Advanced VX and anti-backlash

[Tudor Chibacu]

As a newb in astrophotography, I do have a couple of questions. I'm using a Celestron AVX mount and I do believe I have quite a bit of Dec backlash.

Using the anti-backlash feature helps, but I'm not sure if it would interfere with PHD guiding.

Should I disable the mount's anti-backlash when using PHD ? Also, I do not use a polar scope. I usually go directly for the drift alignment in PHD.

What is the correct order of things? Running calibration and then drift alignment or the opposite ?

[Andy Galasso]

Hi Tudor,

Although backlash compensation ("BLC") can sometimes help, it can also lead to over-correcting and oscillations.

We recommend disabling the backlash compensation in the mount and using PHD2's built-in BLC.  This is better than the mount's BLC since PHD2 can detect over- and under-correcting and adapt BLC dynamically.

To use PHD2's BLC, you run the Guiding Assistant and select "Measure Backlash Compensation".  After the guiding assistant completes the measurement it will give you the option to enable BLC.  After you enable it make sure you let PHD2 guide for several minutes to allow the BLC to stabilize.  You only need to do the measurement & stabilization once.

Regarding calibration: the drift align tool requires a calibration to be done first as that is necessary to learn the orientation of the camera (RA and Declination directions).

Andy

[bw_msgboard]

Hi Tudor.  To follow up on what Andy said, if you have a Celestron mount you may have more Dec backlash than any sort of software compensation can handle.  If you run the PHD2 guiding assistant, you can measure the Dec backlash directly and decide what to do about it.  Making mechanical improvements is always the best course of action, but if that’s not feasible, you may need to guide in only one Dec direction (uni-directional Dec guiding).  The 2.6.2 help file has a lot of information about Dec backlash and techniques you can use to deal with it.

 

Good luck,

Bruce

 

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[Tudor Chibacu]

Thank you, Andy and Bruce for your fast and detailed responses.

I will go ahead then and measure my backlash with PHD ( with the anti-backlash feature off in the mount ).

Depending on the amount I can then decide what to do.

Andy, in regards to the polar alignment. Does it matter if I run the calibration ruffly aligned North so then I can proceed to drift align? 

Should I re-run Calibration again after the drift alignment to get a more acurate reading ?

Thank you again.

Tudor

[Andy Galasso]

On Fri, Aug 19, 2016 at 1:32 PM, Tudor Chibacu <tudor.vla...@gmail.com> wrote:

in regards to the polar alignment. Does it matter if I run the calibration ruffly aligned North so then I can proceed to drift align? 

Should I re-run Calibration again after the drift alignment to get a more acurate reading ?

Right, start with a calibration after you are roughly aligned.  If you have not rotated your guide camera then you can re-use your calibration from the last session.

You can re-do the calibration after the drift alignment if you had to make large alt/az adjustments.  In general guiding is not very sensitive to small changes in calibration, but it does not hurt to re-calibrate after the drift alignment is done to get it more precise.

Andy