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Sky Brightness Measurements

This page contains analysis for Sig-R3 and Sig-R4.

See some further study of when Standards can begin by Douglas, at Standard_Star_Evening_Twilight_Tests

General Information

Initial estimates of sky brightness:

Filter mag per sq. arcsec Moon Alt
g 22.27 -2.12
r 21.12 -1.67
i 20.13 -1.28
z 19.12 -0.74
Y 18.59 18.

These are based on data obtained on the first night of SV.

Moon Illumination and Separation from SV Fields

This figure shows the Moon's illumination fraction and separation from various SV fields during the SV dates. These calculations all corresponds to 11pm local time.

Moon Illumination and Separation from Fields vs. SV Date

Sky Brightness vs. Moon Separation and Illumination

Several fields were observed during bright time to measure the sky brightness in the izY filters as a function of moon separation angle. The observations on November 20, 22, and 24, 2012 were obtained when the Moon Illumination was 55%, 75%, and 90%, respectively. These datapoints are shown in the figures below. The dashed, horizontal lines are the measured sky brightness in these filters in dark time. The main results are:

  • The sky is at least 0.5 mag brighter (per square arcsec) in izY when the moon is up and at least 50% illuminated.
  • The sky becomes noticeably brighter (by 0.5 mag) within about 45 degrees of the moon (about 1 mag brighter than dark skies).

Sky Brightness vs. Moon Separation and Illumination

-- Paul Martini, 26 Nov 2012

Sky Brightness vs. Time

Below are plots of the sky brightness (in DN/s/pixel) in each filter as a function of time since sunset. The approximate time of 18 degree twilight is shown, along with the median (and 4x the median) dark sky rate. For the ugr filters these calculations are only based on data obtained when the moon was down. The red points are based on data obtained at HA within 2 hours of the meridian, while the blue points are based on data obtained at larger hour angle. All of these data points are based on observations from the first two weeks of SV.

u-band Sky Brightness vs. Time
g-band Sky Brightness vs. Time
r-band Sky Brightness vs. Time


Sky Brightness vs. Sun Altitude

Here are plots for izY as a function of the Sun's Altitude. The sky brightness correlates better with the Sun's Altitude than minutes after sunset. These plots indicate that the sky is within 2x the dark sky value in (i, z, Y) when the Sun is approximately (-15, -12, -10) degrees below the horizon.



-- Paul Martini, 2 December 2012

OBSTAC sky model fits

Here is some analysis from Eric Neilsen from an email on 8 December 2012:

Model magnitudes for the dark sky at zenith:

Filter mag/arcsec2
g 22.09
r 21.21
i 20.10
z 18.95

The levels are divided into "wide", meaning that the measured mag/asec^2 is within one of the model dark zenith value, "SN" meaning that the mag/asec^2 is within two or the model dark zenith value, and "neither" when it is more than two mag/asec^2 from the model dark zenith value.

Model refit to observations
Original model refit to observations

Requests for Special Observations

Night 1 SV_11_1

Please observe these two fields and three standards at the end of the SV time (about 1am local time):

Field MoonAngle Airmass RA DEC
Moon15 15.3 1.69 04:05:00 +15:00:00
Moon22 22.6 1.44 03:45:00 +09:00:00
SDSSJ0200-0000 49.6 1.16 02:00:00 +00:00:00
SDSSJ0320-0000 33.1 1.22 03:20:00 +00:00:00
MaxVis 82.2 1.64 06:30:00 -58:45:00

All scripts should take about the same amount of time. The three standards can be the standard star observations for the end of the night.
Note the Moon Altitude is 22.6 degrees at this time.

Script files for Moon 15 and Moon 22

sv_moon15.json
sv_moon22.json

The other script files are listed here, under photometric standards: Calibration Observations

Night 20 SV_11_20

The Moon is at RA = 22:15:49 Dec = -04:34:51 and is 54% illuminated.

Please observe these fields right after the end of twilight (about 2200 local time):

Field RA Dec Altitude Note
Zenith 00:30:00 -29:00:00 90 40 deg from Moon
20deg 23:15:00 -19:00:00 72
10deg 22:45:00 -12:00:00 62

Observations in izY (20s each) would be most useful. Please observe in all six filters if time permits.

These observations step from zenith toward the moon along approximately constant azimuth angle.

Night 22 SV_11_22

The Moon is at RA = 23:52:08 Dec = +04:31:39 and is 74% illuminated (10pm local time). Altitude = 54, Az = 345.
Please observe these fields right after the end of twilight (about 2200 local time):

Field RA Dec Altitude Note
Zenith 00:30:00 -30:00:00 90 36 deg from Moon
20deg 00:10:00 -15:00:00 74
10deg 00:00:00 -05:00:00 70
90deg 04:30:00 -75:00:00 37

Observations in izY (20s each) would be most useful. Please observe in all six filters if time permits.

Night 24 SV_11_24

The Moon is at RA = 01:26:43 Dec = +12:38:44 and is 89% illuminated (10pm local time). Altitude = 45, Az = 18.
Please observe these fields right after the end of twilight (about 2200 local time):

Field RA Dec Altitude Note
Zenith 00:30:00 -30:00:00 90 45 deg from Moon
20deg 01:10:00 -07:00:00 65
10deg 01:20:00 +03:00:00 55
90deg 04:30:00 -75:00:00 37

Observations in izY (20s each) would be most useful. Please observe in all six filters if time permits.