In article <5bfra0$1oa@hpwin055.uksr.hp.com>, dierick@cygnusx1.belgium.hp.com says...

>I would like to find which type of Nikon focus screen I need for an
>F3HP which gives me a clear images for astrophotography.
>I know for Canon F1 there is a I type screen which gives a fully clear
>spot in the center, and I would like to know what Nikon has to offer
>here.

There are a couple of F3 screens with clear centers, the "C", with
a 4mm clear circle and a cross-hair (for aerial parallax focusing
of micro or astro subjects) in a matte-screen field; and the "M",
with a 5.5mm clear circle with double cross-hair, in a matte-Fresnel
field with a vertical and a horizontal line with mm marks.
Hope This Helps