Filter Focus?

[orestis]

Hi guys:hi:,

i thought this was the right Forum to ask you guys about my filter.

Well the filter is a 80a blue filter and when i have my eyepiece focused on Jupiter and then insert the filter i have to refocus.My question is why that is.

Thanks in advance
Orestis:thumbsup:

[mithrandir]

Light changes direction when it enters the filter (closer to perpendicular) and changes back to the same direction but with an offset when it leaves. That moves the focal point further out in the focuser.

This is exactly the same reason why a refractor works. The difference is that the two faces of the filter are flat and parallel, and in a refractor they are curved which provides the magnification.