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A Brief Description of Standard DES/DECam Exposure Scripts

Standard exposure scripts are loaded from the Observer Console GUI.

  1. Select Exposure Control and then click on Load Exposure Script
  2. Click on the right button labeled Select DECam Script
  3. A pop-up window shows you the available scripts from the DES and DECam areas. Scroll up or down until you find the script you want to load. Select it and click Submit
  4. The exposures defined in the script will be appended to the Exposure Queue. If the queue is empty and enabled, processing of the first exposure will start right away

The nightly observing plan will specify which scripts to use and when they should be used.
Here is a brief description of the different categories. This table summarizes the frequency and duration of the scripts.

Task Time to completion Frequency Conditions
Pre-night calibration 52 min Every afternoon Telescope at white spot, dark dome, daytime ok
Post-night calibration 5 (52) min Every morning Dark dome; at white spot if pre-night domes failed and post-night domes are required
Standards 5 min per field Every clear AM and PM 3 fields each AM & PM nautical twilight; 1-2 fields at end of DES half-nights
Darks 30 min Weekly to monthly Telescope parked, dark dome, daytime ok
PTC 60 min Monthly to seasonal Telescope at white spot, dark dome, daytime ok
Star flats 2 hrs Monthly to seasonal Clear conditions, not-too-variable seeing
SN variable Rarely Done when there is a need to override OBSTAC SN scheduling
Eng. Regression 4 hrs Unclaimed engineering time Need to be able to see stars

Pre-Night Calibration

These scripts are used to take a sequence of zeros (bias) and dome flats in the late afternoon before the dome is opened.
For DES we take 20s dome flats in z to satisfy a requirement from the SN survey to have 10x the photons in flat exposures than sky photons in the 1 hour long deep SN field exposures.
We also take u and VR band flats for community use.
These scripts include the VSUB junk exposures [check: set propid and dtsqueue to keep this exposure out of the data stream].
Default script name (check observing plan): DECam/prenight_zeros_flats.json
At the end of the script the instrument will be placed in safe mode (Vsub off, block filter, LEDs off)

Post-Night Calibration

If the prenight_calibration script was executed successfully, run the DECam/postnight_zeros.json script after the standards are taken, the dome is closed and dark, and the telescope is parked. This script will take a short series of bias frames, then safe the instrument by inserting the block filter and shutting off VSUB.

If the prenight calibration was not successfully executed, run the DECam/postnight_zeros_flats.json script to acquire dome flats in the morning. The telescope has to be at (or able to move to) the white spot and the dome has to be closed and dark. The camera will be placed in safe mode at the end of the script.

Standard Star Fields

We try to observe standard star fields at low, medium and high airmasses every evening during nautical twilight and again at low, medium and high airmasses during morning twilight. It is important to start the standard field scripts on time to get this done before the end of astronomical twilight when we begin DES observations. If the standard star field sequence was started late or for some other reason is not finished in time it has to be aborted. At least for now we have decided to give survey observations higher priority.

Use the DES/standards/Yzirg scripts (which start with Y-band and end with g-band) for evening twilight; use the DES/standards/grizY scripts (which start with g-band and end with Y-band) for morning twilight or for a half-night "pre-switchover" observation. (For those rare nights when we also observe in u-band, please use the corresponding "DES/standards/Yzirgu" and "DES/standards/ugrizY" scripts.)

See also Standard_Star_Instructions.

Engineering Night Regression Script

The standard regression script performs blocks of 4 exposures (30s, 30s, 30s, 90s) at fixed telescope positions: HA -3, -2, -1, 0, 1, 2, 3 and declination 30 to -60 [get exact values from script]. Tracking is on. We have three versions of the script: AOS-5, AOS-1 and absolute position. The last version repositions the telescope at the initial HA:DEC coordinates of the exposure block to correct for the position change due to tracking.
The scripts takes 230 minutes and is run during the monthly engineering night.
Default script name: Utilities/Regression_Observations_all.json

Star Flat Sequences

Star flats are taken to calibrate the response of the array to focused starlight and to derive a relative astrometric solution. This requires taking exposures of a rich star field (but not confused) at 22 pointings dithered by up to the field of view. These observations can be taken in bright conditions but should be done in clear skies. They will be repeated at least seasonally, at times determined in consultation with the DES operations group. Any location with Galactic latitude of 10-15 degrees is a good potential target field. These coordinates have been used successfully in the past:

RA Dec Comments
19:00:00 -50:00:00 Good for early LST
06:40:00 -34:00:00 good
07:30:00 -50:00:00 better
13:27:00 -48:45:00 Omega Cen outskirts, best

The procedure for executing the star flats is:

  • Move telescope to the selected starting position. Airmass should be <1.4. Pointing is not critical, within 1 arcmin of designated position is good enough.
  • Execute script DES/starflats/g.json
  • Move telescope back to starting position - again, within 1 arcmin is sufficient
  • Execute script DES/starflats/r.json
  • Move telescope back to starting position
  • Execute script DES/starflats/i.json
  • Move telescope back to starting position
  • Execute script DES/starflats/z.json
  • Move telescope back to starting position
  • Execute script DES/starflats/Y.json

In fact the order does not matter; do the g script in the darkest conditions / lowest airmass. The star flats need not all be done in one night either, but they should use the same field for a given set.
A DES/starflats/u.json script is also available.

Darks

Roughly once per week, run the Utilities/darks.json script to take a series of dark frames interlaced with bias exposures. The telescope should be parked and the dome dark. It is ok to take these dark frames during daylight.
At the end of the script the instrument will be placed in safe mode (Vsub off, block filter, LEDs off)

Photon Transfer Curve (PTC)

The Utilities/PTC_rxxx.json script (for example PTC_201407.json) takes a series of dome flat exposures in r band with bias frames between blocks of exposures in order to measure the nonlinearities of the DECam detectors. Run this with the dome darkened (daytime is ok), and the telescope parked at the white spot. At the end of the script the instrument will be placed in safe mode (Vsub off, block filter, LEDs off)