Need a grid of 100 lines per inch

[legswilly]

Hi, I need a Ronchi screen of 100 lines per inch. That is vertical lines of alternating black and no colour, the lines of which are 0.0025 inch wide. This would give 100 black lines per inch.
Could someone show how to do this, or even better, make a file that is for an A4 paper filled with the screen and I could get it printed on transparent film. I have a laser printer which I could try.
I would also post the file on a telescope maker, astronomy forum, if that is OK.

Please let there be a person that has pity on me.

[rich2005]

Gimp is a bitmap (raster) editor and works in pixels, not real world units even though it can display rulers, sizes, in inches / centimetres etc.

The minimum unit is therefore a pixel and you want that to print at 0.0025 inch. For 100 of those per inch I make it 1 line + 3 spaces.

In Gimp you might make it like this.

Make a small image 4 pixels depth x (say) 4 pixels wide (1 pix wide would do) Use the pencil tool for hard edges, there is a tiny brush in the standard Gimp brush set. Set the width to 1.  screenshot: https://i.imgur.com/yZFGGjy.jpg
Ctrl-c copies that to the clipboard.

Create a new image, and start with a A4 template. This is set-up for 300 ppi printing so expand the  'Advanced Options' and increase the ppi to 400.   https://i.imgur.com/2D2lzHo.jpg

Now fill that new A4 size image with the little pattern you just made. You can click and drag from from the clipboard pattern (first pattern)  https://i.imgur.com/Pab8l5Q.jpg

Printing: Using linux, all depends on the printer. If it imposes margins, then you will get a shrunken A4 smaller than required. The usual fix for this is crop off maybe 5 - 6 mm all round so it prints 100% size. As long as you can get 400 ppi and not 407 ppi which often happens.  https://i.imgur.com/IXeKFSr.jpg

There is a whole page on the importance of printing resolutions here: https://www.gimp-forum.net/Thread-Image-size-in-Gimp

Edit: Oops, Not paying attention. Vertical lines required. Exactly the same but with a vertical 1 pixel line in the pattern. Make sure you use the pencil tool.