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.