or merge functions. If this test shows that you do not have proper drive control from the box, then (using the 'Control box' input setting for the software) check that the lamps work properly and, if so, try using a new RJ11 cable, or test its continuity with a meter. If the lamps do not work, then it could be that the STAR unit is not initialising correctly, a problem which has shown up with some fast computers.
If the control looks good, make sure that the long axis of the camera chip is reasonably parallel to the RA drift direction (E-W). Start with a short focal length (say with an F3.3 compressor) and try guiding on an isolated star (not too bright) with high values for the pixels per second settings. This makes the correction rate slow and is less likely to cause 'hunting' of the drive, but may result in slow 'wandering' of the image. Make sure that the guide rate setting for the 'scope is low (2x sidereal max.) and then set the system guiding.
Fast rates will lead to overshoot and the kind of comment 'It's worse with STAR 2000 than it was without' . Overshoot is also caused by too low a pixels per second setting in the guiding parameters -- start at 20 pixels per second and reduce until the corrections are rapid but do not overshoot. Always start with very gentle correction rates and work up, that way you won't get overshoot and should just get a gradual improvement in the guiding until it is near perfect (remember, high pixels per second values give gentle guiding corrections).Closely watch the guider frame and observe the movement of the star and the correction errors. If the errors are very small (<1 pixel) then not much will happen between frames, but larger errors should be followed by indicator flashes from the STAR interface and a steady reduction of the error figure. If the errors grow larger and larger in one direction, until the star is lost, then that axis direction setting needs to be reversed in the defaults menu. Once the star remains within the guider box and appears to be reasonably stable, then try reducing the 'pixels per second' rate on the RA axis until errors are corrected in only one or two passes. Making the rate too low will cause oscillation of the control loop and the star will be thrown rapidly from side to side until lost from the box. Because DEC drifts are generally slow and constant in direction, it is often not worthwhile reducing the pixels per second rate for DEC, but you can try to optimise this as well, if you want.
- Guiding at higher F numbers is more challenging, as the errors are more severe and more difficult to correct. You will need to increase the pixels per second number in proportion to the increase in focal length and use shorter guider exposures to increase the error correction update rate.
Some sources of trouble to watch for are:
- Camera not aligned E-W,
- Trying to guide with a very long effective focal length (unstable),
- Mirror flop in SCTs,
- Camera cables dragging on ground (strap to mount and bring out from near polar axis).
- I don't recommend using the 'training' option to set the pixel and backlash rates, as the results can be erratic. Just work down from high pixels per second rates until the best results are achieved.
- Bright guide stars give problems with the centroid calculations -- make sure that the star image in the guide box is unsaturated.
After these checks have been done, you should have achieved stable guiding, but there will always be some mounts which are just too erratic without mechanical servicing.
Dark Frames
Proper dark frames are important with STAR2000 to subtract out the amplifier glow from the many guiding readouts during the exposure. The important points here are to make the dark exposure the same as the light and to continue running STAR2000 during the dark frame (just uncheck the X and Y guide boxes so the scope doesn't move!).
Dark Frames & AstroArt
AstroArt's implementation of the dark frame capture routine does not allow for the drive correction time during dark frame capture and this results in too much amplifier glow in the dark frame. Most users are correcting this problem by setting the guide exposure time about 10% longer than for the 'light' frame when taking the dark frame image. This reduces the number of corrections by about the right amount. The USB interface certainly helps here, as it cuts down the time that the amplifier remains 'ON' during download.
Changes to STAR2000 Requirements (2004)
The STAR2000 module is no longer required to run an LX-200 via the RS-232 port. It is still required for interfacing via the ST-4 protocol since the STAR2000 module is what generates the ST-4 control signals. This change should be reflected in new versions of AstroArt3, MaxIm DL/CCD, and all Starlight Xpress software released in 2004.
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