With a star catalog generated, we can properly align our mosaic panels with the Star Alignment process.

Star Alignment

To run star alignment, we need to have our individual mosaic panels loaded. They should be cropped to remove the dark edges and have the background gradients removed.

Mosaic Star Alignment
Mosaic Star Alignment

To use Star Alignment for mosaics, we need to change how the process functions:

  • Reference Image: This should be the generated star field image we created
  • Registration Model: Thin Plate Splines is used for mosaic images as it allows us to correct for greater distortions typical with multi-panel mosaics.
  • Distortion Correction: Enable. This helps with mosaic as the sky and panels become curved
  • Local Distortion: Enable
  • Working Mode: Register/Union – Separate. We need to align our mosaic panel with our star catalog but to not have our star catalog pixels appear in the resultant image. This working mode aligns the mosaic panel and then places is on a black canvas the size of our reference image.
  • RANSAC Tolerance: This defines how far away a star can be from the location in the star catalog. This should be increased for mosaics. Starting with 5 works well.

Apply this process to the image by dragging the triangle icon to the image.

This process can take some time, but when complete, you will have to images. One includes the mosaic frame properly placed in a canvas the size of the star catalog. Save this file.

Mosaic Panel
Mosaic Panel

The second file is the star catalog canvas. This can be deleted.

Do this process for the remaining mosaic panels.


Sometimes, the Star Alignment process will display “Unable to find an initial set of putative star pair matches”. This is either because there is too much noise in the image and the process is unable to find real stars, or that the identified stars are not in the exact correct position as the star catalog. This is quite common for mosaics.

There are a few things you can try

Star Detection:

You can try adjusting the star detection parameters to help the process identify stars better.

  • Detection Scales: Increase the number to find larger stars
  • Noise Resolution: Increase to 1 or 2 to help reduce false positives
Star Alignment: Star Detection
Star Alignment: Star Detection
Star Alignment: Star Matching
Star Alignment: Star Matching

Star Matching:

The next thing to try is to adjust the star matching algorithm.

  • RANSAC Tolerance: Increase to maximum to allow for greater differences between the stars actual position and expected position. As the mosiac gets larger, this can help.
  • Descriptors per Star: Increase this to 100 or 200 to help match stars. However, this will increase processing time, so only do this for cases where alignment continues to fail.


This process can take quite some time to complete. To automate it, do the following:

  1. Create an Image Container: Right-click on the PixInsight workspace and hit “Image Container”
  2. Add Files: Select the folder icon half-way down towards the right. Select all mosaic panels.
  3. New Image Container Instance: Drag the triangle in the lower-left onto the workspace. This finalizes the container
  4. Start Process: For the Star Alignment process, drag the triangle in the lower-left onto the newly created image container. Star Alignment will get applied to each item in the Image Container.
Image Container
Image Container

What’s Next

Once all panels have been aligned, we can bring them together with Gradient Merge Mosaic.