Re: Syncing/Align Fundamental Question....

Khalid Baheyeldin

On Sun, Oct 24, 2021 at 05:41 PM, Martin Bonfiore wrote:
I must confess that the more I read about "alignment" and the more confused I become.  If I understand correctly, Onstep's alignment using say, three or more stars, generates a model that accounts for errors such as cone error. 
And the alignment and model building only happens at the start of the session (from when you tell OnStep to start the alignment process, and slew to the first star), and ends shortly after the last start has been synced/aligned.
OnStep takes some time to calculate the model, and refine it using some iterative calculations.
I recall it was a minute or so on the Blue Pill, longer on the Arduino platforms.

From that point on, there is no alignment active or anything. Only the calculated corrections in alt, az, and some cone error (If I remember correctly).

When I read about other so-called alignment tools/software, it is not clear to me that their alignment routines acccount for cone error (for example)...that they can achieve highly accurate gotos but only if there is no cone error i.e. they assume a cone-error free mount (through mechnical adjustment or construction).  I think I can see that if you have a cone-free mount and do an excellent polar alignment then a one star "align" is all you need to have the system understand the spatial relationship between the mount coordinates and the celestial coordinates.  One of the beauties of Onstep would seem to be that its deals with that mechanical imprecision by some kind of warping/transformation??
Many of the alignment algorithms are based on Toshimi Taki's equations and BASIC code that was published back in 1989.
They do include cone error, which he calls fabrication error.  The original page is now lost to the bit heaps of years gone by, but you can still find the source code here.

OnStep was initially based on these same equations, but Howard rewrote those a couple of years or so ago to what they are now.

The one alignment tool in particular I was studying was connected with the Ekos software.  I assume you use the onstep alignment, not the alignment feature in Ekos? 
I don't use either alignment. As I said, a rough polar align using a polar scope along with autoguiding gets me where I want with the least time and effort. Previously, I tried various methods to refine polar alignment, and they were just too time consuming (and I am impatient). So autoguiding cut the time needed to get setup, and saves dark time.

Bottom line is that I am chasing my tail trying to understand the interplay between onstep alignment and an alignment tool such as part of the Ekos software?  
They are two different paradigms.

Ekos alignment is a tool to help you get better polar alignment. So it goes through its steps, then gives you a result and expects you to use its various methods (legacy, or the newer method) to correct for those errors by using the alt and az knobs on your scope. Ekos DOES NOT provide logic to correct pointing or tracking based on the errors it shows. I don't think that Ekos has cone error. Or maybe it is there, but hidden from the user interface.

OnStep's alignment takes that one step further: it calculates the errors, remembers them, and does active corrections to  compensate for them in pointing when doing Gotos (and somewhat in tracking, though it is not perfect, and probably can never be, not too many people using that feature).

I have been using Stellarium but I just took a look at Kstars and think I might give it a try.  It certainly looks like it would require less CPU bandwidth than Stellarium and per your comments, can be better integrated into the mount control.
Stellarium is CPU intensive because it prioritizes 'looks'. It wants a realistic view of the sky.
One way to reduce its CPU load is to change its config.ini. Find maximum_fps and set it to 3, and minimum_fps to 1.

But KStars/Ekos has better overall integration, build in plate solving, sequence capture, and so on. It is just a very pleasant experience overall, and well thought out. Its planetarium is not the best, but still functional.

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