The Danubia Moon filter is suitable for use with the Meteor 31 on the 1 inch eyepiece and reduces the brightness of the moon, so it is more comfortable to view through the telescope. It is similar to a Neutral density filter, as it reduces the brightness of the moon and makes it much easier to view. It screws to the eyepiece and offers higher contrast and better detail and will aslo reduce the amount of time needed in adjusting night vision between the moon and stars for example.