MESSIER INDEX - Herschel II INDEX -
Seeing and transparency are two elements to describe the observing conditions. Seeing measures the steadiness of the air and transparency measures the darkness.
Seeing Measurement Method
The quality of the image is affected by the steadiness of the air, telescope focus and by the telescope mount's ability to track the object during the CCD camera exposure. All of these affects are measured by the FWHM (Full Width Half Maximum) of a star. The FWHM measurement is on the calibrated image and a 13 to 14 magnitude star in the image. A smaller FWHM number indicates better seeing.
Transparency Measurement Method
Transparency measures the darkness which is dependent upon the star's brightness and the brightness of the background caused by the moon, faint clouds, sky polllution, aurora, etc. Transparency measurements are the peak value of a known 13 to 14 magnitude star and the backgroud level measured on the calibrated image. A higher difference between the peak value and the background level indicates better transparency.
Astronomical Seeing by Jeff Beish
© 1998-2003 David Haworth