The used teleobjective is apochromatic and it is quite convenient for astrophotography. However, I have discovered that the field is not flat enough. Although I cropped the image the distortion is clear. As Miguel commented, it is an empirical assessment about something that astronomers knew long time ago: you must use a telescope, i.e. a large optical length to achieve a flat field close to the axis.... It has been sacrificed in standard photography... so, forget to use all the photographic stuff for astro!!!!!
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