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Y6 global photometry program

Sep 6 2018, Gary B

One of the poor-weather programs for DES Y6 is an attempt to tie together the photometry across the full survey footprint by having DECam visit widely spaced positions across the FOV in a time that we hope is short compared to the timescale of transparency variations in the atmosphere. This page describes the execution of this program.

Description of a sweep

The program will be activated when the observers decide that conditions are too poor for the DES Wide survey to continue under ObsTac control, but meet the criteria below for this program. The observers should then begin this program by loading one of the JSON observing scripts prepared for this evening. They will have names like photosweep20180912-0300.json. The numbers give the UTC date and time for which the script was prepared. Each script contains approximately 1 hour's worth of exposures - pick the one whose time will occur within the next hour. For example, pick the 0300 script if you're starting any time between about 2:00 and 3:00. Once the script has begun, the observers should monitor conditions: if the seeing gets good enough to resume the Wide survey, halt the script and restart ObsTac. If the script ends and conditions are still right for doing the global photometry program, go ahead and start another JSON script.

Here's what the script will do: move the telescope to a series of positions that span all parts of the DES footprint that are currently at airmass < 1.8 and and least 30 degrees from the Moon. At each position, 15-second exposures will be taken in the g,r,i,z, and Y filters. Then the telescope will do a fairly long slew to the next position - somewhere between 15 and 90 degrees. These positions are ordered by azimuth to minimize the dome motion required. Essentially the telescope will loop around the zenith when running the sequence.

Some of the fields it hits will be from the DES standards list.

Necessary conditions

  • Photometric - do not bother with this program if clouds are visible on Rasicam. We are trying to get millimagnitude accuracy!
  • Seeing <2 arcsec. If seeing is worse or highly variable, the varying aperture corrections make it difficult to get to mmag accuracy.
  • A photosweep observing script is available for the hour. If there is no script, it means that not enough of the footprint is observable to make the program worthwhile (or you are too close to twilight).
  • There is at least a half-hour of observing time left.

Preparing the nightly scripts

The photosweep.py program attached to this page will generate the JSON observing scripts for the night. A script should be run each day by the run manager to delete the old photosweep*.json files and produce new ones for the coming night. This script (run with no arguments as user sispi -- the same account used to run nightsum) moves the old photosweep json files, creates new ones for the upcoming night, and copies them into place:

/usr/remote/user/sispi/y6_global_photometry_program/photosweep.sh

To run the script that just creates the json files but does not copy them into place, go to the /usr/remote/user/sispi/y6_global_photometry_program directory and run:

./photosweep.py [YYYY MM DD] [-p sweep.pdf]

Here the YYYY MM DD gives the calendar date at the start of the observing night. It can be omitted and the current UTC date will be used (be careful - if you do this when the date has already rolled over in Greenwich, you will get a plan for the following night!). The -p option allows one to specify the name of a pdf file that will be generated, showing the path that the telescope will take for each script (as viewed from the zenith), and the location of the moon and DES footprint. The photosweepYYYYMMDD-HHMM.json scripts will also appear in the current directory. Examples of both outputs are attached here.

Two input files are necessary for the program to run, both are attached. The photosweep.radec file lists the coordinates of potential observing spots on the sky. The round17-poly.txt file outlines the DES footprint and is only needed for the plotting.

photosweep.py -h will list other options for the program, which include changing the names of the input or output files, the maximum airmass, and the minimum and maximum number of observing points that can go into a single script.