Sky Brightness Data

These statistics are now included in the Observing Statistics page.

OLD INFORMATION - do not trust/use!!

Sky brightness is defined in magnitudes in the given bandpass per square arcsecond. To limit the overhead in the computation of the sky brightness we will limit the HET sky brightness statistics to Johnson 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
Johnson V
0 (dark) 21.3
7 (gray) 20.4
14 (full) 18.2

The above numbers are brighter than the "normal" McDonald numbers as measured from the 30" and yield dark time numbers in the range of 21.3-21.8. The source is likely scatter from the HET mirrors.

HET Sky Brightness Statistics

We are now collecting sky brightness numbers from the LRS g1 and g2 spectra using absolute spectral photometry. This has been done since August 2008. The typical sky brightness numbers are consistent with the numbers given in the HET table above.

PI Applicable Sky Brightness Magnitudes

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 default maximum V sky brightness figure of 21.0 in your target constraints, which HET software employs to signify as fully dark.

NOTE: ALL greyed out text is outdated and should not be trusted. Updated sky brightness information is available on the
Observing Statistics page.

Last updated: Fri, 29 Dec 2023 12:08:15 +0000 stevenj


The Telescope

Technical Overview

Object Observability


Non-sidereal observations



LRS2 Summary

LRS2 Details

LRS2 Fiber Layout and Position Angle

LRS2 Throughput

LRS2 Observing details


VIRUS - Summary

VIRUS - Setting up on targets

VIRUS - Misc details

VIRUS - Throughput and sensitivity

VIRUS - low surface brightness sensitivity

VIRUS - Dithers, IFUs, Tiling

VIRUS - Tiling observations

Habitable Zone Planet Finder (HPF)

HPF Details

HPF Setting up on targets

HPF Throughput and Exposure Meter

HPF Data Reductions

HRS-2 (in development)

HRS-2 Summary

HRS-2 Details

HRS-2 configurations

HRS-2 exposure meter

HRS-2 Position Angle and Fiber layout

HRS-2 Throughput

Old Instruments

HRS - old

LRS - old

MRS - old

Program Preparation

Web Management System