Proper PHD2 settings for a Planewave L mount?

[Ed M]

I have the L-500 mount from Planewave which has no periodic error or backlash.  It has absolute encoders as well.  So the mount is very stable and reliable.  I work with a NASA exoplanet group that requires that the same set of pixels remain on the star for an entire period of 6 to 10 hours.  So the settings in PHD2 are very important.  

Are there other users who use the Planewave L mount?  And what settings have you found work best for such a mount?  Thanks,

Ed

[Bruce Waddington]

Hi Ed.  I see you haven’t gotten any input on this which doesn’t surprise me.  Because the mounts are very accurate, the required guiding should be minimal although probably not zero. It’s essentially done to clean up whatever residual tracking errors there are and to correct for polar alignment error, atmospheric refraction, and mechanical flexure of the OTA and device assembly.  I think you may have to think through is how long the mount can go unguided and what are its limiting factors.  If you’re using a fine-grained, continuously correcting pointing model, you can further reduce the amount of guiding activity needed.  There are a number of approaches that can be taken with PHD2 – the question is which will work best for you:

 

1.       Don’t do it at all if the combination of encoder+tracking-model is sufficient for the purpose.

2.      Use an AO device with the expectation that the mount will never need to be bumped.   Many of the top imagers use this approach but it is expensive and the requirements for back-focus can be a problem.

3.      Use what some call a “bump” guiding approach: use typical 2-3 second guide camera exposures plus a PHD2 “time-lapse” value that forces a delay between guide camera exposures and their corresponding guide commands.  For example, you might use a time-lapse value of 8-10 seconds to keep the guiding activity to the minimum necessary.  You would also set Min-Move values that should keep the number of guide commands low.

4.      Use longish (5+ sec) guide camera exposures while setting sufficiently large min-move values to avoid frequent guide commands.  In that case, the min-move value might be something like 80% of the maximum tracking error that can be tolerated

For either (3) or (4), you would use either LowPass2 or Hysteresis for the guiding algorithms.  I think every mount in this class may require something slightly different.  Hopefully, you can get your system dialed in quickly.

Good luck – let us know where you end up.

Bruce

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[bval...@gmail.com]

Hi Ed

I have a similar mount in the AP1600 with absolute encoders. We looked at the L series but at the time (a few years ago) we decided to stick with the GEM

however the absolute encoders make guiding with our mount similar to your L series direct drive

After a lot of experimentation, we ended up with a style called "bump guiding", which is Bruce's #3 above. We use 4 second exposures followed by 7 seconds of delay (by setting Time Lapse setting in Camera tab to 7000). This is giving us about 0.3" guiding over the night down in Chile. 

Regular cadence guiding (i.e., take a guide exposure every 2-5 seconds, correct, take the next one, etc.) resulted in degraded guiding/tracking, maybe 0.6-0.7" which for a long focal length can be significant

I hope that helps with some experience in this area

Brian

[Ed M]

Bruce and Brian,  Thank you very much for your advice.  The mount I have is rather unusual so I did not want to set it with standard parameters.  I appreciate your help!

Ed