Quote:

---

Steve: I would point out that the color produced in the diffraction pattern of a mask is not due to chromatic aberration, but is a product of diffraction itself.

---

Thanks for the comments Steve. I was also a Physics teacher, which probably explains why I could not rest until I had figured out exactly how the Bahtinov mask worked.

When I mentioned spherical abberration I was not referring to the colours seen in the spikes as you move away from the central diffraction blob (the zero order). As you say, those are a product of the diffraction. What I meant was the fact that when spherical aberration is present, those colours will be diplaced laterally from the spike. My reflector always show the colours in a neat straight line, but the refractor shows the red displaced to one side and the blue to the other. This is because they are brought to focus above and below the green position. The effect is only visible at short exposures like 1/40 sec, but it is a useful test to see if a refractor really is 'APO'.
This shows the effect

Note how the blue is shifted upwards and the red downwards.

As for the calculations of slit spacing I am not too sure. Closer spacing and more slits will cause the diffraction orders to be spread further from the centre. The Russian website where Bahtinov first showed his mask goes into detail about spacing, but I think that is only important if you use the 2nd or 3rd order to focus with, using a short exposure. Most people just use the elongated spike with a longer exposure.
Very narrow slits are hard to cut by hand, and even the laser cutter has problems because heat tends to make plastic melt and distort.
I am hoping someone will give my design a try. It needs a longer exposure and a fairly bright star, but the results seem very good.

Edited by GeorgeFC (04/01/09 04:18 AM)

Post Extras