Auto N Point Alignment

[Skyewatcher]

Jim, to align you say to take image, Solved and then Sync. As the "Auto" button in Point Craft carries out a
Shoot -> Get Scope Position ->Solve -> Sync, is choosing "Auto" acceptable to obtain an alignment?

Regards,

Mike.

[JST200]

Hi Mike,

In a word, yes.

Any method of creating an alignment point is acceptable, as long as you do so away from the celestial pole.

The points behind this thread are to get people away from the old Star Alignment method, where you use an "alignment" star, and realise that with platesolving you can point anywhere. This enables using Alt Az coordinates. Which in turn means you can automate it, using a universal plan that will work ar any time anywhere (line of sight obstructions not withstanding).

Cheers, Jim

[Skyewatcher]

Jim,

Thank you for your prompt reply, this is hopefully going to ease alignment no end. I have a polar mounted Meade LX200 & presumably don't have to carry out the alignment procedure with the Meade handset, just go straight to APT Point Craft as soon as the scope is connected.

Mike.

[JST200]

Hi Mike,

If you were using the EQMOD driver for Skywatcher mounts I could tell you definitively. But, unfortunately, I don't know the Mead system, so I am less sure. However, I can take an educated guess at an answer.

I would think that once your pc is connected to your mount, the handset controls take no part in controlling the mount. Everything is done through the moumt driver. So no, you don't need to create any Alignment/Sync points using your handset.

Cheers, Jim

[Skyewatcher]

Thanks Jim, no chance to try out a yet - v.poor weather. When connected to the mount, scope is control by
APT so it looks like you're on the mark.

Cheers, Mike.

[pmodi]

Introduction

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 result of any inaccuracies is that when a GoTo is executed the requested location will not be centred exactly in the centre of the scope's field of view.

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.

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 exampleLight Plan that creates just that:

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

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.

From APT V3.65 onwards, you can use the Plan Editor to import this file as an example:

• for a DSLR camera: DSLR - Auto 4 Point Alignment Plan.xml
• for a CCD camera: CCD - Auto 4 Point Alignment Plan.xml

Creating a more accurate "whole sky" 6 Alignment Point Model

Of course you can use as many points as you wish. A step up from the four point model is to create three points in each East/West hemisphere, e.g.

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 75 90
15	1	#SolveSync Near
0	1	#GotoAltAz 45 315
15	1	#SolveSync Near
0	1	#GotoAltAz 45 225
15	1	#SolveSync Near
0	1	#GotoAltAz 75 275
15	1	#SolveSync Near

Cheers, Jim

Jim,

It seems that for this to work, there has to be some software that uses the transformation model for goto. I'm using an iEXOS-100 mount using an ASCOM driver with APT. As far as I can tell, neither APT, ASCOM driver for my mount or mount firmware are using the alignment transformation model. Could you please explain if I'm incorrect? EQMOD does not seem to support my mount.

[JST200]

Hi,

For anyone interested in this thread, I have just made the following changes to the initial post:

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:

1. A note has been added to the "4 Alignment Point Model" section about an issue that can arise when it is used.
2. The placings of the recommended alignment points of the "6 Alignment Points model" have been improved.
3. A recommendation for the setting of "EQMOD/EQASCOM > Alignment / Sync section > Point Filter" has been added.
4. An example "CCD - Auto 6 Point Alignment" light plan has been added

Cheers, Jim

[Drac0]

Just came across this post (thanks Jim) and setting up an Auto 6 Point Alignment but it's got me wondering about using Auto Sync in PointCraft.

Will using Auto Sync interfere with the alignment results in any way? I have EQMOD Point Filter set to "PierSide Only" for whole sky alignment & Proximity Range at 15°. I'm guessing that means that any Auto Sync within 15° of a current alignment point won't be added so using it in PointCraft won't be an issue and any points outside the 15° range will be added to the alignment? Or should I disable Auto Sync & manually sync only when doing a GOTO++ ?

Cheers,
Mark

[JST200]

Hi Mark,

I was with you up to the point wherevyou said "Or should I disable Auto Sync"

I took your first use of "Auto Sync" to mean the "PointCraft > Sync" button. Which then makes no sense of your sentence above, because you can't disable the button. Then i realised you meant "PointCraft > Settings > No Auto Sync" option. Doh!

I would be interested if unselecting "PointCraft > Settings > No Auto Sync" makes any difference as to whether Sync points are actually created for you. The way to check is to look at "EQMOD > Spanner>>> button > Alignment/Sync section > Notpad button > Alignment list". Whatever I set "No Auto Sync" to makes no difference. I don't get Sync Points from a GoTo++.

I think this might be my fault from a long time ago. I discussed the issue of having lots of Sync points close together with Ivo and he changed the way it behaved. It was only after this did I discover the "EQMOD > Proximity Range" slider. Since then I've never understood why the option still exists, because it doesn't seem to do anything. I have discussed this with Ivo before, but I can't remember the outcome.

Anyway, to answer your question...

Providing you have the "Proximity Range" set to a distance that is greater than the distance that GoTo++ can hunt around the target, then only 1 sync point should be added. This is fine. Even if you have already done an "Auto 6 Point Alignment".

The issue with creating loads of points is that EQMOD makes a series of triangles from each point to every other point. So the number of triangles it has to process rapidly increases, but to no great advantage in accuracy. But going from 6 to 7 (or 3 to 4 if you have "PierSide Only" selected) is inconsequential.

As I explain in my "Auto 6 Point Alignment" post, there are two approaches to Alignment. One to preset Sync points around the hemisphere and one to only create sync points near a target. Actually there is a third; doing both. I've wondered for some time whether this third option us the best of both worlds.

To use either the second and third option we need GoTo++ to create sync points.

So, Ivo, maybe it's time to reinstate Ivo's original code? Perhaps when you deselect "No Auto Sync" there needs to be a warning about the proximity issue of Sync points and a pointer to how to set a "Proximity Range" if the mount driver has one:-?

Hope that all makes sense?

Cheers, Jim

[Drac0]

Thanks Jim. I'll test it out if I get a chance - clear skies are few & far between down here for the last 6 months or so, certainly not a normal Aussie spring/summer. From the sounds of it the auto-sync shouldn't matter much at all if enabled. As I said I now have "PeirSide Only" active along with "Proximity Range" and if GoTo++ adds some target sync points, maybe it will be the "best of both worlds" - it's not as if I'm using GoTo++ to jump around the sky all night.

Cheers,
Mark

[Drac0]

You know, it was only last night I realised I was royally screwing up the alignment process, despite using it for the last 6 months. I run the 6-point all-sky with a park command at the end & it all goes well. But just before I start an imaging session I'll go out & check everything is correct - no cables in the wrong place etc. One automatic thing I have been doing is making sure the mount is in the correct home position - everything aligned north-south as it should be at the start of the night.

This of course kills anything the alignment has done - the mount is parked in a position to make allowance for the misalignment & I have just totally nullified it all...
6 months of not realising I was being a total idiot...

Cheers,
Mark, the Astro-Fool...

[JestinD09]

Nice write-up, thanks for putting this together — the “Auto N Point Alignment” method in APT looks really solid. I like the idea of using 6 (or more) alignment points spread around the sky so your GoTo accuracy stays good even when moving between distant targets.
If you don’t mind another tip: when I tried a 4-point model it worked quickly, but accuracy improved noticeably once I switched to a 6-point model & used the “PierSide Only” filter in EQMOD / EQASCOM — that seemed to avoid meridian-crossing distortions.
Also — if you plan to image multiple objects in one session — saving the alignment as a preset after park/unpark really saves time instead of doing fresh alignment each time.