There are two related questions regarding PA and calibration that have entered my mind over the past year, and I finally have the answer. The longer standing and more perplexing question is how can calibration yield a result that says that my mount’s Ra and Dec axes are not orthogonal? The other is a chicken or egg question regarding polar alignment and calibration – which comes first?
The answers are related, because polar alignment and calibration are interdependent.
The reason that a non-orthogonal calibration result perplexes me, especially a large descrepancy, is I can look at my mount and readily see that the two axes are orthogonal. Scott Losmandy, a highly skilled machinst with decades of experience, doesn’t know how to make a mount any other way. If the axes are mechanically orthogonal, then how can calibration yield a non-orthogonal result?
I found the answer to this question, surprisingly, while trying to answer the polar alignment vs calibration question, so lets answer that question first.
Polar alignment comes first, then it might come last. The reason for this answer is that a good polar alignment is a fundemental component of a good calibration result. Without a good polar alignment, drift will be present during the calibration procedure, and the drift will cause a non-orthogonal calibration result.
If polar alignment comes first, then why might it also come last? A resonably precise polar alignment is essential to a good calibration result.
I approach polar alignment in two stages: initial and precision.
Initial polar alignment starts when I put the tripod on the ground facing north. There are many non-precision tools (e.g., compass, smartphone app, polar alignment scope) that can be used at this stage to refine the polar alignment. When more precision in polar alignment is needed, other tools like drift alignment (whether using PHD2’s tool, or not), apps resident in capture software, or hardware/software similar to PoleMaster.
It is when PHD2 DA tool is the precision polar alignment tool, then polar alignment also comes last. In this case, start by using the best on-precision polar alignment tools available. Now do a calibration run, which will likely yield a non-orthogonal result, and then use the DA tool to attempt a precision polar alignment.
If the DA polar alignment was based on a non-orthogonal calibration result, then the polar alignment precision may be compromised. I this case, iterate through calibration and DA polar alignment another time or two to tighten up both results.