There are a number of techniques used to determine when an image is in focus. A common
technique is to calculate the Full-Width Half Maximum (FWHM) of the star. The FWHM of a
star will get smaller the closer it is to critical focus. Here’s a MaxIm screenshot showing two
images of the same field of view. The image on the right is closer to focus than the image on
the left
Note the two yellow highlighted fields in the Information Window. In the Autoguider image to
the right, the maximum pixel value inside the blue rings has a count of 3380. The FWHM is
1.817 pixels. In the out of focus image to the left, the values (not shown) are a maximum of
910 counts and FWHM of 4.655 pixels. The total amount of light recorded from the star is the
same, but the energy is spread out over more pixels in the out of focus image.
Note:
The Inspect tab next to the Focus tab can help you achieve focus by
analyzing the area around the brightest point light source in an image. Review the
MaxIm LE online help for additional details about focusing.
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