During the Night

Where is the data?

Tools to Check the Quality of the Data:


All of your images are copied over to the observer2 computer. They are located in this directory: /home4/images/fits/your_proposal_ID, except for the DES project which has a different location.

Images in this directory are in compressed format (.fz).



A suite of commands to quickly examine DECam images were developed by Steve Kent and are known as the Kentools. To start the program:

  • Open a terminal in observer2
  • Type observer
  • Then, you can run several useful commands:
Command Explantion
commands Lists all available commands
inventory [n]

Lists the observations made during the current night, or n nights ago. This is a useful tool to produce a log of the observations.

inv [n] Short of the above
invPrint [n] Same as above but prints the results to a file (in DECamObserver's home directory; file is named: yyyymmdd.inv)
load [exp num] [ccd] Displays a ccd in DS9 (it stars DS9 if needed). If no arguments are given, it will display chip S4 from the latest image
bigload [exp num] Displays all 62 CCDs after a quick bias sustraction and decimated by a factor of 8
center [exp num] Performs astrometry to determine the offset of the telescope pointing
seeingall [exp num] Determines the average seeing in the 8 central CCDs

In the above commands, if [exp num] is omited it will default to the latest delivered images. An exposure can be identified by just the last few digits of the exposure number, without having to specify the sub-directory of the full file name.

Other useful commands are:

Command Explantion
seeing Click on a particular star and it will produce a radial plot and delivered the seeing
ephem Gives the ephemerides for the night (sunset, sunrise, twilight, Moon, etc)
skyview Looks up field
ned Looks up galaxy
simbad Looks up star
camFbsPlot This shows:
1. the shape of the stellar ellipticity for every DECam CCD,
2. ellipticity with mean ellipticity subtracted, and
3. the relative fwhm to the mean. This command takes a while to run.
flatCheck [n] Checks the nominal values taken during the current day or n days ago.

More documentation on kentools is available here.

Database query

To obtain a summary of the quality of your data you can use the command godb. This command queries the telemetry database and lists data for your exposures, including the seeing.

To run it, open terminal in observer2, type “godb”. There are two commands that you can use “inv” and “qcInv” (or their print-to-file versions, invPrint and qcInvPrint).

“qcInv” is the most interesting command, as it lists both the seeing (consistent with running the “seeing all” command on individual images) and airmass. The command “inv” is similar to the one in Kent's Tools. Other values listed with this command are sky, cloud and teff.

  • sky gives the sky background expressed in magnitudes relative to a dark sky at zenith for the particular filter.
  • cloud gives a measure of cloudiness (in magnitudes) relative to clear sky at zenith. This value should be used with care. It is computed by comparing stars in an exposure to either the NOMAD or APASS catalogs. These catalogs have not been validated over the entire sky and are known to have issues, sometimes at the level of several tenths of a magnitude.
  • teff is the effective exposure time one would need to use when pointed at zenith with a dark sky and median site seeing in order to have the same limiting magnitude for point sources as the actual exposure. It is normalized to the actual exposure time.

Be aware that sky and cloud have not been calibrated for the u band yet.

Note that if you are doing very short exposures, values may be missing from the table as the database can have missing entries in those conditions. In general it is useful to run “qcInvPrint” at the end of a night and then for the few missing values (if any) just run “seeingall” (Kent´s tools) and add those values to the table.


IRAF can be used in observer2 for image inspection and analysis. In order to be able to display and/or examine an image in IRAF you need to decompressed it first. In a terminal:

> funpack image.fits.fz

Some useful comands in IRAF:

To display a CCD:

> !ds9 (to open a ds9 window)
> display image[XX] (displays the CCD XX, where XX is a number from 1 to 62).

You can then use imexam to examine the image.

To find out which is the CCD of a given extension:

> imhead image[XX] l+ | grep DETPOS

To extract a single CCD from a DECam image:

> imcopy image[XX] output

Weather Pages

The following tools are only available on site or though the internal VPN.

The Tololo Environmental page (internal access only) updates automatically every few seconds and contains weather data as well as DIMM seeing measured at the observatory. It also displays the Night Almanac. It is a useful tool monitor the environmental conditions during the night and the altitude of the Sun. Keep an eye on the wind. You may want to close the louver windows around the dome floor if the winds increases above 15 mph.

Clouds can be monitored using the all-sky infrared camera RASICAM (run rasicam.sh in a terminal in observer2) and the Tololo external webcam which looks South (optical) .

Also useful is to look at this camera (Blanco Prime Focus webcam) which is pointing straight up from the telescope prime focus and allows views of the sky as currently observed by the telescope.

Useful forecast pages for Cerro Tololo and weather in nearby Cerro Pachon can be found here.

Updated on May 24, 2022, 4:45 am