On Thu, 26 Aug 2004 02:05:50 GMT, Lurking Luser
wrote:

Centro de Observação Astronómica no Algarve has a little program to make
diffraction gratings with a home printer. But it has some limitations.
First
the resolution is maxed at 720 dpi and second the pattern is round
wasting a
large amount of the transparency.

I have access to a HP 2200 laser jet printer that has a dpi of 1200. I
assume that is horizontal and not necessarily vertical but that should
not
matter since you can make lines up and down just as easily as back and
forth, or for that matter set the printer to landscape. The idea appeals
to
me because it is cheap, 50 cents for an 8 by 10 inch sheet of grating,
and I
don't have to wait or pay for shipping. (Kind of along the lines of the
person who posted about frugal astronomy.)

So I have several question for the group.

1. Is the idea feasible?
2. Is 1200 dpi achievable or even desirable?
3. Would I be better off use 600 etching per inch in both quality and
defraction of starlight?
4. Are you better of putting the grating at the eyepiece or the
objective?
5. Is there a simple way or producing this pattern in PhotoShop?

Thanks in advance and clear skies,
James King


1. I've made transparency gratings in the past, with mixed results.
Using a parallel-linear pattern on a 300 DPI printer, the finest
resolution was 150 lines per inch.

When placed over the objective, this caused a small spectrum angle which
was just right for a low power telescope ( 100x). The downside is that
the lines were very coarsely drawn, smearing out the spectra instead of
allowing a desirable resolution.

Two limiting physical constraints seem to be involved:
a. the ability of the Laser Printer to deliver smooth lines
(of whatever shape) at high resolutions.
b. the optical quality of the transparency film (also quite rough).

You might be able to do better if you use an optical cement to
glue the rough side of the transparency to some relatively
smooth plate glass. The cement should fill in the hollows
of the film, thus reducing the optical smearing introduced.

A very slightly curved set of lines may provide the widening
of the spectra to where you can make out emission and absorbtion
lines.


2. 1200 DPI is probably NOT desirable, unless you have a really
wide-field scope. Beside the finest grating you can print will be 1/2 of
the max resolution. You need gaps between every line pair to transmit
light!

3. Whatever resolution you use, the limiting factors will be your
telescopes magnification and how smooth the grating lines are.

4. I like the objective grating because you can then change eyepieces
to match the magnification and field size with the diffraction angle.

5. Instead of Photoshop, I used a CAD program. A more precise way might be
to send graphics commands to the printer, but that would
require a suitable knowledge of programming.

Cheers,
larry g.


--
Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/