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Q: Do I have to align the autoguider with the telescope axes?
A: No. When you calibrate the autoguider, MaxIm DL/CCD notes the angle of motion of
the star, and automatically compensates for it. This means you can attach your autoguider
camera in any orientation, without any impact on guiding.
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Q: I have two cameras. Can I use one to image
while using the other as an autoguider?
A: Yes. All cameras will work as autoguiders. Some cameras include
built in relays for commanding the mount; if not, many mounts can be
controlled via the Telescope Control, through the Starlight Xpress relay
box, or by a direct connection to the PC parallel port. Please note that
connecting two cameras usually requires separate ports. You should be aware
that excessive shutter wear may occur with some cameras, and that a few camera
models are too slow for effective autoguiding.
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Q: Can I adjust the autoguider settings during
an exposure?
A: Yes. Changes to the settings do not get used until
you restart the autoguider, but you can just hit the "Stop"
button and then the "Start" button again. Guiding will continue
as if there was no interruption. This is possible because the autoguider
controls are completely independent of the camera controls. Note that
for situations where the main camera and autoguider share a shutter,
you may have to turn off auto dark frame subtraction.
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Q: When I start guiding, the star initially
stays put, but then starts moving rapidly back and forth until it leaves
the guide box. What is wrong?
A: The autoguider is not calibrated properly. Go to the Guide tab Settings
and set the X and Y backlash to zero. Set the X and Y Cal. Times to 20
seconds, or the highest you can use without the star going off the chip
during calibration. If selectable, set your mount to 0.5X guide
rate. Find a bright star and run the calibration procedure. Now try
guiding on that star. If it still oscillates, try reducing the
Agressiveness setting.
Remember, if you change Declination, you must either use the
declination compensation feature or recalibrate. If you are not using the
declination compensation feature, just leave the setting at 0.
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Q: What does the Aggressiveness setting do?
A: It reduces the amount of correction applied. When the
setting is 10, 100% of the error is tracked out. When it is 8, 80% of the
error is tracked out.
Why would you want to reduce the aggressiveness? If the agressiveness
could be set over 100%, the software would overcorrect and the tracking
would go unstable; the star would bounce back and forth until it left the
guide box. Calibration accuracy is never perfect because of backlash in
the mount; decreasing the aggressiveness ensures stable guiding. A lower
aggressiveness will also prevent the mount from following small random
motions caused by scintillation and wind loads.
Generally speaking, reducing the aggressiveness improves the stability
of tracking at the expense of response time. Usually a value between 5 and
8 provides the best tracking; you should experiment to see what works best
in your setup.
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Q: What are the backlash settings for?
A: Almost all telescope mounts have some degree of
mechanical backlash in their gear systems. If you reverse the direction of
motion, there will be a slight delay until the gears re-engage. On some
mounts, the telescope will even go in the wrong direction briefly!
The backlash is measured in seconds; i.e. when you change directions,
it is the duration in time while pressing the button before the telescope mount starts
moving.
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Q: What is stiction and how can I overcome it?
A: The term “stiction” comes from “static friction.” Static friction occurs
when two objects in contact are at rest or moving very slowly relative to each other. Dynamic
friction occurs when the two objects are in motion. Static friction is always larger than dynamic
friction, so when an object starts moving, the amount of friction present can change very rapidly.
An example of this happens when you brake your car. Just before the car stops, you have to let up
on the brake pedal; otherwise the car jolts to a stop (if you’ve been driving for years, you may
not even be aware you are doing it).
Stiction occurs between the teeth in a gear system. Of particular interest
here is the effect on worm drives, which are typically used in astronomical telescopes. When the
drive is stationary, the gears “stick together.” When the worm starts rotating, it pulls on the
teeth of the worm wheel. In this situation, the wheel can bend, or the mounting or bearings can
flex, at right angles to the normal rotation. The resulting motion is usually backwards – you try
to move the mount North, and it goes South! This only happens for a brief period of time after
reversing directions; eventually the worm turns enough that the gear is forced to move in the
correct direction.
Stiction problems can be complex mechanically; it can often be hard to determine
what the root cause is. Many telescope mounts – even high-end models – suffer from this problem, and it
may vary between different samples of the same model. The problem has a terrible effect on guiding
because it forces overcorrection, and it cannot be effectively solved in software.
If you experience bad guiding in declination and cannot resolve it through
adjusting the calibration or aggressiveness, you may have a stiction problem. One solution that
works is to note the average drift in declination, and disable corrections in that direction. The
guider should only be able to push against the average drift, not with it. This works surprisingly
well. Some users deliberately adjust their polar alignment slightly off to ensure that the drift is
in a consistent direction. You should avoid large offsets that may cause field rotation.
You can disable one guide direction by clearing the appropriate Output
checkbox in the Guider Settings section of the Guide tab Settings dialog. These check boxes
only affect tracking corrections, not calibration, so you do not have to re-enable the direction
in order to calibrate the guider.
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