What is N-Point Alignment?

What is N-Point Alignment?

EQMOD driver supports the N-Point Alignment algorithm.

N-Point basically is a mount stepper coordinate to sky coordinate affine/taki matrix transformation process. The driver internally plots the sky using a minimum of three alignment points (anchor points) as reference points. Pointing accuracy is increased by simply adding more anchor points on different parts of the sky automatically compensating for any mount alignment errors (polar, cone, etc.) yielding more accurate pointing.

Think of the coordinate space of the mount+EQMOD as a rubber sheet with the star database marked on its surface. You place the rubber sheet on top of a star map (your sky) and align the stars on the sheet with the ones on the map. You then lock the sheet with push pins on top of the map. The push pins are now your “anchor points” or “alignment stars” in the real world. To align the markings on the rubber sheet with respect to the catalog points on the map means stretching/ rotating/ scaling specific areas of the sheet and locking them to the star map using the anchor points (push pins).

The affine/taki matrix coordinate transformation routine is actually the one implementing the “stretching/rotating/scaling” functions and needs at least three points as anchor points to define the process.

When you have aligned on more than 3 points, the EQMOD driver dynamically selects 3 anchor points or alignment points from the list of anchor points (N-Point) to apply in the coordinate transformation during a GOTO process.

Accuracy is best within the bounds of the 3 anchor points and degrades dramatically (due the centering errors) as it goes outside the three points. However, if the stars are centered dead-on, even the GOTO's outside the area of the three anchor points should be accurate.

It is best to use points in the area where you most likely to do your GOTO's. If you imagine one big triangle formed by the 3 points, GOTO's will be most accurate within the triangle area.

Other Information about N-Point alignment
  • avoid syncing or aligning on stars at the meridian itself as the driver will block such operation. At least 5 degrees away from the meridian is recommended. If the star is too close to the meridian, alignment will be aborted and a message displayed.
  • in theory, the driver can handle up to 1000 alignment points
  • you can add additional points to the alignment data. This works really well when you need more accurate pointing for a hard to find object. Adding a near-by star to the N-Point set greatly improves the local pointing accuracy.
  • alignment points should be separated as far as possible in the area of the sky you plan to observe.
  • alignment points can be distributed on both side of the meridian, however, most observers will probably set the “PierSide Points Only” to avoid issues with inaccuracies caused as the mount crosses the meridian. This means that only points on the object side of the mount are used.
  • the [Map] button displays a screen with graphical information about N-Point alignment points
/home/welshdra/public_html/eqmod/data/pages/what_is_n-point_alignment.txt · Last modified: 2009/05/21 00:46 (external edit)
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