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# Why are ronchis sometimes blurry slitless?

30 replies to this topic

### #26 MKV

MKV

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Posted 25 January 2019 - 03:57 AM

but it doesn't elude to the quality of correction, where a ronchi can.

Even so, the Ronchi is less sensitive. Take k-e readings at the paraxial focus and the edge and you should have a good idea where you are and how much you have to go. The sagitta is D2/8R, where D is the mirror's clear aperture diameter and R its radius of curvature.  The Foucault is based on LSA, which which at COC = D2/4R; the  Ronchi is based on TSA, and TSA = sinU(LSA), where sinU is the sine of the light cone angle, numerically equal to 0.5/effective F#. Thus, for an f/3.3 mirror at COC, the effective F# = 6.6, hence TSA = (0.5/6.6)(LSA), or only 7.6%  of LSA. At COC, In real numbers, for D = 254 mm and R = 1676.4 mm, LSA = 9.6 mm, and TSA = 0.73, so the Ronchi test is 13 times less sensitive.

In short, when testing a mirror at CO,C

(1) LSA = Dz2/4R

(2) TSA = Dz3/8R2

Where Dz = the effective zonal diameter of the mirror. Clearly, for full aperture Dz ≡ D.

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### #27 Mark Harry

Mark Harry

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Posted 25 January 2019 - 06:08 AM

Good description.

### #28 Lognic04

Lognic04

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Posted 25 January 2019 - 06:55 AM

Sorry - what the hell are you guys talking about?

### #29 Mark Harry

Mark Harry

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Posted 25 January 2019 - 07:32 AM

-SEE?-

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### #30 Gleb1964

Gleb1964

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Posted 25 January 2019 - 08:22 AM

given by you equations predict infinity advantage of measuring LSA against TSA for close to axes zones.

In short, when testing a mirror at CO,C

(1) LSA = Dz2/4R

(2) TSA = Dz3/8R2

Where Dz = the effective zonal diameter of the mirror. Clearly, for full aperture Dz ≡ D.

See, if you take relation between LSA and TSA, it is LSA/TSA = 2R/Dz . With Dz approaching 0 relation tends to infinity.
What is problem with such definition?
TSA can be measured with some limited uncertainty. Uncertainty of TSA transferred with the same relation into LSA uncertainty, so the relation between measured parameter and its uncertainty is constant.
However, LSA can be measured with higher precision because additive error of micrometer would be relatively less.

Gleb

Edited by Gleb1964, 25 January 2019 - 08:29 AM.

### #31 Pinbout

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Posted 25 January 2019 - 08:56 AM

Sorry - what the **** are you guys talking about?

absolutely nothing about why ronchi's are blurry inside RoC

if you want them sharper inside roc, use a smaller aperture stop

Edited by Pinbout, 25 January 2019 - 12:33 PM.

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