Plate Solving 2 – Automated Alignment

The Plate Solving 1 article described how Plate Solving software uses pattern matching to determine the stars and other objects that appear in an image. Plate Solving provides additional benefits when used with a computer connected mount – including accurate gotos with automated alignment.

The usual setup routine for using a goto mount is to first roughly polar align mount so that the mount’s polar axis is pointing at true north. This is often done with a small polar scope attached on the mount. An alignment process with the following steps is then repeated on several stars:

  • Select an alignment star that is visible from a list provided by the mount
  • Slew to the selected star
  • Use the hand-controller to center the selected star in the finder and eyepiece.

Plate solving can automate the alignment process. It requires a computer that controls the mount’s movements and can take images through the scope or finderscope. Many astronomy apps such as Stellarium, Carte du Ceil, or KStars are capable controlling a variety of mounts from vendors such as Celestron, Meade, iOptron, or Skywatcher. The process starts by selecting a target star or object in the App – then the computer takes over:

  • The computer tells the mount to slew to the target.
  • An image is taken through the scope and downloaded to the computer.
  • The computer uses Plate Solving to determine that region of the sky that the scope is actually pointing to.
  • The computer issues a sync command to update the mount’s alignment model to where the scope is pointing.
  • If the scope is not pointing at the target then the computer again tells the mount to slew to the target and the above steps are repeated

I automate alignment with my Celestron CGEM mount, Edge HD 8 scope, and a small Raspberry PI computer. The computer controls the mount and is also connected to a ZWO ASI178 camera on a piggy-backed 60 mm ZWO guidescope. The Raspberry PI is velcro’d to the mount and runs the Stellarmate OS but I connect to it remotely from a Macbook laptop running the KStars astronomy app over a wireless connection.

KStars displays a map of the sky for my location and time. I normally start by selecting a bright star relatively close to Polaris – making sure to pick one that is above 45 degrees in the North or North-East to avoid being block by the hedges or house in my front-yard. I then using KStars to have the mount “goto” to the target – in the image below my target was the bright star Mirfak in Perseus.

Screenshot of Kstars goto with Celestron CGEM mount.
Initial Goto the star Mirfak with KStars connected to a Celestron CGEM Mount

The EKOS alignment module in KStars handles the automated alignment procedure and plate solving.

EKOS Alignment Module Screenshot
Plate Solving Settings for EKOS Alignment Module

The main settings that I use are are highlighted with orange oval boxes in the screenshot above.

  • Select the “Slew to target” radio button to repeat a slew to the target if, after plate solving, the scope is not actually pointing at the target.
  • Set the Scope selection to “Guidescope” because I do automated alignment and plate solving using my guidescope.
  • Select “ZWO ASI178” in the CCD drop-down as that is the camera I have attached to the guidescope.
  • Set “Exp: 2 sec” to use a short 2 seconds exposure time. I occasionally increase this if not enough stars are visible.
  • Set “Bin: 4×4” so binning is used to combine pixels and decrease the size of the images.

Then I just click the “Capture and Solve” button. After an image is taken and plate solving is done, the image and results of plate solving, including the RA (right ascension) and Dec (declination), is displayed on the left hand side. On this night, plate solving succeeded despite the presence of the significant cloud cover seen in the image.

EKOS Screenshot after Successful Plate Solve and Alignment
EKOS Screenshot after Successful Plate Solve and Alignment

I use automated plate solving with my guidescope when doing visual observing – it turns my guidescope into an automated electronic finderscope that is faster and more accurate than doing a manual alignment.

When imaging, I extend the procedure to do plate solving with my primary scope and a Nikon DSLR camera. One great feature in KStars is that it shows the camera FOV on the sky map with its rotation after plate solving. That makes is easier to rotate the camera and compose the image so that it includes additional interesting objects.

Screenshot of Kstars with FOV plus rotation after Plate Solving
Field of View plus Rotation shown in KStars

Try it out – for technolophiles, doing automated alignment with plate solving with inexpensive hardware and free software is pretty cool stuff.