Astrography under a good astronomical seeing is the main desire for astronomers all around the world. Seeing is the most important factors for DSO or planetary image quality. Practically all professional observatories are located on locations with sub-second seeing, where seeing values are bellow 1 arcsec, but amateur astronomers are doing astrography under the sky where is the common seeing between 2 – 5 arc seconds. What does this mean in the practice? Seeing conditions can be estimated by several types of scales e.g. Antoniadi Scale, Mt. Wilson Seeing Scale, Pickering scale, Tombaugh/Smith Double Star Scale, Environment Canada Scale, etc. One very popular is the Pickering scale which is divided into numbers from 1 to 10. Number 1 shows a very poor seeing and number 10 shows excellent seeing conditions. However, measuring of angular sizes of the almost pointy stars is more professional than measuring seeing by Pickering scales. For example in astrography, seeing can be measured in arc seconds with FWHM (Full Width Half Maximum) method with an appropriate software. FWHM is determined in arc seconds where higher value of FWHM means worse observing conditions.
Let’s see one practical example. At our ZWO observatory we have made one simple test, where we have been capturing a series of Messier 106 (Messier 106 is an intermediate spiral galaxy in the constellation Canes Venatici.) images with 30-cm astrograph and ASI1600MM-Cool and LRGB filters. The image resolution was 0.65 arcsec/px and exposure of a single image was 200-seconds. We analyze all of our images with a software and got very interesting values.