|
Chris - a step down adaptor (or "ring") is device to let you put a smaller filter on the lens - male thread on one side and female on the other. They could be 77mm to 67mm. H is just suggesting you use this as an external mask.
An external mask is just a piece of something (can be cardboard) with a hole cut in it - think of it like a Bahtinov mask that has a hole in it, rather than the grid. The aim is to have a circular hole for the light to pass through, rather than the geometrical shape (straightish lines) that the inbuilt aperture blades create. These straightish lines can create diffraction spikes on your final image. You vary the size of the hole to vary the "aperture" of the lens.
I saw an article in S&T about using a hexagonal mask on a telescope to allow you to better see a double star where one star is 10-100x brighter than the other.
DT
|