polar alignment in Kstars/Ekos compared with Astrometry.net
I have a simple setup as just starting out - an old Canon DSLR (600D) attached to a Sky Watcher EQ-AL55i equatorial mount (no scope), using Kstars/Ekos to control on an old Macbook Pro running Ubuntu 24.04. The camera has a Canon EF-S18-55mm f/3.5-5.6 lens. Currently I'm just exploring the Milky Way and other stars looking south while trying to learn the software (and starware!)
After some struggling with Kstars/Ekos I am trying its polar alignment module, which I think is pretty impressive. Last night it aligned to about 40 arc-seconds which I was very happy with. So I then took some images of the SCP without moving the mount and uploaded them to Astrometry.net to check (what a great service!). I used their grid overlay which shows where they calculate the SCP to sit. I've attached a screenshot of the output.
According to Astrometry.net the SCP is not at the centre of the image, yet Ekos says I'm within 1 arc-minute of polar alignment. It's possible that the camera isn't perfectly aligned on the mount, as it's done by eye, but it's the same for both procedures.
What am I missing here? Should I not even expect them to show the same alignment?
Thanks for any thoughts and comments,
Tony
I wouldn't worry too much about the difference between the two.
Why?
Because the polar alignment process takes into consideration any misalignment between the camera and the mount whilst the astrometry solving only looks at the alignment of the camera and where it's pointing.
Polar alignment involves (usually) three images at different angles to calculate where the centre of rotation is - which is where your mount will be rotating about.
An alignment error of 40 arc-seconds is quite good, and for your focal length (at most 55mm) any error will not be noticeable in your images.
I hope this makes sense - it makes sense in my head as I type this...
Thanks Steve - yes that makes sense, and as you point out the error at 55 mm focal length is probably ignorable. But I will have my 135 mm Samyang F/2 lens very soon and expecting a big improvement in sharpness!
Tony
