Quote:In your case it's the f/4 that's at the heart of your problem. That's why even some top mirror makers will not produce f/4 but will make mirrors no faster than f/4.5, even though they have test equipment.
Mike Lockwood - Owner, Lockwood Custom Optics. 20" F/3 MX Starmaster, 14.5" F/2.55 self-built Newt., nine self-built scopes, 4.25" to 30" http://www.Loptics.com/
Quote:f/4 is no problem...
Quote:No doubt a cheap, simple, easy to use null test for parabolids is the holy grail :-).I would completely trust Mike ability/skill to Foucault test fast mirrors.... comes with extensive experience. The thing is, I doubt *mine* :-). Can I tell the difference in a zone null with the steep slopes of a f/4, within 1-2 thousandths?.... not so sure... :-). I like the Ross null as a gut check and as a qualitative way to put the initial 5 or so waves(!) of correction in. In conjunction of course with the Foucault. I think the Ross test is a fantastic tester for more "reasonable" ATM mirrors (e.g. 14 f/5 and such).
Quote:Ofner null, can be read about in Reflecting Telescope Optics II, manufacture, testing ......... on google books. It hardly more complicate than the Ross and just needs another lens but does account for higher order aberrations so is suitable for faster mirrors.
Quote:The only requirement is to set the lens to mirror distance very, very accurately, and keep it that way. One measurement, versus hundreds for a Foucault test.
Quote:Now that the construction and other issues of the Ross are handled, one of my favorite bench tests to analyze a null, is the bench star test, and that should be useful in the Ross. Anyone tried it?
Quote:I was hoping to find a way "around" it by fixing the KE to lens distance and then move the whole stage back and forth to get the best null I can as I work the mirror (just like if it is was a sphere).
Quote:Well, coming from someone who professionally specializes in under-f/4 mirrors of up to 60-inch diameters (and won't even touch anything smaller than 14-inch disks), I'd say that for your experience and tooling it's no problem for sure!
Quote:On the subject of the Foucault, I am curious, since you deal with much larger optics than commonly encountered among amateurs, why do you think the professional community abandoned the Foucault way back in the 30's for their large observatory mirrors and bothered to devise alternative testing methods?
Quote:I would completely trust Mike ability/skill to Foucault test fast mirrors.... comes with extensive experience. The thing is, I doubt *mine* :-). Can I tell the difference in a zone null with the steep slopes of a f/4, within 1-2 thousandths?.... not so sure... :-).
Ed Jones ........... The best data is an interferogram, the best analysis is from an honest optician.
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Quote:No the KE/lens air space is the more critical airspace in the Ross. For example a 20 inch f/4 set up with a Ceravolo null lens has .015 wave nominal wavefront test error. If you would make a 0.05 inch error in the mirror/lens airspace and find focus with the KE then the wavefront error increases to .23 waves. However a 0.05 inch error in the KE/lens airspace results in a .54 wave front error.
Quote:As far as I am aware the only feasible way for most people to measure the radius of curvature is via a knife edge anyway
Note I said KE/lens airspace not Ronchi. Where there are no bands with a Ronchi is at focus and that's where you want the KE to be a null, then you can leave the lens fixed and put the Ronchi where you want. Perhaps I should have said lens to focus airspace. You can set it up either way but the focus/lens airspace has the tightest tolerance and the one I would want to know is most correct.
I don't use this test often but I have used it in the past. My preferred test is the Offner.
Quote:Why Offner? More glass, more spacings..