Testing secondary mirrors

[torana68]

Ended up buying a new secondary the other day as I couldn’t find anyone who could test my used ones (prior to recoating) . Is there a place in Australia that can?

[Merlin66]

A cheap Ronchi grating will give you a good ideal of the smoothness and flatness of the secondary.

[Adox]

Flats can be tested using a reference flat. The reference surface and the surface to be tested have to be without reflective coating. When the surfaces are one on top of the other leaving a little air gap, the Newton fringes will appear showing the error.
I am going to get a lambda/20 flat mirror soon but I cannot do this test.
I don't think there is anybody in Australia who can test flat for amateur telescopes.

[torana68]

Ok , thanks guys , guess I’ll leave the secondarys in the cupboard and just buy new ones. Not that there is much $ difference between recoating and real good new ones. Might silver them one day and try them in something, if I find time :)

[dave brock]

How?

[Merlin66]

Dave,
If you test the primary with the Rochi grating (or Foucault testing) to determine any defects, then introduce the secondary in the same test, any additional defects seen must originate on the secondary....

[Wavytone]

Ken, aha - there is a way - and it should be quite good.

Assuming you have (i) a large concave telescope primary mirror and (ii) the gear to do a Foucault or Ronchi test, set this up using the secondary mirror flat as you would in a double-pass autocollimation (DPAC) test:

Light from light source to concave mirror, reflected to the flat and back to the concave mirror then reflected back to knife edge or ronchi screen. Knife edge/ronchi should be close to the light source, and both at the centre of curvature of the concave mirror.

Although technically you need a concave parabolic mirror, the aperture of a secondary mirror is much smaller so only a small portion of the concave is used, ie it will effectively be working at a very long focal ratio. Defects in the primary such as spherical aberration won’t matter much with one exception - turned down edge - and in this respect as long as the light path avoids the edges and utilises the central section of the concave mirror, it should be happy days.

You could also test the primary beforehand to identify which areas are the best ie smooth without significant localised surface errors.

Defects seen in the DPAC test will be due to the secondary, and as only 1 bounce is used off that, wavefront error will be as measured (not half as when DPAC is used to measure a scope or primary mirror).

[ausastronomer]

Mark Suchting can test them, whether he will spend the time doing it is another matter. You would have to contact him and ask him.

Davis Collis Bird or Wayne Stewart could also do them. But the same question applies to them, as applies to Mark. Both of them haven't made optics in a while, but I am pretty sure they both still have all the gear.

Cheers
John B

[torana68]

Thanks guys :)