Some basic questions about Focault and Ronchi testing.

[kbrown]

I've never done a Focault knife edge or Ronchi grating test but I'd like to learn how to do both in the view of maybe one day grinding my own mirror(s). I've read a lot about the subject online and in books and I've now decided to make a tester so I can start experimenting with my existing 10" F/4.7 skywatcher mirror. However there's a few basic questions I'd like some clarifications on:

1. Some sources say the light source should be cut by the knife edge or go through the ronchi grating and then reflect back from the mirror. But I swear I've seen designs that have the light source to the side or above or below knife/grating. What's the deal here?

2. If the light source is not cut by the knife edge or by the grating, should it be behind a narrow slit, like I've also seen in some cases?

3. Some designs use laser diode without the collimating lens as a point light source. Surely this doesn't need to be cut by the knife edge or the grating or even be behind a slit? I am aware that this is not suitable for visual testing especially if the diode is actually lasing...

Edited October 23, 2020 by kbrown

[skybadger]

Foucault designs started with two half slits, one for the source and one for the sensor., one fixed, one moving independently to cut the return cone to detect the focal point for each coude zone. 

Then a bright spark realised that the same slit blade could do both, if the two positions were side by side or on top of each other, removing alignment problems. This further simplifies if the source is small enough to not need a slit. This arrangement is the moving slit because the source and slit a re on the same stage, using the same blade. 

Check out the stellafane web pages for a much better description. 

The problem with ronchi is it's qualitative while with Foucault it's very much quantitative. Typically glass pushers might use simulated  ronchi patterns as comparisons to grind towards. While Foucault and the bath interferometer will tell you exactly where you are. 

I'd recommend if you are starting out to take a look at the bath. It's easy to build , quantitative, has a support community and easy to use. On the other hand, Foucault and ronchi will give you lots of practice aligning optics. 

 

[johninderby]

Interesting read on the subject of making your own testing setup.

https://stellafane.org/tm/atm/test/tester-1.html

Edited October 24, 2020 by johninderby

[kbrown]

2 hours ago, skybadger said:

This further simplifies if the source is small enough to not need a slit.

Am I correct you still need a slit in front of the sensor?

[skybadger]

I ...think... so. You need something to cut the cone of returning light  to measure the zones.

I have two Foucault devices, one from the stone age with a separate slit source and blade and a moving source one on a tripod stage.  The second is much easier to use .