IMHO, a vector-based program such as Adobe Illustrator or Corel Draw
would be better for making a diffraction grating.
Vector-based graphics are much more scalable than raster graphics

Matthew Ota

I think a CAD program is probably best, they are designed to print exactly to
the page and setting the line spacing is super easy. If everything is set
correctly, aliasing should not be a problem.

But the limitations of the printer are significant, dots are not squares so the
lines may have quite rough edges. 1200 DPI does not necessarily set the dot
size at 1/1200th of an inch, especially with inkjet printers. It just means
that there are 1200 dots of some size, probably greater than 1/1200th....

jon