Re: Hanging a deco window from a plaster ceiling
Never hang anything from plaster, you're only asking for cracking and damage. Find a joist and secure it there.
Locating a joist usually isn't too difficult. If you have a light fixture, lightly tap with a hammer all the way around the fixture. The joist will produce a stronger "thunk" than an open area. Once you find which side of the light the joist is on, typical joist spacing is 16" on center. Measure out to where you want to hang the window and lightly tap your hammer again to locate the joist. To find the center you tap back and forth over the joist and note the point at which you notice a difference in the "thunk" on each side. The center point between those marks should be close to the center point on the joist.
Another method would be to not worry about where a joist is, drill a hole where you want to hang the item, then stick a long nail or screw driver through the hole. Go up into the attic and locate that spot, then span the top of the joists with a 2x4 and install all-thread rod from the 2x down through the hole to your hanger hook.
A third method would be to go into the attic and locate the joists. Take reference measurements. Transfer those measurements to the ceiling from below.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!