Focussing Masks

[tim26792]

Hi,

Could someone tell me what the purpose of focussing masks are?

[Demonperformer]

They are aids to help you achieve perfect focus.

For example, a simple Y-mask, creates a diffraction pattern - an X with another bar passing through it. As you change focus this other bar moves. When it exactly bisects the X, you have achieved perfect focus.

A lot easier than trying to determine when the dot is at its smallest.

[tim26792]

ok thanks for that

[ollypenrice]

Important to undrstand that focus in imaging is far more critical than in visual observing. Personally I still think that FWHM measurement can refine what is possible in a Bahtinov mask.

What you absolutely cannot do is get an imaging level focus by looking at a magnified stellar image on a laptop screen. It simply is not good enough.

Olly