EQASCOM Alignment Overview.


  • EQASCOM provides a pointing model using a list of calibrated points.
  • The number of points can vary from 1-1000.
  • The process of adding a point to the list is referred to as “Alignment” and involves moving the mount to a known location (typically a star) in the sky and adjusting the mount to centre this point in a high power reticule eyepiece (or camera image).
  • The point remains as a fixed position In the sky with respect to the observer (i.e. it does not move with the related star). Using the same star one hour later would add a completely new point.
  • Some means for slew control that can be used at the mount is required. A simple game pad is a good choice.
  • EQASCOM provides two alternate interfaces for alignment, one method uses a dialogue based from EQASCOM and the other uses ASCOM syncs using your planetarium program. The ASCOM sync method is highly recommended. However, if your chosen planetarium program cannot provide a mechanism for initiating a “sync”, you will have to use the EQUASCOM dialogue method.
  • To provide pointing/goto corrections EQASCOM applies a coordinate transformation algorithm using data from the alignment points list. There are two alternate transformation algorithms,“Nearest Point” and “3-Point+Nearest”.
  • If the “PierSide Points Only” checkbox is enabled, only those points on the same side of the meridian as the target point will be used.
Alignment via ASCOM Syncs

This method allows for an alignment list and pointing model to be built directly from any planetarium program that can issue ASCOM slew and sync commands.

The procedure is:

  1. Using the planetarium program, slew to a star
  2. At the mount, centre the star with a high power reticule eyepiece (or use a camera “bulls-eye”). This is best accomplished with a game pad but you can also use EQASCOM's nudge buttons / spiral search etc.
  3. Using the planetarium program, issue a 'sync' command on the target star. A new alignment point will be added.
  4. Repeat 1-3 for each alignment star.

With this approach, it is very easy to add new point should you feel 'goto' performance is suffering in a particular area of the sky. Just 'goto' a star, centre it and 'sync'. If you use a 'game pad' for slew control there is no need to open up EQASCOM.

Alignment via EQASCOM dialogue.

In this mode the user initiates alignment from EQASCOM.

The procedure is:

  1. In EQASCOM initiate the Alignment dialogue by pressing the “Add Point(s)” button.
  2. In the planetarium initiate a slew to an alignment star.
  3. Centre the star using your game pad / EQASCOM Nudge controls using a high power reticule eyepiece / camera 'bulls-eye'.
  4. In EQASCOM Press the [Accept] button.
  5. Repeat 2-4 for at least two further stars. In EQASCOM press the alignment dialogue [End] button when all stars are done.

You can add more points to the list at any time by calling up the Alignment dialogue

When operating in 'dialogue' mode, alignment syncs received from the planetarium will result in entire pointing map being 'shifted' so it corresponds with the sync point .The map itself isn't changed.

Transformation algorithms.

EQASCOM provides two pointing transformation algorithms, 3-Point+Nearest Point and Nearest Point

Nearest Point Algorithm

Nearest Point simply searches through the current alignment list to find the alignment point nearest to target position. Once the nearest star is found a transformation is performed based upon the data associated with that point.

3-Point Algorithm

3-Point iterates through all triangle combinations possible using the 50 nearest alignment points to establish which triangle has its centre closest to the target position. The three alignment points that make up that triangle are then used for pointing correction. The diagram below illustrates this process. There are four points in the alignment list and as a result there are four possible alignment triangles.

The centre of the triangle formed by alignment points 1,2 & 3 is the one used since the centre of this triangle is closest to the target of the slew (the blue dot).

3-Point vs Nearest

The drawback with these triangle based transformations is that they are less accurate for targets falling outside the bounds of all possible triangles. To compensate for this the “Nearest Point” mechanism is automatically applied for these areas and so this method is referred to as 3-Point + Nearest.

Nearest Point is always applied if less than three points are present in the list.

PierSide Points Only

Another potential problem is where triangles span the meridian, particularly if you have a reflecting telescope where mirror flop many be significant. The “PierSide Points Only” checkbox will force using only points on the same side of the meridian as the target point in transformation calculations.

Nearest Point Zone

The following diagram (with 4 alignment points) shows the area of the sky (green) where the 3-point algorithm will be used and the area of the sky (red) where the nearest star algorithm will be used. You should remember that the 4 points are fixed in the sky with respect to the observer and only coincide with the stars used at the time of the alignment. In theory, points 1 & 2 could actually be the same star at different times of the night.

Alignment FAQs

Q: My Synscan provides 1-Point, 2-Star and 3-Star alignment. What modes does EQASCOM provide?

A: EQASCOM provides MultiPoint Alignment. It's up to you how many alignment points you want to use. If you want the equivalent of “1-Point” then you simply clear the alignment data and add a single point to the alignment list.

Q: Can I change between Sync and Dialogue based alignment methods once an alignment list has been started?

A: Yes, you can change the alignment method/interface at any time.

Q: Can I switch between “Nearest Point” and “3-Point+Nearest” at any time.

A: Yes.

/home/welshdra/public_html/eqmod/data/pages/alignment_overview.txt · Last modified: 2009/05/20 20:08 (external edit)
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