Exposure Time Calculator (ETC) and Overheads

A new version of the Exposure Time Calculator (v7) is available (March 2016).

  • DECam_ETC-ARW-RCS7.xls: This version of the spreadsheet works only in Windows excel and allows interpolation of the sky brightness given an age of the Moon
  • DECam_ETC-ARW7.xls: This version works in all systems (including OpenOffice) but requires to enter manually the sky brightness from a table

Follow instructions written at the beginning of the spreadsheet.

New in v7:

  • An error in the calculation of the CCD read noise contribution to the total S/N was corrected (thanks to Nigel Metcalf (U. Durham) for pointing this out). We have also taken the opportunity to put a better (lower) value for the read noise. Read noise is only a factor in the total S/N for faint objects in images taken with bluer filters in dark skies with short exposures. For example: for 20s exposures, no Moon, Mag AB = 21

ETC Ver 6 (incorrect):  S/N, u,g,r,i,x,Y  = 17, 43, 32, 23, 12, 3.7
ETC Ver 7 (correct):  S/N u,g,r,i,z,Y = 9, 38, 31,23,12,3.6

  • Several minor descriptive (non-code) corrections.

New in v6:

  • Revision of the quantum efficiency of the CCDs. Major changes from previous version are in the g band which changed from 70% to 59% (see Estrada et al, 2010, SPIE 7735)
  • Change of the center wavelength in the u band from 355 to 375 nm. This causes a minor effect only.
  • The telescope primary reflectivity changed to be closer to real-world measurements, not fresh aluminum


One of the nicest features of observing with DECam is that overhead times are kept to a minimum. The telescope starts slewing to a new position while reading out the CCDs. Overhead estimates are as follow:

  • The largest between readout time (20.6s) and slew time (a function of the slew angle, see plot)
  • 5-8s for hexapod movement, filter change and others

No overhead is produced by the acquisition of guide stars since that is automatically done within the first 30s of each exposure.


Updated on May 28, 2021, 9:56 am