Collinder 399: The Coathanger as 2x2 Mosaic

01 June 2022

This article is not so much about the picture I took as it is about the steps needed to be taken to capture a mosaic.

Phase 1: Planning the Panels

The first thing that needs to be done when shooting a mosaic is to plan the panels properly. Sure, this can be done by hand but using a service like telescopius.com makes it much more enjoyable.

Just set your telescope settings, camera settings, and mosaic settings according to your equipment and choose your target. Then export the panel settings as CSV file or copy the content to your clipboard.

Example CSV data for the Coathanger:

Pane, RA, DEC, Position Angle (East), Pane width (arcmins), Pane height (arcmins), Overlap, Row, Column
Pane 1, 19hr 29' 26", 20º 36' 40", 0.00, 100.80, 68.40, 10%, 1, 1
Pane 2, 19hr 22' 58", 20º 36' 40", 0.00, 100.80, 68.40, 10%, 1, 2
Pane 3, 19hr 29' 25", 19º 35' 07", 0.00, 100.80, 68.40, 10%, 2, 1
Pane 4, 19hr 23' 00", 19º 35' 07", 0.00, 100.80, 68.40, 10%, 2, 2

This will be required for the next step.

Phase 2: Shooting the Panels

Before the panels can be shot the individual targets have to be programmed. The steps to do just that are described by the screenshots below:

Don't forget to add bias-, dark- and flat frames for the session.

Phase 3: Stacking Panels

After all the required data has been collected it's time to stack the individual panels. A properly configured instance of the weighted batch preprocessing makes this step very easy, although I ran into some trouble selecting the required registration reference image selection mode. I hope that this is only a bug that will get fixed in future versions.

1. SCRIPT > Batch Processing > WeightedBatchPreprocessing
   • Grouping Keywords checked
   • Keyword PANEL
     • Pre unchecked
     • Post checked
   • Registration Reference Image
     • Mode auto by PANEL
       (If not available, make sure grouping settings are set correct, exit script, and restart PixInsight then it should be available)
   • Check Output Directory
2. Add files (bias, flats, darks, lights)
   • Ensure that the Post-Calibration tab is multiple rows, one for each panel.
3. Run the script and wait for the process to finish

Phase 4: Stitching the mosaic

After the individual panels have been stacked it is time to stitch the mosaic. This will be done in three steps:

1. Acquire an astrometric solution for each panel.
2. Register the panels with each other and within a large enough image.
3. Merge the registered panels.

Optionally one might need to perform a step zero where each panel is treated individually with a dynamic background extraction but this might not be required and depends on the actual image quality. Also cropping might be required if the overlap of the individual panels is too much which might cause problems when stitching the mosaic later on.

Astrometric Solution

To know where in the final mosaic each panel needs to be placed, an astrometric solution must be acquired for each panel. This can be achieved using the following steps:

1. SCRIPT > Image Analysis > ImageSolver
2. Add the panel files
3. Configure Image Parameters
   • Search for the coordinates
   • Input date and time
   • Set focal length
   • Set pixel size
   • Check Distortion Correction (Important!)
4. Click OK

When this process is finished, there should be a corresponding *_WCS.xisf file for each and every panel.

Registering Panels

Registering the panels within a large enough image can simply be done via

1. SCRIPT > Mosaic > MosaicByCoordinates
2. Add all of the *_WCS.xisf files.
3. Click OK

The result is one *_WCS_registered.xisf file for each panel where the actual image data of the respective panel sits in the correct place of a larger image.

Merging Panels

The last step specific to mosaics is merging the panels. In PixInsight there is a process called GradientMergeMosaic that does the job:

1. PROCESS > GradientDomain > GradientMergeMosaic
2. Add all *_WCS_registered.xisf files.
3. Adjust Parameters
   • Type of combination: Overlay
   • Shrink radius 8
   • Feather radius 80
4. Click Apply Global

When this process finishes the merged mosaic will be opened as a new picture.

Phase 5: Image Processing

After the mosaic has been stitched properly the following steps to finally process the image are no different from any other deep sky image and thus will be left out in this article.