Setting backlash Question


Burton
 

Hi, so I've never quite understood how backlash works and i think I realized why that is. I'm pretty sure I've not ever had it dialed in right before. 

So I use a dobsonian.

My question is that in the instructions in the wiki for how to set backlash, it says to set a .5x guide rate. What does this mean? Because I know and can test that what I have my go-to speed set at effects the guide rates' speed for a given multiplier. 
I just don't get how I'm supposed to calculate the number to enter for backlash because of this confusion. In the instructions it says it's a 7.5 arcseconds per second, but how do I know that my rate is?


Khalid Baheyeldin
 

There is a simpler way to adjust backlash than what is in the Wiki.
No calculations needed ...

Basically, during daylight, use a faraway stationary terrestrial object.
For example a communication tower, a nail on a neighbour's chimney, or something like that.

Start OnStep normally, then slew to that object.
Turn off tracking.
Now using trial and error, try to refine how the object is centered when you move E and W.
If it jumps, then the backlash value is too high, reduce it.
If it lags, then the backlash value is too low, increase it.
Keep doing this until you don't observe any jump or lag when you reverse direction.

Repeat with N and S buttons for the other axis.


Henk Aling
 

On Wed, Jul 7, 2021 at 01:02 PM, Khalid Baheyeldin wrote:
There is a simpler way to adjust backlash than what is in the Wiki.
No calculations needed ...

Basically, during daylight, use a faraway stationary terrestrial object.
For example a communication tower, a nail on a neighbour's chimney, or something like that.

Start OnStep normally, then slew to that object.
Turn off tracking.
Now using trial and error, try to refine how the object is centered when you move E and W.
If it jumps, then the backlash value is too high, reduce it.
If it lags, then the backlash value is too low, increase it.
Keep doing this until you don't observe any jump or lag when you reverse direction.

Repeat with N and S buttons for the other axis.
I like that, and will try it myself.  However backlash may not be the same for each target.  Backlash due to a gearbox will likely be constant but backlash due to a non-round cog wheel or play in the RA and DEC axes, which every gear-based mount has, will depend on the position of the target in space and time.  It's best to just focus on the gearbox if possible and compensate for that part if you have a controller that allows it.  Or use stepper motors that don't require a gearbox.  But then you have to deal with noise as a result of the low torque, which I am looking into now.