Polar scope alignment on NEQ6

The best polar alignment is a minimum requirement for astrophotography. In a permanent observatory this can be easily achieved with several methods, but for those imagers who move to dark sites, you have to rely on your polar scope precision.
As my NEQ6 arrived I performed some ‘quality checks’ and I immediately noticed that the reticle wasn’t ok.
The first step was trying to align it aiming at far object, and despite the uncomfortable procedure I got quite good results.
A second attempt with the support of a webcam improved the precision, but since the small circle where you should put the Polaris is not aligned with the central cross (at least in my mount), I had problems in defining the ‘home’ position, either it is at 3, 6, 9 or 12 o’clock.
Why not using the webcam and some math to find the best centering of the reticle?
Said that, I developed a small application to show the video from the webcam, click few points and get a good centering of the reticle and ‘home’ position.

PolarAligner
Download the installer from the link at the end of this page, install and run it. This is how the window looks like.

click on the images for full resolution
 

First select your webcam from Devices list. As you select one device, the Frame size list will be populated with possible resolutions for that webcam.
Select the frame size and then click on Connect button.




 

With the webcam well focused and placed in front of the polar scope, you should see the reticle on the screen. Next step is to find the rotational center of the RA axis. I found useful to align the RA circle at 24 o’clock and rotate the RA by 8 hours. Pick one point on the screen and a small cross (or circle, depending on your marker selection with the Radio Buttons) will be drawn.
Pick three times the same point rotating the RA by 8h. In math, at least three points are needed to find a circumference.




 

Don’t worry if you don’t pick the point exactly where it should be. Click on the point coordinates in Centering points list and use the arrows to move it where it should be. When you are happy of point position, just hit ESC key to release the point from ‘move mode’.


 

As you have placed the three points click on Calculate button. On the screen will appear a circle passing through the points and a cross that shows the rotation center.

 

If your reticle is not centered (as mine) you’ll see that the yellow cross do not match with the reticle cross. Align it using the grub screws.
Click on Clear lists and repeat the sequence until you are happy of the result.

 

I usually align the mount to Polaris with the EQASCOM Polar scope alignment tool (project page, under documentation), so I need to know the ‘home’ position of my reticle.
If you have accurately levelled you mount it’s enough to align the small circle of the reticle with the yellow cross, otherwise this program can help you:
clear the points list, lock RA axis, then move the elevation screw of the mount up or down. Pick one point with left click while pressing SHIFT key.
A point will be added in the Orientation points list.
Now move the elevation screw in the opposite direction and pick the same point while pressing SHIFT key (the more the points are distant, the more is accurate). These two points identify the line that passes through 12h and 6h of the reticle.
Move back the reticle roughly at the window center and repeat the centering. Pick three points at 120° and finally click Calculate. In the below picture I used T-Octans circle.
On the screen you’ll see the circle, the yellow axis and a blue line. The yellow-blue cross is now rotated to be parallel to the line passing through the Orientation points. Rotate the RA axis until the Polaris circle is perfectly on this blue line and mark the ‘home’ position of the reticle.
The blue line crosses the Polaris circle (around NCP) on both sides, so be sure your home position is 6 or 12 o’clock depending on which side you align. You could also align the Polaris circle on the yellow line, and in such case your ‘home’ position will be 3 or 9 o’clock.

 

A comparison of the ‘home’ position that I got aiming at a far building edge (long mark) and the new ‘home’ position found. Alignment errors calculated by EQASCOM are now smaller and also the autoguide graph shows less corrections.

 

Here two pictures of the setup I used. Honestly, at the end I glued a couple of magnets under the webcam and I used a small ‘L’ shaped metal bracket to hold it so all the pictures are rotated by 90°. The mount up direction is on the right of all previous images.

 

As mentioned before, in the interface there are also two option buttons named ‘Cross‘ and ‘Circle‘, and a text box where you can set the Size of the point markers, expressed in percent of the image height.
The shape of the markers can be changed at any time and possible values of markers size can vary from 1 to 25% of the image height.


 

Here you can download the installer:

SW

	Polar Aligner Installer for Windows




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