What is vignetting?
Vignetting is a reduction in image brightness at the edges of the field of view. There are two possible causes for this, the lens/mirror design or an obstruction in the light path.
Telescopes bend the light that passes through them, forming a cone of light that forms a circular image at the focus point but there is always a brighter central region in the cone leading to light fall-off towards the edges.
Anything that intrudes into the light cone will attenuate the light so telescope designers go to great lengths to produce focusers and adaptors with wide apertures.
If you’re finding errors in your field of view, read our complete guide to optical aberrations in your telescope and how to fix them.
Email Steve your astronomy queries to firstname.lastname@example.org and they could be answered in a future issue of BBC Sky at Night Magazine.