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Regions and Catalogs

When looking at astronomical images, you rarely just want to stare at the pixels. You usually need to mark specific objects, measure areas, or cross-reference what you see with known databases. FViewer handles this using Regions and Catalog Overlays.

Working with Regions

Regions are shapes you can draw directly over your FITS image to highlight areas of interest, measure backgrounds, or define extraction boundaries. If you have used ds9 in the past, FViewer’s region system will feel familiar.

Drawing and Editing

You can access the region tools from the Region Toolbar or the top menubar.

  • Shapes: You can draw Circles, Boxes, Ellipses, and Annuli (rings).
  • Interacting: Once a shape is drawn, you can click and drag it around the image. You can also use the drag handles to resize or rotate the shape precisely.
  • Pan vs. Draw Mode: By default, clicking and dragging on the image pans your view. To draw, simply select a shape from the toolbar. Once you are done drawing, FViewer automatically switches back to Pan mode so you don't accidentally draw shapes when trying to navigate.

Saving and Loading

FViewer is fully compatible with standard DS9 Region Files (.reg). * You can save your drawn regions to a text file to share with collaborators or use in other analysis pipelines. * You can load existing .reg files into FViewer. FViewer understands complex coordinate systems (like converting RA/Dec to image pixels on the fly) and will preserve your specific colors, line styles, and background tags.

Catalogs and TAP Queries

While regions let you manually mark objects, Catalogs let you overlay thousands of known sources from remote databases directly onto your image.

FViewer includes a powerful TAP (Table Access Protocol) client. This allows you to query massive remote astronomical archives—like HEASARC, Chandra, or XMM-Newton—from inside the viewer.

Searching Archives

  1. Open the Catalogs menu.
  2. You can perform a Context-Aware Spatial Query: If you are currently viewing an image with built-in coordinates (WCS), FViewer can automatically search a remote database for any known sources that fall within the exact footprint of your current screen.
  3. If a region is selected from the image when making the query, the catalog will be searched for soruces within that region.
  4. Manual ADQL: For advanced users, you can write custom ADQL (Astronomical Data Query Language) queries to filter exactly what you want to download.

Interactive Overlays

When you load a catalog (whether from a remote TAP query or by opening a local VOTable file):

  • FViewer automatically plots the catalog sources as circular markers directly over your FITS image.
  • The center workspace splits in half, showing the image on one side and the catalog data in a spreadsheet on the other.

Bidirectional Linking

The catalog markers on the image and the rows in the data table are perfectly linked: * Click a marker on the image, and the data table will instantly auto-scroll to the exact row for that source, highlighting it in green so you can read its properties. * Click a row in the data table, and the image will automatically snap and center onto that specific source.

Because FViewer uses an optimized, zero-dependency data grid, this linking happens instantly, even if the catalog contains hundreds of thousands of sources.