Please note:
The post was updated on the 1st Nov 2021.
Through use and a clearer understanding of the issues I have made four significant changes:
- A note has been added to the "4 Alignment Point Model" section about an issue that can arise when it is used.
- The placings of the recommended alignment points of the "6 Alignment Points model" have been improved.
- A recommendation for the setting of "EQMOD/EQASCOM > Alignment / Sync section > Point Filter" has been added.
- An example "CCD - Auto 6 Point Alignment" light plan has been added
The Problem
The software that controls your mount (whether it is in a handset or your PC) has an "ideal" model of the sky. This model is used to calculate the GoTo command require to slew to a given location. The accuracy of this model, its relationship with the "real" sky and therefore any GoTo command, is affected by the accuracy of the setup of the mount, e.g. whether
- it is perfectly level,
- it is perfectly Polar Aligned
- there is no Cone Error
- there is no flexure errors
- the mount's stepper motors and gears are all perfectly manufactured, with no backlash
- etc.
The Solution
The solution is for the software to also contain a Transformation Model. The Transformation Model is then used to do a coordinate transformation between the known coordinate of an Alignment Point and the actual location of that point. Traditionally the Alignment Points, required to create the Transformation Model, are created by requesting the mount to slew to a selected star, using the mounts controls to slew it until the star is in the dead centre of the scope's field of view and then selecting Sync.
With APT's Point Craft functionality the creation of the Transformation Model is simplified somewhat. The mount can be slewed to any location in the sky, an image taken, Solved and then a Sync issued.
Types of Transformation Models
There are various levels of complexity that the Transformation Model can take. At it's simplest it can be a single point and, in EQMOD's implementation at least, a "Nearest Point Transformation" is used. If you have three or more points then the model uses an "N Point Transformation", which triangulates the desired coordinates from within the triangle created by the closest Alignment Points. If the coordinate is outside of any available triangle of Alignment Points then a "Nearest Point Transformation" will be used.
Which Transformation Model to use?
There seem to be two schools of thought, within Astrophotography circles, as to whether to use a single Alignment Point close to the imaging target or whether to create multiple Alignment Points, ideally that encompasses it.
The advantage of the single Alignment Point is that it means you can start imaging sooner; time is saved slewing, imaging and solving multiple points in the sky. The advantage of multiple Alignment Points, spaced around the sky, is that if you are imaging multiple targets then the GoTo's to those targets are likely to be more accurate first time. APT's Auto Meridian Flip would benefit from this approach.
Notes
What is important to note is that the Transformation Model, created by the Alignment Points, is ONLY used when a GoTo is executed. It is NOT used whilst an object is being tracked.
Chris Shillito (EQMOD Developer) has written an excellent document on the way Transformation Models are used within the EQMOD/EQASCOM software: http://eq-mod.sourceforge.net/docs/eqmo ... models.pdf. Another document is here: http://eq-mod.sourceforge.net/tutindex.html and look for "Developer > 3-Star Alignment" link. Both well worth a read.
How do I do an Auto N Point Alignment?
A particular type of Light Plan
If you decide that using an "N Point Alignment Transformation Model" is appropriate for your requirements, then APT has a way to automatically create the Alignment Points. It is done by creating a Light Plan in a particular way.
To automatically and consistently create Alignment Points you need to be able to go to an ALT AZ coordinate, rather than an RA DEC one. Using APT's #GoToAltAz command enables this. It allows you to slew to a consistent sky location, take an exposure, do a #SolveSync, then repeat.
This process allows you to pick sky locations that are visible at your site and, of course, it isn't affected by time.
If you are unsure about your mount's 1st GoTo accuracy, then just do a #SolveSync Blind for the 1st and perhaps 2nd Alignment Points. You can, of course, change the exposure time and Altitude and Azimuth values to suite your requirements. You could also end with a #Park command.
If you have read Chris Shillinto's document (mentioned above) then you will know that the simplest Transformation Model to cover the whole sky is a Transformation Model with four Alignment Points (Sync points) equally spread around the sky.
Creating a simple "whole sky " 4 Alignment Point Model
Here is an example Light Plan that creates a 4 Alignment Point Model.
But please note:
One of the problems of a 4 Alignment Point Model is that it requires the triangles that form the Transformation Model to cross the Meridian. This can cause problems when the a transformed point is being calculate from a triangle that is predominately in the opposite hemisphere.
Therefore, it is strongly recommended that you use a minimum of a 6 Alignment Point Model (see the "6 Alignment Point Model" section below).
Code: Select all
Don't dither this plan > checked
Exp Cnt Command
0 1 #Unpark
0 1 #GotoAltAz 45 45
15 1 #SolveSync Near
0 1 #GotoAltAz 45 135
15 1 #SolveSync Near
0 1 #GotoAltAz 45 315
15 1 #SolveSync Near
0 1 #GotoAltAz 45 225
15 1 #SolveSync Near
- for a DSLR camera:
- for a CCD camera:
Creating a more accurate "whole sky" 6 Alignment Point Model
Of course, you can use as many points as you wish. However an excellent compromise is to create a 6 Alignment Point Model, i.e. three points in each East/West hemisphere. To cover the whole sky the following LightPlan places the points to do just that.
When using a 6 Alignment Point Model, by default, there will be some triangles that form the Transformation Model that cross the meridian. This can cause problems when the a transformed point is being calculate from a triangle that is predominately in the opposite hemisphere.
To avoid this the transformation model should be set such that triangles are only used that lie within the same hemisphere as the point being transformed, if your mount driver supports this.
In EQMOD/EQASCOM, this is done by setting "EQMOD/EQASCOM > Alignment / Sync section > Point Filter" = "PierSide Only"
Code: Select all
Don't dither this plan > checked
Exp Cnt Command
0 1 #Unpark
0 1 #GotoAltAz 45 170
15 1 #SolveSync Near
0 1 #GotoAltAz 45 90
15 1 #SolveSync Near
0 1 #GotoAltAz 45 10
15 1 #SolveSync Near
0 1 #GotoAltAz 45 190
15 1 #SolveSync Near
0 1 #GotoAltAz 45 270
15 1 #SolveSync Near
0 1 #GotoAltAz 45 350
15 1 #SolveSync Near
- for a CCD camera:
Correction made. 
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