How do you create a lum layer mask in PS

[Jessun]

Hello, in PSP4 it comes up as an option straight away to create a luminance layer mask if I remember correctly but I wonder how to do it easiest in Photoshop?

The idea would be to catch a galaxy or Trapezium etc in some simple way.

Greatful for any input.

[Spikey]

Copy your colour image and de-staurate it, then apply as a layer blend mode luminance?

[Jessun]

Thanks for the tip Simon, will give it a go one rainy eve.