I often read in Astrophotography forums about trouble with guiding in DEC. Folks put in the time to get a great polar alignment and even though only very small adjustments need to be made, the guider seems to have troubles… non-response, then overshoot, periodic corrections that seem to come too late and allow too much drift. I’m not going to try and discuss all the possibilities, but rather the most likely causes of the trouble.
First off, guiding in DEC is a little different from what happens in the RA axis…. With the mount/scope properly polar aligned, there should be very little motion of the guide star in DEC. There is no steady constant motion like there is in RA. Of course there is some steady drift in DEC, but it is very small and slow. This can actually makes corrections difficult due to the effects of static friction and backlash.
When guiding in RA, we balance the load so the gears that are driving the axis are always lightly loaded in one direction. And since the RA axis is always moving to keep up with the sky, this means the friction effects are pretty steady and as such the load on the gears is very even. However, the DEC axis doesn’t have this advantage. It may need to be moved slightly one way, then the other. This brings backlash into play, and varying amounts of friction since moving and static friction coefficients are significantly different and may also vary depending on direction of rotation.
Let’s talk about backlash a little more. For this discussion I’d like to break it into two terms, Clearance and Preload. Clearance is the usual way we think of backlash. Gently rock the axis back and forth and you’ll feel the knock knock of the gears moving through the tooth clearance. However, there is more to it. When a servo or stepper motor tries to move the axis, it turns and first takes up the basic tooth clearance. Then, because the axis is not frictionless, as the motor turns some more, the axis doesn’t turn immediately, but the little gears all load up like little springs until enough force is generated to overcome the static friction and move the axis. We’ll call this wind-up the motor preload.
Manuals typically don’t address balance for the DEC axis except to balance the load (to ensure the motors aren’t overloaded). So for guiding, there are some things that make guiding the RA direction easier than DEC.
- The RA axis is always moving (darn slow, but it’s moving), the DEC is not. When surfaces are in the process of sliding (like gear teeth), the friction is is much less than it is at the moment when it starts to slide. This difference in friction is big enough that engineers divide computations into startup and running values, and the friction coefficients are completely separate (static and sliding). Since the DEC axis often static, when forced to move the axis will have to overcome static friction and make the transition to sliding. This makes the forces involved unstable.
- Because the RA axis gears are always biased/loaded in one direction, there is no backlash. This means that guiding corrections take effect immediately. The gears don’t have to take up any slack, or dead band in the drive.
- Additionally, whenever the drive is starts to move an axis from rest, or change of direction, the gears will have to load up or become preloaded enough to cause motion. This is kind of like backlash, but accounts for flexure (strain) in the mount and drive parts, which are to some extent variable in relation to the friction in the assembly, the load on the mount, and imposed balance.
In some ways, guiding in DEC is more difficult to control than RA.
So, what can we do:
If you’re seeing some erratic behavior in DEC, my first suggestion is adjust the scope balance in DEC to be either a little tail or front heavy. This should at least keep the DEC biased slightly in one direction so problems due to backlash are minimized.
If problems are still present, try adjusting the DEC guiding parameters in your software to see if you may be over or under correcting with each pulse.
Beyond that, I’d suggest looking at lots of data. I always log my guide data, and if I’m not happy with how my images look with regard to tracking, the guide logs is where I look. There are several programs available that can present your guide log data in graphical form. PEAS, PecPrep, and PemPro, are a few. However I don’t know if they all will display DEC guiding graphs. I use PEAS, and I know it can do it, but I don’t know about the others. If necessary it’s pretty easy to converted the guide logs into comma separated variable format and graphed the data in Excel. Anyway, looking at the data should give you some hints whether the problem can be addressed by adjusting guide settings or if other technique or adjustment is needed.