First of all make sure that the autoguider camera works reliably as a basic imager, by running it in continuous mode.
Telescope Not Moving
To verify whether your telescope is being commanded to move during guiding, click the Move button on the Guide tab. Use the Manual Control to move the motors in several second bursts. You should be able to see motion of the image. If you are doing this indoors, on most telescope mounts you can hear a difference in the motor noise levels when you click a button (you may need to hold your ear up to the motor).
Make sure you have set Control Via properly (Guide tab Settings). This determines how the autoguider commands are sent to the mount. If it is set incorrectly, the commands will not reach the mount. Please refer to Guider Settings for more information.
An extremely common failure point is the cable that sends autoguider pulses to the mount. These often run from the guide camera directly to the mount controller. These cables usually use telephone-style jacks, which are not terribly rugged. If the mount does not move in all four directions, a bad cable is probably the cause. These cables look very much like telephone cables, but they use all six pins whereas most telephone cables have only four pins. Also the cables can be assembled with the connectors in two possible orientations, and if the connectors are attached incorrectly the wires will be reversed. If reversed this can cause various symptoms including no motion, continuous motion on one axis, and in some cases hardware damage. Please note that some older drive controllers require an isolation relay box on the guide port; be sure to check with the manufacturer before making the connection.
Star Not Found
Always take an Expose-mode image first, before you select Tracking. This allows the software to auto-select the guide star it will use for tracking. At this time it also measures the brightness of the star, and this is used to detect guide star fades. If you wish to manually select the guide star, click on the star once, do not drag a box. Also remember to start tracking reasonably promptly after the Expose-mode image, to ensure the guide star does not drift away in the meantime.
You will be able to guide on a fainter star than will work for calibration. The calibration routine is deliberately more discriminating, to avoid it chasing after the wrong star or image artifacts.
Please note that some autoguider setups work better with 2x2 binning, while others work better with 1x1 binning. Longer focal length guide telescopes may require 2x2 binning, especially if the stars look like big faint blobs.
The guider calibration algorithm attempts to locate "real" stars, in order to avoid chasing hot pixels and other artifacts. Check the star with the Graph Window in Star Profile mode. If the star has a big, flat top, then it is excessively saturated. If the star has a large, flat top it won't be recognized as a star, even though you can see it in the window. The solution is very simple: use a shorter exposure or a fainter star.
Some low-cost autoguider-only cameras and all webcams have 8-bit A/D converters. It is extremely easy to oversaturate stars with these cameras. Again, look for flat tops on the image.
Star recognition during tracking is deliberately more forgiving that during calibration, since the track box is usually small enough to isolate a single star. It is quite common to be able to track on a star that you cannot calibrate on.
Calibration Does Not Produce "L" Shape
Please note that you do not have to recalibrate all the time; in fact, this is discouraged. The guider settings are remembered, and as long as you turn on Auto Pier Flip (for German Equatorial Mounts only) and Auto Declination Compensation, then recalibration is not necessary. In fact, if you don't change your equipment configuration between sessions, you can reuse the same calibration for months or even years. (Some mounts behave a little differently during pier flip; if you find the tracking doesn't work after a pier flip then check the Advanced Settings.)
The autoguider calibration procedure performs four moves, and traces the movement of the star for each move. This is traced on the screen with a red "L" shape, if it is working properly. The lengths of the sides of the "L" can be different, but they should be at right angles.
The star needs to move at least five pixels during each calibration step. If it does not move at all, see Telescope Not Moving, above. If it is not moving far enough, then either increase the Cal. Time setting(s) or increase the mount's guide rate.
If you have a wide field autoguider, so there are many stars in the field, MaxIm DL will automatically use auto star matching instead of the single star detection method. This matches all of the stars in the field, which produces a more robust result.
If your autoguider has a narrow field of view, MaxIm DL will look at a single star in the frame. If there are a small number of stars this can potentially confuse the calibration, particularly if stars are moving in and out of the frame during the motions. You may wish to find an isolated, reasonably bright star (say 6th magnitude) and center it in the chip before starting calibration.
Autoguider Oscillates
If the guide star seems to bounce back and forth rapidly, your calibration is likely wrong. Try increasing the Cal. Time setting so that the calibration moves the star a larger distance on the guide sensor. If it only moves a small distance, mechanical problems like backlash may make the measurements inaccurate.
Also try reducing the Aggressiveness setting in the Guide tab Settings dialog box. Generally speaking, correcting 100% of the error (Aggressiveness = 10) is unwise because in theory it is on the critical edge of instability. A lower setting will make the autoguider corrections more stable and prevent overcorrection. In most cases it is recommended that you use a setting no higher than 8. For mounts with very smooth tracking you may wish to back the aggressiveness down to 3; this has the advantage of smoothing the guider corrections, effectively averaging successive guider measurements.
You can try experimenting with the backlash settings; however, it may be better to use the Anti-Stiction feature rather than play with backlash settings. The backlash features works by sending an extra-long pulse out to the mount whenever the declination direction reverses. The amount of time of this extra pulse is entered in seconds. Backlash compensation is usually only required on the Declination axis since the sidereal tracking normally runs out the backlash in RA. Be careful, however, not to set the backlash time too high – that will make the situation much worse. It is far better to set it too low than too high. Also make absolutely sure that any anti-backlash feature in the telescope mount controller itself is turned off. If the mount performs its own anti-backlash functions the star will still be moving when the guider takes the next exposure, and severe instability can result.
Some mounts, such as those equipped with Losmandy Digital Controllers, do not like RA and Dec corrections occurring right after one another. You can set an extra time delay between these corrections using the Guider Settings Advanced Tab. Other ”finesse” features are available, such as minimum move and maximum move parameters. Be careful when playing with these settings since you can render the autoguider non-functional with incorrect settings!
Stiction can cause autoguider oscillation, so try using the anti-stiction feature. Please see the section Backlash and Stiction for an explanation of stiction.
Other Things to Check
The Tracking Error Graph can be extremely useful in diagnosing autoguider problems, as it helps you see what is happening over time.
For some mounts, you must set the guide rate manually. The maximum usable rate is 1X sidereal. In most cases the 0.5X rate will work well. If your mount drifts very slowly and you want extremely high precision, use the 0.1X rate.
Use a calibration time that causes the star to move ~20 pixels or more across the sensor array. This is to ensure an accurate measurement. You may have to increase this number if your mount has significant backlash. Watch the motion of the star during calibration to ensure that it travels in an ”L” shape and returns close to the starting position after the cycle is complete.
Make sure that any backlash compensation in the mount is turned off. If the mount performs backlash compensation, it will keep moving the telescope while the next image is acquired. This will induce a phase lag that can cause the feedback loop to go unstable.
If you are not sure how to set the Guide tab’s Backlash settings, then set them to zero. If your mount takes one second to reverse directions, then the backlash setting should be set below one second; never higher. Note that backlash is generally only an issue in Declination, since sidereal tracking takes care of it in Right Ascension. It is usually better to use the Anti-Stiction feature than the backlash settings.
For most mounts, you should set the Aggressiveness to somewhere between 5 and 8. If the autoguider oscillates back and forth, use a lower Aggressiveness. If the autoguider can't keep up with the drift, use a higher Aggressiveness. You can adjust each axis separately.
Some mounts have significant stiction on the Declination axis. Stiction causes the mount to continue to move forwards briefly when you reverse directions. Consider using the Anti-Stiction guider option.
On most telescopes, the Right Ascension drive likes to have some load pushing against sidereal tracking. If the mount is balanced such that it is pulling the mount forwards slightly, the gear teeth may bounce back and forth resulting in terrible tracking that cannot be corrected by an autoguider. Be sure to always balance the telescope such that it is ”lifting the weight” rather than ”allowing it to fall”. Note that this may require balancing the telescope differently when it is pointed East versus West.
Treat each axis as a separate problem to be solved. It might be working fine in RA and overshooting in Dec, etc. Often the solution for one axis is different from the other.
You can adjust the mount response in RA and Dec separately by tweaking the manual calibration parameters. The number displayed is the rate of star movement during tracking in pixels per second. If the axis is overcorrecting, increase the number; if it is undercorrecting, decrease the number.
Try adjusting the rate at which updates are sent to the mount. Some mounts work best with multiple updates per second, while others work better with a slower update rate. For example, LX200 mounts generally work best with a 3 second update rate. In most cases you can simply increase the guider exposure time to slow down the updates; however, a delay-after-correction function is available if needed.
If you are using the track log to analyze guider operation, it is a good idea to rotate the camera so that the RA is parallel to the camera X axis. This is not required for guiding, but it does make the log easier to interpret. The same advice applies to use of the Tracking Error Graph dialog.
Avoid overloading the mount. A mount that might be quite satisfactory for visual imaging with a certain telescope may be too small to image with that same telescope.
If your telescope mount is not up to quality autoguiding, consider getting an AO guide unit. These devices can literally turn a sow's ear into a silk purse, when it comes to autoguiding.
Please note that Screen Stretch has absolutely NO impact on guider performance.