Calculator for Determiming the Signal-to-Noise of a Star/Pixel in a CCD Image

Use this calculator to make your life easier! Enter values measured in your image into the form below. Click the "Calculate S/N" button. The input variables, and the calculation itself, are explained in the document Measuring the Signal-to-Noise Ratio S/N of the CCD Image of a Star or Nebula. In that document (and in this calculator) S/N is calculated as

where the variables are listed below.
Enter the values of the input variables, as measured in your image:

= = number of counts (ADU) in a box containing the light from the star (or in one pixel in a nebula)
= = number of pixels in the on-star box (=1 for one pixel in a nebula)

= = average pixel value in a sky background box, off the star or nebula
= = rms pixel value in the sky background box
= = number of pixels in the sky background box:

= = gain of the CCD (in units of electrons/ADU)

Resulting S/N =