I've been focusing astrophotos since 1997. If you mean a focusing screen internal to the camera I can't see how that would help when you are looking at the night sky. I had a K1000 with split ring focus aid, the focus screen was useless in the dark.
What I use is a Bahtinov mask from this company:
Bahtinov Mask by Focus-Mask
I have two, $24.95 for my refractor and $34.95 for my SCT. The smallest one they make >might< even work on a lens or you can make a smaller one from plans.
Unlike ALL methods I tried in the past, the Bahtinov:
1. Tells you the direction to go. No more guessing one way to see if it is right, then having to start over the other way if you guessed off wrong. Think about it, that is how we manually focus cameras during the day. Turning back and forth to find the best focus!
2. Tells you when you are already focused. Other methods require you to unfocus to discover you were already at best focus. I check focus on occasion throughout the night. With the Bahtinov mask if the cross bar is centered I'm done without touching the focusser!
If you are on a tight budget Focus-Mask lets you download their actual layout so you can make your own. But, the black plastic ones they sell are hard to beat.
Once I tried black I never went back to the old way of focusing astrophotos.