PHD2 question: backlash compensation


Continuing the discussion from SGP Imaging Run Complete (But Not):

You can read about it in detail in the PHD2 help file.

In a nutshell, run the Guiding Assistant (Tools => Guiding Assistant) and leave the Measure Declination Backlash box checked. Make sure you choose a star near the center of the field since the backlash measurement will require a very large movement of the star. If you want you can skip the guiding assistant measurement phase (click Stop) and let it proceed to the backlash measurement phase. When backlash measurement is complete, there will be an option to enable backlash compensation, and you can click “Apply” to enable it. You can disable it or change the compensation amount later in the brain.

When declination guide pulses reverse direction, a backlash compensation pulse is applied to take up the backlash. The difference between PHD2’s BLC and the BLC in your mount is that when PHD2 manages BLC it can detect overshoots and undershoots and dynamically adjust the BLC pulse size, and that is what we do. (Enabling BLC in the mount is strongly discouraged since it can case overshoots and oscillation.)

As mentioned earlier, make sure you are using 2.6.1dev3 or newer for best results with BLC.



@Andy, this setting is new to me, so I checked my setup and the Use Backlash checkbox on the Algorithms tab is not checked. I guess this is not turned on by default when installing PHD2. Should I be using this, and do I need to follow the process you outlined here, or just check the box? Will I need to recalibrate?


If you are struggling with dec backlash problems, then PHD2’s BLC is a great way to address it. If you’re not sure whether or not you have dec backlash problems, check out Bruce’s excellent tutorial on analyzing PHD2 guide logs.

There’s nothing to lose if you just want to try it out and learn how much dec backlash is present.

Just checking the option in the brain is not enough since PHD2 needs to know the required backlash pulse size (baseline), and this requires measuring the actual backlash. The Guiding Assistant can automatically do the measurement and enable the feature.



Sounds perfect. I’ll give it a try. I have no idea if I need it, but like you say, nothing to lose. My first guess is that I probably don’t need it since I have a 10 pound weight biasing the DEC axis to the east, but I will find out.


My declination does have bad backlash. I will definitely give this a try. @Andy, thank you very much for the information.


I believe you mean that you are biasing your RA axis to the east. We are talking about backlash in the declination axis.


Ah so. Yes, I did mean I am biasing the RA axis to the East. Brain twist going on here. So we are talking about DEC backlash. Will see if I have any.