DECam Flat Fields

Dear DECamers, 

After a very busy, very eventful 5-nights on the mountain this week, with Andrea's expert help for our non-NOAO visitors, we've made a lot of progress this week; not least among our successes, are the psuedo-final settings for the flat-field projector (shown below) settings.

DECam will be calibrated by the DECal system. Designed and constructed by our friends at Texas A&M University (Darren Depoy, Jennifer Marshall, Jean-Philippe Rheault and formerly Jason Wise), and with me acting as CTIO's local "expert", the system consists of three main components:

1. An LED projector cone
2. A photodiode (and thermocouple) 
3. A fibre projection unit

One of DECal calibration units, which total four in number, prior to its installation 

of the top ring of the Blanco 4m telescope. The large black cone houses the seven LED 

lamps that shine onto the flat-field screen inside the dome. The medium-sized black-coloured 

box houses a photodiode and thermocouple assembly, while the small black unit (on the right) is 

the fibre projection unit. 

Each of the four LED projection units consists of seven LEDs, which for are designed for flat-field and internal calibration purposes, are powered contemporaneously. The emitting light level of each LED can be adjusted independently, allowing us to set the flux level of flat-field data in each filter per unit exposure time. The constraining parameters for our "ideal" exposures are for a mean flux of about 15k ADUs above BIAS across the array in a minimum exposure time of 10-seconds. 

Through empirical experimentation, afternoon dome flat-flats are now routinely obtained using the following parameters: 

           LED#                 Peak                   LED 
                                Wavelength             Power

• LED1:        6500 Angstroms             5% 
• LED2:        3650 Angstroms         100% 
• LED3:        7800 Angstroms             5% 
• LED4:        9700 Angstroms            8% 
• LED5:      10300 Angstroms          75% 
• LED6:        9050 Angstroms            8% 
• LED7:        4700 Angstroms          15% 

         Filter:       Exp              ADUs

                          Time        above BIAS

•     u'       15-secs         ~ 13k 
•     g'       30-secs         ~ 14k 
•     r'       10-secs         ~ 17k 
•     i'        22-secs        ~ 17k 
•     z'       10-secs         ~ 19k 
•     Y       10-secs         ~ 17k 

 

For those of you with access to the archives, a good set of flat-field calibration data are to be found in files #136529 -- 136584. 

a la belle etoile, 

David James,                        

djj@ctio.noao.edu

Dark Energy Camera Calibration Team [DECal],

CTIO Staff Astronomer.