Viz Tutorial: Exercise 1

This exercise is intended to take about 15 minutes. Please ask in the Whova chat if you run into any problems or need any clarifications. For “big-picture” questions, there will time for a debrief/Q&A after the exercise.

Setup

1. Have you installed the software and downloaded the sample data yet? If so, great job! If not, do that now. Ask for help in the chat if needed. You won’t be able to do anything in this tutorial without completing the setup first.
2. Open a terminal, activate your software environment, and try running the command toasty. You should get a help message about allowed subcommands.
3. Make sure that you have downloaded the file vlass2.1-ql-T01t30-J145214-363000.fits (106 MiB) into your ~/wwtviz/pywwt-notebooks directory.

Basic tiling

1. Open the VLASS FITS file in your favorite FITS viewer. What are its dimensions? Adjust the stretch (if needed) to locate the bright double radio galaxy, and make a mental note of its approximate position in the image.
2. The toasty command-line tool can break it into tiles. Run:

   toasty tile-multi-tan vlass2.1-ql-T01t30-J145214-363000.fits --hdu-index=1 --outdir=vlass
   

3. This will create a directory named vlass with one subdirectory named 4, indicating a “level 4” tile grid. (“Level 0” is the top level.) How much disk space do the level-4 tiles consume? On most machines you can find out by running:

   du -hs vlass/4
   

4. Open the file vlass/4/0/0_0.fits in your favorite FITS viewer. This tile represents the top-left corner of the input image. Does what you see make sense?
5. The file name format used by Toasty in this example is vlass/{L}/{Y}/{Y}_{X}.fits. What are the X and Y values of the tile containing the bright double radio galaxy?

“Cascading”

1. Now run the command that Toasty suggested earlier to generate the shallower tiles:

   toasty cascade --start 4 vlass
   

2. Investigate the new contents of the vlass directory. How much disk space does the entire tile pyramid consume? How does this compare to the size of the input image? What is the typical size of an individual tile file? (It will probably be faster to determine this empirically rather than analytically.)
3. Open the top-level tile vlass/0/0/0_0.fits. What is the (x,y) pixel position of the bottom-left corner of the image that actually contains data? What is the “efficiency” of this tiling — what percentage of this top-level tile actually contains data, rather than padding?

If you have extra time, try toasty view vlass2.1-ql-T01t30-J145214-363000.fits to explore this image in WWT’s viewer.

You can also go back to the tutorial landing page.