Re: Fine tuning PHD2 settings for 1100 with Encoders #Guiding

Andrew J

Hi Roland.

Thank you for your feedback. I will get the screen shots with no guiding corrections after I let it run for 5 - 10 mins. Unfortunately, clouds have rolled in again and not expected to clear again until next weekend. I will revive this thread once I have the screen shots. 

Some explanation of terms:


Seeing is simply the amount of random motion that a star exhibits due to various atmospheric effects. It is usually expressed in P-P arc seconds. Knowing that value allows setting the Min Move.


P-P is the maximum fluctuation above and below the zero point with drift component removed.


RMS is a good indicator of the amount of error that is added to the theoretical FWHM star size of your optical setup. For example if your system's theoretical resolution is 2 arc sec FWHM and the RMS is 0.5, then the star size will increase to approximately 2.5 arc sec FWHM. RMS is typically 1/4 of the P-P value.


Seeing has zero to do with light pollution or Bortle or transparency or whether you are in the country or in the middle of a large metropolis. Seeing can be superb when the sky is super hazy and you can see no stars but only bright objects like the planets and the Moon. Seeing can be particularly bad when the sky is super transparent and you can see mag7 stars with the unaided eye. Do not confuse seeing with transparency.



Thank you for explanation of the termination around of Seeing . I must admit I still struggle to get my head around this. I understand the concept that we need to let PHD2 run unguided for a period of time to determine the Seeing on a given night and use that information to set the MinMo in PHD2. However, I still struggle with how to get the necessary inputs. For example, once I have an unguided PHD2 Graph, how can I determine my P-P arc seconds seeing? How can I determine what is the theoretical FWHM of my system? Once I know the P-P arc seconds seeing and my systems theoretical FWHM, how can I derive the best MinMo settings for a given imaging session? I thought all I needed was to get the Total RMS from the Guiding Assistant and the use this to determine my MinMo. That said, I have not been able to understand the relationship between the Total RMS and what PHD2 recommends as the MinMo values. Maybe once I publish the unguided screenshots you or some of the gurus here can help explain how to interpret and extract the required information from the graph and feed that information into the guiding algorithms. A explanation using screenshots I think would be really helpful (at least for me).


I recently acquired and setup a Unihedron SQM meter thinking it might help determine my Seeing. My best readings recently have been Naked-Eye Limiting Magnitude (NELM) of ~4.4 and Mag/sq Arcsec (MPSAS) of ~18.5, which translates into a White/City Sky Bortle value. Based on what you have said above, I interpret this to mean that all this meter will do is help me determine is my Transparency. It will not help determine my Seeing. Is this a correct interpretation?


I appreciate that this is not a PHD2 forum, but I trust the expertise and recommendations that I receive here over what I might find on other forums. Plus I really want to see how this applies specifically to AP mounts with encoders. I do think getting a solid understanding of these concepts can help us get the most out of our mounts.



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