One of the challenges with mosaic images is the intensity of the signal between each panel of the mosaic. The panels will get captured across multiple nights with different sky transparency/seeing levels. All of this impacts the quality of the panels.

Before we can effectively merge all panels together, we need to equalize the brightness levels across all mosaic panels. Unfortunately, Linear Fit does not work well for mosaics. Instead, we will use a custom script called dna Linear Fit.

Note: To download the DNA Linear Fit script, visit David Ault’s mosaic tutorial. The script can be found in the Intensity Matching section. Download and install into PixInsight. It will get added to Script – Utilities.

DNA Linear Fit

The DNA Linear Fit process is extremely easy.

First, all images in the mosaic must be open.

Next, simply select one image as the reference and another as the target.

DNA Linear Fit
DNA Linear Fit

Once complete, save these images (ideally as new files)


Doing a 2 panel mosaic is extremely easy. But what happens if you have a much large mosaic? 6-panels? 15-panels?

In this situation, you need to be smart about how you use DNA Linear Fit. Take the following mosaic plan as an example.

Mosaic Plan
Mosaic Plan

When running DNA Linear Fit, the reference image must overlap with the target. No single image does that in this mosaic. This is why you need a plan to help define the reference frames, target frames and order.

OrderReference FrameTarget Frames(s)
1Frame 7Frame 12
Frame 2
Frame 8
Frame 6
2Frame 6Frame 11
Frame 1
3Frame 8Frame 13
Frame 3
Frame 9
4Frame 9Frame 14
Frame 4
Frame 10
5Frame 10Frame 15
Frame 5

As you work through the plan, make sure to save each image as a new file.

What’s Next

Now that the images are aligned and the intensities are matched, we can now merge the separate frames into a single image with Gradient Merge Mosaic.