Creating Mosaics with APT for JWST

This document shows how to create mosaics for JWST observations (click here to see the movie). The project is to mosaic the galaxy M51 using MIRI. In the Mosaic Properties tab, we will put in an initial guess of a 4x4 mosaic. For more information about the other fields, see Mosaic Parameters.

First we need to determine the position angle for the mosaic; note that for most mosaics, you will need to specify an exact position angle (the exception is for rotationally symmetric regions with no guide star issues). The default position angle for the Aladin display is V3PA=0; for more on assigning position angles, see Assigning Position Angles for JWST Observations (document). If we rotate the mosaic, we see the a V3PA of 18 gives better coverage. Note that when you rotate a single aperture in a mosaic, all the other apertures move to catch up when you finish the rotation.

If you need to revise the mosaic, there are 3 options:

  1. Split Tile to New Observation - this will add a new target and observation at the position of the selected tile, and mark the tile removed from the mosaic. This is useful if a given tile in a mosaic does not have guide stars and needs to be execute at a different V3PA than the rest of the mosaic.
  2. Copy Tile to New Observation - this does the same things as Split except that the tile is NOT removed from the mosaic. This is useful if a given tile failed during execution and needs to be repeated.
  3. Insert New Observation into Mosaic - this does the same things as Copy except that the new tile is placed at the center of the mosaic. This is useful if you need a add a new tile to the mosaic but do not need to add a entirely new column or row.

Note that the Split and Copy options are not active until you have selected a tile.

For our project, we also want to include the companion galaxy in our mosaic, so we need to add another tile. To do this, we will insert a new tile.

Note that a new target is generated for the position; the is present because the Category and Description needed to be filled in. For the new tile, a new Observation Group is created that contains the original mosaic and the new tile. The new tile has, by default, the same V3PA as the original mosaic.

With the new tile selected in the Tree Editor, we need to move it to its correct position (by dragging aperture to be on the companion galaxy). When you do this, the Aladin view shows the change to the tile position. When you are happy with the position, click on Commit All to save the change.

With the Observation Group selected in the Tree Editor, you can see the complete mosaic.

If you wanted to remove a tile from the mosaic, select the tile to be moved and change the Tile State to be "Tile Removed".

Last modified: November 24, 2017.