Guiding Error

After trying the autoguider again with the telescope, the result was far from perfect.. 

I added some logs, an checked what went wrong. 

The blue line shows the position of the star, if everything's fine, that should be very close to zero. The red lines are the corrections sent to the motor ( either positive or negative correction, based on the position of the star), and where the red line is missing, it means the motor turns with the predefined speed.

So, the problem is that after the first correction is applied, it takes still like 20 more corrections, before the star 'turns back'. As there are 15 corrections per second, it means that it takes 1.5 sec, which is way too much. So, the star goes up and down from coordinate -4 to 6, which is 10 pixel difference.  

My first thought was the Arduino board, which is controlling the motor driver IC. The Arduino board gets the correction parameter from the Raspberry Pi via I2C. There is a timer interrupt in the Arduino in every 0.5 millisecond, where a counter is increased. If the counter reaches a value (that is the speed value), it makes the stepper motor to take one steps. 

line-01 shows the time spent in the interrupt,

line-03 and line-04 is the I2C communication,

line-06 show the time spent in the function where the speed is modified 

line-02 shows a step of the motor..

It seems everything is normal.. the PI sends the corrections to the Arduino in time, the correction is applied immediately, so it seems there is no problem here, and there must be something wrong with the mount.