Thank you, Paul. I was not familiar with the topic discussed in the link you provided. To be honest, I was thinking about some defect in the corrective plate. I had not considered the effect of atmospheric refraction. So, I understand that I probably never get a good focus precisely because of atmospheric dispersion.
I took that image with the Moon about 30° above the (astronomical) horizon and about 20° above the profile of the mountains surrounding my location. The table and graph in the link are very interesting. As for the atmospheric dispersion corrector, I don’t think my little one deserves it.
Ciao
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Supporting amateur astronomers since 1890