3 -- Measuring Your Sky Brightness from an Image
Brightness Is Critical
The sky brightness is critical to the
computations that SkyTools does. In order to get the the most
out of the model your sky brightness for your observing location
should be set up as accurately as possible. The sky brightness
is one of the parameters for your observing location. It is
measured on a dark night at/near the zenith. SkyTools uses this
as a base measure of your light pollution. Using this as a
starting point it will automatically account for other factors
such as twilight and moonlight.
1: Start with an image obtained under a dark sky
a dark night with no twilight or moonlight take an image of the
sky. The target area should be high in the sky. Don't pick an
area with lots of nebulosity.
the target object, exposure time and filter (if any).
the most accurate results record an estimate of the astronomical
seeing, temperature and relative humidity as well.
2: Subtract the dark signal
your standard image processing technique to remove the dark
signal from the image.
3: Read a typical sky vale from the image in ADU per pixel
an image processing program read a typical sky brightness value,
in ADU, of a pixel away from any nebulosity or starlight.
that most image processing programs will add an offset (or bias)
to the pixel ADU values so that they won't ever become negative.
A value of 100 ADU per pixel is typically added, but you should
verify this in the manual for your image processing software.
Subtract this offset from your typical sky brightness value.
4: Set the Exposure Calculator to exactly match the image
the SkyTools Exposure Calculator for your target object at the
observing location where you took your image.
the start time, exposure time and filter to exactly match your
the astronomical seeing, temperature and humidity to match the
conditions of the exposure.
5: Adjust the sky brightness for your observing site until
SkyTools computes the same sky brightness
the value for the sky brightness in ADU/pixel that has been
computed for the exposure by the Exposure Calculator.
Click on the observing location at the top of the calculator and
then click on the sky brightness.
Click the Advanced button.
If the calculated sky brightness is smaller than what you
measured make the sky brightness value on the dialog smaller
the calculated sky brightness is larger than what you
measured make the sky brightness value on the dialog larger
the locations dialog and note the new value of the sky
the steps above, a through c, to adjust the sky brightness
value on the dialog, changing the value by smaller
this until you get a close match between the calculated sky
brightness (in ADU) and the sky brightness on the image.
you will have a consistent value to use for the sky brightness
of your observing location.
the SkyTools Exposure Calculator
Cameras Manually to SkyTools