Creating Mosaics


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This note documents the steps used to create widefield mosaics, referenced to a starfield generated by a sky charting program. A reference base is particularly important for widefield mosaics due to lens distortions and geometric distortion inherent in covering a large part of the sphere of the sky.

Programs used:

  • Registar
  • Photoshop
  • SkyTools

Creating the Reference Map

SkyTools is used to create the registration reference for alignment of the component frames as well as a labeled reference map for an overlay:

  • Frame area of the sky somewhat larger than will be covered by the mosaic.
  • Use the stereographic projection of the interactive atlas, turning off all objects and labels.
  • Copy the view to the clip board at the highest possible resolution (5000x3000?), create a new document in Photoshop and paste from the clipbloard. Flatten this file, convert to grayscale, and save it as an uncompressed TIF.
  • Turn on labels:
    • Contellation boundaries
    • Constellation lines
    • Milky Way outline
    • Nebulae
    • Clusters
  • Copy via the clipboard into Photoshop and save as an uncompressed TIF.

Preliminary Mosaic Creation

The purpose of this step is to create a small version of the mosaic (sized to the size of reference map) to insure that the mosaic will not fail, and that the reference map includes all of the sky area covered by the frames.

  • First create a test directory which includes uncompressed TIF files of all of the mosaic frames, plus the reference map image (monochrome, stars-only version).
  • Use Registar to:
    • Align all frames to the reference map.
    • Combine all frames against the reference map (use overlay).
    • Insure that all of the frames are completely contained by the reference map.
    • Determine the approximate amount of the scaling factor which needs to be applied to the reference map image to avoid downscaling the original image frames.
    • In order to minimize the memory required by Registar for the final mosaic registration, load the reference map file and the combined preliminary mosaic to determine how far down the registration map can be trimmed. Re-run the preliminary mosaic creation process to verify that the trimmed reference map still covers the entire area of the mosaic.

Aligning Pieces of the Final Mosaic

Once the preliminary registration map has been verified and cropped to a minimal size, it must be upsampled to be at least as high a resolution as the frames of the mosaics. Then Registar can be used to generate and save registered versions of the component frames.

Assembling the Final Mosaic

The final step is to combine the registered frames in a single photoshop layered mosaic. Use masks with each frame to blend the edges.