SkyTools
3 -- Measuring Your Sky Brightness from an Image
Sky
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.
Step
1: Start with an image obtained under a dark sky
On
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. Record
the target object, exposure time and filter (if any). For
the most accurate results record an estimate of the astronomical
seeing, temperature and relative humidity as well.
Step
2: Subtract the dark signal Use
your standard image processing technique to remove the dark
signal from the image. Step
3: Read a typical sky vale from the image in ADU per pixel Using
an image processing program read a typical sky brightness value,
in ADU, of a pixel away from any nebulosity or starlight. Note
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. Step
4: Set the Exposure Calculator to exactly match the image
circumstances Open
the SkyTools Exposure Calculator for your target object at the
observing location where you took your image. Set
the start time, exposure time and filter to exactly match your
exposure. Set
the astronomical seeing, temperature and humidity to match the
conditions of the exposure. Step
5: Adjust the sky brightness for your observing site until
SkyTools computes the same sky brightness Note
the value for the sky brightness in ADU/pixel that has been
computed for the exposure by the Exposure Calculator. a.
Click on the observing location at the top of the calculator and
then click on the sky brightness. b.
Click the Advanced button. c.
If the calculated sky brightness is smaller than what you
measured make the sky brightness value on the dialog smaller
by 0.5. If
the calculated sky brightness is larger than what you
measured make the sky brightness value on the dialog larger
by 0.5. Close
the locations dialog and note the new value of the sky
brightness. Follow
the steps above, a through c, to adjust the sky brightness
value on the dialog, changing the value by smaller
increments. Do
this until you get a close match between the calculated sky
brightness (in ADU) and the sky brightness on the image. Now
you will have a consistent value to use for the sky brightness
of your observing location.
Related
Topics:
Using
the SkyTools Exposure Calculator Planning
Imaging Sessions Adding
Cameras Manually to SkyTools |
