(c) Skyhound

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