Author Topic: Help for coudermask  (Read 72 times)

riaherrvodo

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Help for coudermask
« on: December 28, 2017, 03:45:49 AM »
When I calculate zones with coudermask, there is a clearance between zone 0 and zone 1.
Please see attached picture, outer radius of zone 0 is 40.42, but inner radius of zone 1 is 63.07.

That is 23mm big area which can not be checked for coudermask.

Why? and how to deal with that area?
Thanks.Attached Thumbnails




Thomas Homer

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Re: Help for coudermask
« Reply #1 on: December 31, 2017, 04:23:45 PM »
It may not matter, as long as the other zone measurements are good, and the mirror has a smooth surface thru that empty spot.

That being said, Texereau, when he explains the Couder Screen, he says, "The zones immediately adjoin each other, and the squares of the radii of successive zones differ always by approximately the same quantity. The zones therefore decrease progressively in width from the center to the edge." pg 82

On page 83, he talks about the width of each zone, and not wanting to end up having a center zone too large.  Then, "having decided on the width of the outside zone, we may then calculate the successive outside radii of the other zones, using the simple law that the squares of the outside radii diminish always with a constant quantity." He mentioned not having any zone less than 5/8" wide, or 15 mm.

The inside edge of one zone is the outer edge of the next inner zone. Maybe you did some of the math wrong(?)

quetafulra

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Re: Help for coudermask
« Reply #2 on: January 01, 2018, 02:30:46 AM »
You might like Nils Olof's Couder calculator better: http://web.telia.com...ouder/zcalc.htm

Kyle Johnson

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Re: Help for coudermask
« Reply #3 on: January 03, 2018, 10:27:01 PM »
Quote
You might like Nils Olof's Couder calculator better: http://web.telia.com...ouder/zcalc.htm

Olof's Couder calculater is derived from Texereau.

enmofinwins

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Re: Help for coudermask
« Reply #4 on: January 10, 2018, 01:13:16 PM »
Yeah but it's easier to use.

leypelepha

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Re: Help for coudermask
« Reply #5 on: January 11, 2018, 03:10:53 AM »
Quote
When I calculate zones with coudermask, there is a clearance between zone 0 and zone 1.
Please see attached picture, outer radius of zone 0 is 40.42, but inner radius of zone 1 is 63.07.

That is 23mm big area which can not be checked for coudermask.

Why? and how to deal with that area?
Thanks.

That's because the ctr zone is always abnormally large. It really doesn't hurt to trim it down a little.

All zones should be equal area. That leaves the ctr huge.

Whatever it does bother me.

https://youtu.be/14QvmUgZcMk

grateganir

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Re: Help for coudermask
« Reply #6 on: January 11, 2018, 07:34:21 AM »
Quote
That's because the ctr zone is always abnormally large. It really doesn't hurt to trim it down a little.All zones should be equal area. That leaves the ctr huge.Whatever it does bother me.https://youtu.be/14QvmUgZcMk
All zones should be equal area? I say don't even *use* a center zone - at the very least skip the area hid behind the obstruction. Reason: the center zone does not behave like the other zones, and anyway it is not needed for evaluating the figure. Seehttp://web.telia.com...ouder/zcalc.htmAlso, there is the somewhat surprising real reason for making the other zones equal in area - probably not what you would think.

ceicomfeara

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Re: Help for coudermask
« Reply #7 on: January 11, 2018, 12:19:29 PM »
I like the center hole

Ryan Hernandez

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Re: Help for coudermask
« Reply #8 on: January 12, 2018, 03:17:33 AM »
A slight digression: how does one cut out a Couder mask with great precision? (Please don't say "laser cutter" as I'm in Parts Unknown, KY, two hours from The Middle Of Nowhere, US.)

Best,
Terry

ceicomfeara

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Re: Help for coudermask
« Reply #9 on: January 12, 2018, 08:02:25 AM »
Print it cut it with an exacto

kocewaffre

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Re: Help for coudermask
« Reply #10 on: January 20, 2018, 09:29:17 PM »
Yup. Just pay attention to the lines.  The mask itself doesn't have to have extreme precision for it to work well, the test is looking at all the area inside one hole and comparing it to all the are inside another hole - the accuracy of the cut itself doesn't make much difference compared to those area measures.

unexaric

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Re: Help for coudermask
« Reply #11 on: January 23, 2018, 05:00:18 PM »
I used cardboard and an exacto.

The couder screen has all the cut-outs on the same mask. But I found it very hard to concentrate on the matching areas so I made masks for each zone.
(couldn't find the center zone mask.)


ceplinglittfi

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Re: Help for coudermask
« Reply #12 on: January 25, 2018, 05:32:10 PM »
The reason I asked is that an uncertainty of (for example) 0.5 mm on a 70 mm radius gives a relative uncertainty in that radius of 0.7%... not huge but probably not negligible when calculating the correction.

Best,
Terry

rennlispuring

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Re: Help for coudermask
« Reply #13 on: January 25, 2018, 11:33:56 PM »
Model it in FigXP by setting up an ideal mask and then perturbing it while keeping the readings the same. You'll see it matters very little in practice.