Re: Null corrector for a fast mirror
- Thanks for the offer Paul. I can do that with Oslo. It's an area I have wondered about.
With a moving source tester I could get the source knife offset down to under 6mm. Going on comments by Texereau about his tester design even a larger offset should be ok - with a mask.
One aspect I am not clear on is if the orientation of the knife/source in relationship to the mask has any effect. The easy one of course is the vertical knife source arrangement.
There are alternatives. A paper on the Nasa database points out that the source can be in front of the knife and on axis eg intuitively if the source was 150mm in front of the usual position the return image would be 150mm behind it. Unfortunately it doesn't give any maths for making use of the technique. Pity really as tiny sources are easy these days. Spread sheet type maths for calculating this sort of thing are also strangely absent. Plenty on lenses but zero as far as I'm aware on mirrors.
--- In firstname.lastname@example.org, paul valleli <paulvalleli@...> wrote:
> Last fall, I simulated a point source offset from the OA of a spherical
> mirror to determine the amount of astigmatism at the offest knife edge of
> the classic Foucault Test.
> There was a divergence angle in radians that I put in the setup parameters.
> I would have to look it up. It required a coordinate break at the mirror.
> I'll look it up if you wish.
- For a complete solution source anywhere I think the sag at h is needed which can probably be derived from that link. :-) Not sketched anything out so a pure guess. From memory Sixtests allows the mirror to source distance to be specified but that may be just to reduce errors from normal Foucault readings. I don't think it allows offsets to be specified which are another source of error apart from any aberration effects.
Where some views of aberrations go astray is that when a mask is used the aberrations of the whole mirror may not be important. Instead it's the size of the circles of confusion produced by a series of small off axis mirrors in relationship to their diffraction disc size.
I also suspect that there is another test that could be used these days but no sums done to check how large a sensor is needed for good say 1/20 wave results. Hartman, if that is how it's spelled. That one would just involve taking an exposure with the lens off a dslr, moving it a specific distance along the axis of the mirror and taking another.
--- In email@example.com, "Richard F.L.R. Snashall" <rflrs@...> wrote:
> On 5/12/2013 6:06 PM, John wrote:
> > So what is the equation for the exact slope of a conic versus height?