Exposure Time Estimates

S/N vs. Magnitude Plots

Note: the saturation limits shown in the following plots are rough estimates, assuming a star is centered on a pixel and assuming the CCD pixels have a 130,000 e- full well. It may be that somewhat brighter stars can be observed without saturating.

• PreCam Survey target exposures

• 5-sec exposures

• 10-sec exposures

• 30-sec exposures

• 60-sec exposures

• 90-sec exposures

• 120-sec exposures

• 180-sec exposures

• 360-sec exposures

• 600-sec exposures

PreCam Count Rates

The following count rates (and the above plots) were based upon the following:

• The (now slightly outdated) DES filter response curves from DES-doc#775.

• An assumption of a 0.24m^2 effective collecting area for the Curtis-Schmidt

  • 0.6m^2 effective aperture and 15% obscuration

• 1.43 arcsec pixels

• A moon-less sky with (AB magnitude) sky backgrounds of:

  • g=21.7 mag/arcsec^2

  • r=20.7 mag/arcsec^2

  • i=20.1 mag/arcsec^2

  • z=18.7 mag/arcsec^2

  • y=18.0 mag/arcsec^2

mag=20 point source Sky Background

Filter e-/sec Filter mag/arcsec^2 e-/sec/pixel

g 13.21 g 21.7 5.6

r 13.25 r 20.7 14.1

i 12.26 i 20.1 22.8

z 8.92 z 18.7 60.2

y 2.12 y 18.0 27.3

For a mag=20 point source:

• Multiply the “mag=20 object” count rate by 0.93738 for an aperture of radius = 1.0 FWHM (2.0 arcsec for PreCam).

• Multiply the “mag=20 object” count rate by 0.37968 for an estimate of the count rate in a single pixel (assuming the point source is centered on the pixel’s center). This is useful for saturation limit estimates.