Harry Dalek wrote:Andrew sorry i a bit dim but the lined up holes nipkow hole position i don't understand these look similar to Jenkins used on he's drum monitors i still can't get my head around that one unless your using a rotating square of polygon mirror for the framing
OK, first they are NOT holes. They are places for holes. So on each of the 8 slices, we have all 32 possible hole positions. We don't want 32 holes on each slice, but we DO want just a single TYPE of slice that can be used to make a complete disc. After assembling 8 identical slices, the hole markers are designed such that you can poke-out (say, with a pin) 32 of them and form a perfect Nipkow spiral. Which holes to poke out? It's easy... if you consider the radial 'rows' of holes visible in the pictures I posted, then there are 4 'rows' of holes per slice. Start with any slice, and put a pin through the innermost hole on each 'row'. Now you have 4 holes on the slice. Each of those is ever so slightly staggered in distance from the centerpoint. Now move onto the next slice. Instead of poking the innermost holes, we now poke out the holes 2nd from inermost in each 'row'. And for the 3rd slice we poke out the holes 3rd from innermost, etc. The hole positions are designed such that this process will give you a perfect spiral with holes spaced correctly.
Another way of thinking about it - the complete disk actually has eight complete spirals on it, and the positions of the holes for all of those spirals are marked on the pie-slices. Each slice has 32 holes, so 8 slices = 256 holes = 256/32 complete spirals. We need to poke out the holes for just ONE of those spirals. It so happens that the holes are perfectly spaced such that you can do this - as you start from the innermost hole on one slice, the next hole 360/32 degrees around is slightly further away, and the next 360/32 around is slightly further away again. This happens for the 4 holes in a slice, and there will be a position for a hole in the next slice in exactly the right place.
Yet another way to think of it: Take a disk with a complete spiral of 32 holes. Divide it into 8 pie slices. Put them exactly on top of each other and mark on one of them exactly where ALL the holes on ALL of the slices are. You get the 'line' pattern that I have on my pie slices. Each single slice has enough holes to allow recreation of the entire disc from just that one slice.
The reason for doing it this way is that you only need a single type of 'pie slice' to enable you to recreate an entire disc. Hope this is clearer!