As explained before, you can actually use the same star all over again in populating the N-Point table as it traverses across the sky.
What N-Point basically does is it converts all RA and DEC coordinate values into stepper values and uses the stepper values themselves in the transformation process. If we use the equatorial coordinates in the transformation, we need to utilize a third parameter which is the time of measurement. Including time data in the transformation actually complicates the conversion.
In this case, the time data is only used during the equatorial to stepper conversion and the converted stepper values are the ones stored in the N-Point table. The time component is not stored (although we store them for window display reasons).
The good thing about this method is that the same RA/DEC Equatorial coordinates of sky object which is fixed have different stepper values as it traverses across the sky after a given time period. Since we are using the stepper values in the transformation process, we can actually align on the same object again on a different time and use the generated stepper motor values as a separate N-Point entry.
The Stepper values are actually ALT/AZ coordinates in nature in which you can “map” a stepper count value (RAx, DECy) as a fixed point ALT/AZ coordinate in the sky.
N-Point Data processing uses several different different components:
Download a jpg image here:
This converts the RA/DEC coordinates to its equivalent stepper value using the current time as the 3rd parameter in the conversion. This means you will get different results as you execute this function on the same RA/DEC values as the time parameters vary. Also take note that the nutation is also accounted here.
The stepper coordinates are not linear and are spherical/circular in nature. In this case we need to convert them from a vector based coordinate representation to Cartesian to allow us to transform them using TAKI or AFFINE Matrix conversion.
If you remove this module from the system, computations will be more complicated as you will have to introduce the vector coordinates within the affine/taki matrix. This was actually the original approach of the the TAKI.bas routines where a matrix cell has sine and cosine functions in them.
(This was explained a lot on various threads on the news group).
Use matrix conversion/computations for coordinate transformation. It accounts for the 3 basic transformation operations; rotation, shift, magnification/reduction
Reverse computations from Cartesian back to Polar
Referring to the legend, it shows colored boxes that represent operations performed within the N-Point module;
- Current Scope Position COORDINATE DISPLAY and ASCOM Coordinate Presentation
The yellow colored boxes/arrows represent the data flow during a goto execution. The green ones are those used to obtain the current position of the telescope. The Light Blue ones are those used during the alignment process.
The diagram also shows the effect of using the same star all over again as an alignment data. As the star moves across the sky, you can actually create a separate N-Point entry when you align on the same star after (say) 2 hours of movement. That point where the star is located is converted to stepper values; Eqmod simply gets the current stepper count (Measured) and The Catalog Stars supposed to be stepper count at time(t) or time of measurement. These four values (ra stepper catalog, ra stepper measured, dec stepper catalog, dec stepper measured) are stored in the N-Point table. The 2nd measurement which is already located at a different position can now be used as one of the Affine anchor point of the transformation triangle.