Sky Brightness Data

Sky brightness is defined in magnitudes in the given bandpass per square arcsecond. To limit the overhead in the computation of the skybrightness we will limit the HET sky brightness statistics to Harris V at the present time.

Theoretical Sky Brightness Data

The following table are not HET skybrightness values and represent very clear sky dark site magnitudes. The HET is currently performing worse than these numbers but we keep them listed here for reference.


Moon Phase
days from new
0 (dark) 22.7 21.8 20.9 19.9
3 22.4 21.7 20.8 19.9
7 (grey) 21.6 21.4 20.6 19.7
10 20.7 20.7 20.3 19.5
14(full) 19.5 20.0 19.9 19.2

The table above can be used to judge how lunar phase and different band passes will interrelate. The R and I band passes are dominated by the bright OH emission lines so the sky brightness is less affected by the scattered moon light.

HET Sky Brightness Data

The following table represents a very small sample (1) of HET sky brightness in the different HET filters.

Moon Phase
days from new
Harris V
0 (dark) 21.2
7 (gray) 20.4
14 (full) 18.0

The above numbers are brighter than the "normal" McDonald numbers. The source of this is still a mystery at the present time.

HET Sky Brightness Statistics

At the present time we do not collect sky brightness values in the night report. This may change in the near future.

What Magnitudes the PI should use

The PI should base their sky brightness criteria on the actual HET sky brightness values not the theoretical values.

It is very important that the PI's remember that with slit and fiber spectroscopy the size of the slit or fiber must be considered. For example, the 3" slit will have 7" of area over which to collect sky thus the affective sky brightness in the fiber aperture will be 2.1 magnitudes brighter. So if a PI wants only a 1% contamination from sky for a 14th magnitude object with the 3" HRS fibers then the sky must be

14 (object mag) + 5 (1 part in 100) + 2.1 (skyfiber size)

21.1 magnitude per square arcsecond. This means the object would not be observed in bright time. The PI could reduce the contamination a bit by using a smaller science fiber 2" has only 3.1" of collecting area but also will collect a less flux from the science target.

Also please keep in mind the actual HET sky brightness numbers measured above are brighter by about a magnitude than the theoretical numbers. Please do not exceed the maximum possible sky brightness magnitude as your target will never be observed.