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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Caustic vs. Pinstick
      #5742439 - 03/19/13 08:43 AM Attachment (38 downloads)

Bummed. I got a mirror test and it doesn't jive with all the work I did using "Foucault Test Analysis V2.08".

Can someone just review my numbers and see where I went 1/3 wave off?

Pin location /Moving slitless readings
2.50/.071
4.36/.115
5.62/.159
6.63/.205
7.13/.226
7.59/.250

I had used the star test to correlate my figuring earlier, and when I saw it had some discrepancies, I doubted my results, and started relying on the easier method of figuring to the above program.

I started using Figure XP, and I get the same results...should have been an exceptional mirror. See picture.

I set up my Caustic tester, and it's showing the error. Closer star testing does show error.

So why are these Foucault programs giving me bad numbers?
Could I have misinterpreted the term "effective radius"? Isn't that the center of a mask aperture or exact pin location?

I must be a dummy. Calculating r^2/2R gives me about 80% smaller readings. Why didn't I just do it the old way and skip the software?


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5742473 - 03/19/13 09:14 AM Attachment (20 downloads)

Here's the input values.

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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5742501 - 03/19/13 09:38 AM Attachment (20 downloads)

and your using effective radius as opposed to inside/outside?

this is what I got from you numbers.

Edited by Pinbout (03/19/13 09:42 AM)


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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: Caustic vs. Pinstick new [Re: Pinbout]
      #5742508 - 03/19/13 09:45 AM Attachment (21 downloads)

and

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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: Caustic vs. Pinstick new [Re: Pinbout]
      #5742628 - 03/19/13 10:49 AM

You can verify the transverse abberation on a calculator. If it doesn't compare favorably with XP's result, I can understand the possible discrepancy. A transverse error of 8.0 is around 1/.3-something wave, roughly 3 waves total error. fwiw,
M.


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: Pinbout]
      #5742629 - 03/19/13 10:50 AM

Right. I used "effective radius" setting in the XP program, and plugged in my numbers.

In the Foucault v2.08 I specified "FoucaultTest (Pin Stick)", and plugged in my numbers.

With a pin, there's no inside/outside measure. I like the process, and am convinced it's an accurate way to measure the zones. If I had hand calculated the KE values, and just figured from that I'd have been OK. Bungled somewhere in using the software.

Where? Should have persevered and trusted the star test, but shlepping the mirror in & out was tedious. It's a beautiful mirror, just wrong.

Now that I've got the ol' Caustic rig out, I'm using that from now on. For this checkup, I used the poorman's version which showed the overcorrection generally. When I get it set up, I'll use the official mask and statistical measurements.


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: Pinbout]
      #5742646 - 03/19/13 10:55 AM

Quote:

and your using effective radius as opposed to inside/outside?
this is what I got from you numbers.



Look what your version did to the mask/pin locations? Not at all what I used.

C


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: Mark Harry]
      #5742654 - 03/19/13 11:00 AM

I don't use the transverse values function, at least in cases of such great error.

What's wrong with using wavefront analysis, at this stage at least?

Why are there such discrepancies in the software values I got and the actual ideal numbers?

What box did I fail to click? That's where I'm at now.


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Pinbout
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Reged: 02/22/10

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Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5742664 - 03/19/13 11:04 AM

I know I'm trying to make it look worse.

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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: Pinbout]
      #5742680 - 03/19/13 11:11 AM

Worse? Oy gevalt!
I figured a really fine 12.0" f/3.0 without software ...pinstick and autocollimation. Live and learn.


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Pinbout
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Reged: 02/22/10

Loc: nj
Re: Caustic vs. Pinstick *DELETED* new [Re: ccaissie]
      #5742724 - 03/19/13 11:36 AM

Post deleted by Pinbout

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Pinbout
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Reged: 02/22/10

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Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5742763 - 03/19/13 11:51 AM

when I enter your numbers it seems way off when I hit fit best conic, it makes the graph for -.5 conic, it looks like your really good.

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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: Pinbout]
      #5742847 - 03/19/13 12:32 PM Attachment (10 downloads)

Yeah, I get a -.5 too, if I set the KE as fixed.

No, 3 software programs (XP, v2.08, and Zone.exe) I have tried give me great results with the numbers I plugged in.

It must be something about the radii measurements, no?
But when I use a mask setup in XP, and set the inner and outer at +/- 0.1" my pinstick values, it gives an effective radius of my pinstick value, confirming that the effective radius is the mean of the mask aperture radii... and the same fine excellent wave readout.

If I compute zones by r^2/2R, I get different numbers, smaller.


All 3 programs are happy with a 7.62 radius... my zone 6, giving a KE of .250" with moving source.

So, I use the pinstick setting in the software, or effective radii, and specify the exact location of the pins.

I find that I get the same averaged measurements whether I call the setting at the first pair of shadows hitting the pin, or the mid-gray zone, or visualizing the crest in 3-D, or by calling it as the last bit of light crosses the pin.

The attached photo is not great, the crest on the right is not set right, and there's vignetting, but you get the idea of what I mean. just a .001 change in the KE setting makes a difference in how the shadow crest hits the pin. so that's OK, I think...averaged measurements and all that.

Something I did very wrong, related to software assumptions, and it's making me want to can the software route and go back to my antediluvian methods. My mirrors made back then are still excellent.


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Mike Lockwood
Vendor, Lockwood Custom Optics
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Reged: 10/01/07

Loc: Usually in my optical shop
Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5742868 - 03/19/13 12:42 PM

Quote:

Isn't that the center of a mask aperture or exact pin location?



Yes, you are correct for the case of a pin stick.

Quote:

I must be a dummy. Calculating r^2/2R gives me about 80% smaller readings. Why didn't I just do it the old way and skip the software?



Using your numbers and r^2/2R (moving source) I get the same thing as FigureXP for the outer two zones:

(7.59^2/285.4) - (2.5^2/285.4) = 0.20185 - 0.0219 = 0.180"
(7.13^2/285.4) - (2.5^2/285.4) = 0.17812 - 0.0219 = 0.156"

Not sure what error your caustic or star testing is showing.


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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: Caustic vs. Pinstick new [Re: Mike Lockwood]
      #5742884 - 03/19/13 12:53 PM

If Danny's ML graph is correct, it's way undercorrected in the outer half of the mirror, with a hole (in the center)
M.


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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: Caustic vs. Pinstick new [Re: Mark Harry]
      #5742908 - 03/19/13 01:02 PM

no my ml graph is for the wrong zonal measurements.
but when I use his radii its even worse.


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: Mike Lockwood]
      #5742957 - 03/19/13 01:26 PM

"Using your numbers and r^2/2R (moving source) I get the same thing as FigureXP for the outer two zones:

(7.59^2/285.4) - (2.5^2/285.4) = 0.20185 - 0.0219 = 0.180"
(7.13^2/285.4) - (2.5^2/285.4) = 0.17812 - 0.0219 = 0.156"

So, why do my entered measurements of .250" and .226", respectively, in my 3 programs result in such a marvelous wavefront chart?

My Caustic measurements were rough, (x= +/- .0005), but confirmed significant overcorrection. Star test showed earlier secondary shadow breakout outside focus at 540X.

I'm missing something here in the software.


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Mike Lockwood
Vendor, Lockwood Custom Optics
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Reged: 10/01/07

Loc: Usually in my optical shop
Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5743006 - 03/19/13 01:44 PM

Quote:

So, why do my entered measurements of .250" and .226", respectively, in my 3 programs result in such a marvelous wavefront chart?



Those are the ideal readings according to three programs AND by the classical/proper formula - I don't understand why the wavefront shouldn't look good!?

Quote:

My Caustic measurements were rough, (x= +/- .0005), but confirmed significant overcorrection. Star test showed earlier secondary shadow breakout outside focus at 540X.
I'm missing something here in the software.



What is significant?

Accurately-figured mirrors usually show overcorrection in the star test because the mirror is usually cooling. This is why lab tests are often more accurate than star testing.

I would recommend testing the mirror with two different masks and comparing the results with the pinstick. If they all agree to reasonable accuracy, I'd be suspicious of the star testing results.


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Pinbout
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Reged: 02/22/10

Loc: nj
Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5743028 - 03/19/13 01:54 PM

ok I finally inputted your numbers correctly and got the same result in figurexp.

as i get older I become more adhd.


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: Pinbout]
      #5743288 - 03/19/13 03:47 PM

"If they all agree to reasonable accuracy, I'd be suspicious of the star testing results."

Or the coater's 1/3 wave .73 strehl test? Will this ever end?

Made a Couder screen, tested it, 4 sets of measurements...overcorrected by 1/5.4 p-v.

I star test at various times...8 p.m., 5 a.m. so temps are not always cooling, but you're right, I've seen most of my scopes go a bit over while cooling. Except my 6" f/11 sphere which gets perfect.

I'm fixing my Caustic setup and will get a good set of numbers from that.

Thanks all for the feedback. I was hoping that someone would say, "oh yeah, these 3 programs can't calculate pinstick setups...use paper and pencil only."

Obviously, I don't know how to put the numbers into XP correctly because of the discrepancy in the "ideal figures" column, yet got great wave shape. Screwed up on v2.0B, and ZONE.exe is wrong somehow.

Flummoxed.


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Mike Lockwood
Vendor, Lockwood Custom Optics
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Reged: 10/01/07

Loc: Usually in my optical shop
Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5743368 - 03/19/13 04:23 PM

Quote:

Made a Couder screen, tested it, 4 sets of measurements...overcorrected by 1/5.4 p-v.



Evaluated with a program, or with a calculator? How does the figure look when using FigureXP?

Quote:

Thanks all for the feedback. I was hoping that someone would say, "oh yeah, these 3 programs can't calculate pinstick setups...use paper and pencil only."



My experience shows that they work with pinsticks.....

Quote:

Obviously, I don't know how to put the numbers into XP correctly because of the discrepancy in the "ideal figures" column, yet got great wave shape. Screwed up on v2.0B, and ZONE.exe is wrong somehow.
Flummoxed.



I'd like to see the comparison of the Couder mask data to the pinstick data using FigureXP, and how all of that compares to your calculations.


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Dave O
sage
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Reged: 12/21/11

Loc: Sri Lanka
Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5744277 - 03/19/13 11:33 PM

Quote:

"If they all agree to reasonable accuracy, I'd be suspicious of the star testing results."

Or the coater's 1/3 wave .73 strehl test? Will this ever end?




Perhaps you should have it tested by a different optician? If your testing indicates it is a good figure, I would not rush into changing anything until I was sure it needed changing (and what the necessary changes are).


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: Dave O]
      #5744829 - 03/20/13 10:34 AM

Yes. I'll sit on it for a while, more thinking than anything, and when I'm finally convinced, I'll act.

Supposed to be a hobby, not an obsession.


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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5744969 - 03/20/13 11:51 AM

Quote:

Supposed to be a hobby, not an obsession.




yeah but an issue like this is one that really gnaws at you, or at least me.


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Norm Meyer
sage
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Reged: 02/08/09

Loc: Warren, ME 04864
Re: Caustic vs. Pinstick new [Re: Pinbout]
      #5745046 - 03/20/13 12:26 PM

That's why I like spherical systems when possible or
null testing. I guess I'm lazy.

Norm


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Pinbout
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Reged: 02/22/10

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Re: Caustic vs. Pinstick new [Re: Norm Meyer]
      #5745085 - 03/20/13 12:52 PM Attachment (6 downloads)

thats similar to autocollimation. no numbers.

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Tom Stokes
newbie


Reged: 03/19/13

Loc: Mission Viejo, CA
Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5746254 - 03/20/13 08:52 PM

Scuse me. I joined CN in the last hour. Are you sure this isn't cockpit error? Why are the output numbers different from the input numbers? I ran the output numbers through my program Parab and got a Strehl of 0.99+ and a P-V of 1/76. I have never seen a better mirror.

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gregj888
professor emeritus


Reged: 03/26/06

Loc: Oregon
Re: Caustic vs. Pinstick new [Re: Tom Stokes]
      #5746539 - 03/20/13 10:56 PM

Sorry, dumb question. When you place the shadow, is it centered (equal brightness) on the pin or in the space between the pins?

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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: gregj888]
      #5747042 - 03/21/13 08:13 AM

Quote:

Sorry, dumb question. When you place the shadow, is it centered (equal brightness) on the pin or in the space between the pins?



The shadow crest...try to visualize the shadow shape as a 3-d donut curve... should appear to be just at the pin. The pin is, in effect, at the center of an equivalent mask aperture. This came up earlier when I was trying to convince myself that the pin location was also the "effective radius" of the zone, which is one of the software options when entering zone locations.

That means that you judge the mid-grey of the shadow as it crosses the pin. But, as I mentioned earlier, I get the same results if I measure from the first wisp of shadow crossing the pins, or the last hint of light in the shadow crossing the pin. As long as you use the same reference points for all measurements. In the photo of the mirror and pins, I'd say that the left pin is in the middle of the shadow region. The right shadow..hard to see because of the *BLEEP* photo..is not quite to the pin. This is probably a KE position of .002 too close to the mirror...wasn't meant to be a photo record of a KE setting, just an illustration.


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: Pinbout]
      #5747046 - 03/21/13 08:15 AM

Quote:

thats similar to autocollimation. no numbers.




I just love null testing. Or spheres.... That's a beauty of a Ronchigram, there, buddy.


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Ajohn
sage
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Reged: 12/03/07

Re: Caustic vs. Pinstick new [Re: gregj888]
      #5747071 - 03/21/13 08:35 AM

As I don't make mirrors very often I try checking a mirror several ways. One which can give good results is a mask with few holes in it ideally sized as per the caustic test suggestions in ATM. R/100 but larger works out too. Smaller may even be better. I have one central, one 70% and one near the edge. As the holes are small it's much easier to position the knife edge accurately and get the shadows moving exactly as they should do. One thing I find with coudre masks is that getting a really good central reading can be difficult. This goes back to my 1st mirror. I used several masks of a similar nature not entirely covering the mirror and found it much easier to do. When I finished that one I knocked up a Dall null test and couldn't see any errors at all. This was a 220mm F6 - wish I still had it.

Parab by the way is the only test software that allows people to work this way where the user dictates where the zones are. Others need dimensions of the mask or even dictate them. You might say what I do is a variation of makes sure zones are corrects and that the mirror is smooth between the ones that are chosen rather than completely cover the mirror with holes of one sort or another. I found that adequate for early figuring but hopeless at the end.

John
-


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5747091 - 03/21/13 08:48 AM Attachment (5 downloads)

OKAY. Got my caustic rig up and running, ran 3 series of 3 measurements...different times of day with temps rising and falling in the lab a tiny bit.

I get 1/9.1 wave overcorrected.

Here's the numbers. I had to cheat because one of my mask apertures is slightly over the edge, so I called the mirror a 17.0" dia. to trick the software, which wouldn't let me enter a 1.25" dia hole at 7.5" radius.

Mirror R=142.7"
Hole dia. 1.25"
Z1 1.5"
Z2 3.0"
Z3 4.5"
Z4 6.0"
Z5 7.5"

Fixed source, adjustable slit to tune the diffraction effects around the wire.

I use a Mitutoyo Dial indicator reading in .0001". My X measurements are repeatable to about .001", so I'm working on accuracy, but the averages of 9 sets of numbers are:

Values are Y, X

Z1 .047, .0010
Z2 .186, .0048
Z3 .426, .0194
Z4 .757, .0435
Z5 1.183, .0838

I'll be doing more measurements and refine my technique, and maybe tweak my setup...as you can see, it's a bit crude. Caustic Tester sits on my Hendy lathe apron, on a piece of 12" SCH40 pipe. Mirror is seen through the aperture of my polishing room.


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5747148 - 03/21/13 09:16 AM Attachment (4 downloads)

Sorry, X measurements are:
.0004
.0026
.0093
.0215
.0417


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5747286 - 03/21/13 10:33 AM Attachment (3 downloads)

Parab by the way is the only test software that allows people to work this way where the user dictates where the zones are.
******
No, I find that the other programs allow different locations. Foucault analysis v2.0B, and Figure XP allow input. Or did you mean Couder? I'm not crazy about that setup, as I agree, finding the center radius seems just too "hazy". Maybe with practice, but there are a lot of moving shadows in the 6" hole. Probably make a mask with more zones, but this was the one that Foucault analysis v2.0B stated. I could throw a cat through the center aperture.


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Pinbout
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Reged: 02/22/10

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Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5747411 - 03/21/13 11:39 AM

you need to drive it to delmarva this weekend as a walkin, what is it $25 and have steve look at it in his rossnull.

you could pick me up on the way down.

but yeah you need another type of test other than KE or caustic to verify your results.

your going around in circles at the moment.

Edited by Pinbout (03/21/13 11:40 AM)


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: Pinbout]
      #5747430 - 03/21/13 11:48 AM Attachment (3 downloads)

Yeah, circles....

This screenshot is from entering values derived by r^2/2R.

Looks right. Nice figure, etc.


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5747447 - 03/21/13 11:58 AM Attachment (3 downloads)

This one is from an entry where the only thing I changed was entering measurements that are supposedly targets for my figuring .070, .115, .159, .202, .226, .250.
As I figured it, the numbers approached these values, and the wavefront improved from 1/4 wave.

WHY did the program report different numbers?!!!

And a fine mirror.

These are about the same numbers I put into figure XP.

Why did that program ALSO report different numbers?!!!

Also a fine mirror.


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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
Re: Caustic vs. Pinstick new [Re: Pinbout]
      #5747459 - 03/21/13 12:04 PM

Quote:

you need to drive it to delmarva this weekend as a walkin, what is it $25 and have steve look at it in his rossnull.




Hey! That's a good idea! Build a Rossnull! Another set of dimensions and measurements to question...

Going 'round the twist here...


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Pinbout
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Reged: 02/22/10

Loc: nj
Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5747674 - 03/21/13 01:46 PM

nobody said build it, go have him check it.

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gregj888
professor emeritus


Reged: 03/26/06

Loc: Oregon
Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5747782 - 03/21/13 02:54 PM

Just checking...

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Ajohn
sage
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Reged: 12/03/07

Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5747931 - 03/21/13 04:07 PM

Quote:

Parab by the way is the only test software that allows people to work this way where the user dictates where the zones are.
******
No, I find that the other programs allow different locations. Foucault analysis v2.0B, and Figure XP allow input. Or did you mean Couder? I'm not crazy about that setup, as I agree, finding the center radius seems just too "hazy". Maybe with practice, but there are a lot of moving shadows in the 6" hole. Probably make a mask with more zones, but this was the one that Foucault analysis v2.0B stated. I could throw a cat through the center aperture.




I only looked quickly but one package gave coudre mask radii to rather a lot of decimal places and seemed to insist that the have to be used. I looked at fringexp. There may be no problem specifying apertures that don't fully cover the mirror but it's getting rather old and I understand if a plot is printed another package is needed as they are jpg's or bit maps. Parab prints directly from the package. I tested it with some oddly spaced perfect readings and it handled them without any problems. It's also a just does what is states on the can type package - I like that. It processes readings and that's it. One I came across also has virtual foucault images. If some one wants to see what these can look like maybe youtube would be best.

I also came across 2 packages that use shadow intensity via a camera. One is reckoned to be good to 5nm.

More recently I came across another approach where the source is placed some way inside the radius of curvature and the SA at a point the other way is calculated and checked. That one can be on axis without a beam splitting arrangement.

I have also seen a video of a Ritchie test mask in use. I had previously discounted those because of diffraction effects but it looks relatively easy.

Actually I doubt if Coudre would approve of a 6in hole on anything other than an extremely slow mirror. If he was around I suspect he would be annoyed that his name had been attached to it. The idea of a mask is that the apertures approximate a sphere. It's difficult to reach ideal sizes on that basis but it does seem to be possible to get close.

John
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Pinbout
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Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5747984 - 03/21/13 04:25 PM

did you ever think about setting up a bath if?

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Ajohn
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Re: Caustic vs. Pinstick new [Re: Pinbout]
      #5748187 - 03/21/13 06:05 PM

All sorts of add ons tend to creep into bath- F3 mirror so the bi convex lens gets difficult - several answers to that it seems but more gear. There has also been some debate from memory as to if it's suitable for this sort of ratio. Small none polarising cubic beam splitter last time I looked were hard to find cheap. Then comes cameras. Ideal in my book for figuring - testing and so one would be a web cam with a suitable lens. Adapting web cams for that isn't that easy - fitting a suitable lense. I do have one that will take board lenses though. Some don't like trying to get a camera close to the kit so along comes another lens. I'm not knocking it. Just pointing out that it tends to get more complicated than initially appears. Add in multiple averaged readings of one sort or another..... but yes I may build one at some point but I'm inclined to feel it might be more suitable for final testing. I've also toyed with the ideal of asking Dale if he checks for distortions in the camera lens. Barrel and pin chushion etc. Even some of the expensive lenses have it - usually processed out within limits in the camera. But use a DSLR? I do have some software that allows a fairly rapid series of shots to be taken and downloaded immediately with even an original 5D.

To start off with I want to try working the way Waland did on fast mirrors - pins and hopefully fairly intense shadows touching adjacent pins. Then using a null lens to control the shadow density to get an overview. The accuracy of the lens doesn't matter too much for that. Failing that a Ritchie type mask. There is a video on youtube relating to quartz mirrors showing one of those in use. No details of the tester though - suspect a fairly large slit or source Failing that a Dall null test along with some form of pellicle beam splitter. That might just be a microscope cover slip. I have a light source with a 25w halogen bulb in it on my original tester so light loss doesn't really matter. I favour Dall as it references the centre of the mirror - knife position to that and then the null lens unit adjusted to bring the slit image to the same point.

I also have the slides of an old tool makers lathe so could use the caustic test. I knocked that up to see what it did and found that the method where fringes are centred round the wire to be very sensitive. To see them I just used a 10x eye cup type loupe. Using what I had to hand I would say it's sensitive to a small fraction of a thou. Play in the feed screws on the slides will make it more difficult to use as measurement will have to be taken in one direction winding well back if there is any over shoot. Much bigger handles than on my lathe and I manages to set cuts to sub thou's on that.

The software null test makes a lot of sense for initial work but up against the web cam lens problem again. Since I have seen this software I am going to do some sums to see what sort of lens I would need. One package resides on atm_free and the other on sourceforge.

I'm on a mystery tour really. I'm not sure I can even figure an F3 mirror let alone a 220mm one. What tests works is one of the items of interest. I will get a better feel for it all when I figure to a sphere but will need a finely controlled foucault tester for that.

John
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gregj888
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Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5748821 - 03/21/13 11:58 PM

At F/3 the Bath might have a problem but it works on a 20" f/3.5 or seems to. You do need a reasonably good DSLR, but then you need one of those anyway... right :-)

Ccaissie, can you post a photo of the front of your foucault tester? I'm courious how far apart the source and slit are?

Assuming the shadows stay centered when moving in and out (optical amd machanical slide axies are aligned)this is kind of a mystery.

One thing you might try is to set up another set of zones centering the shadow between the pins. The results should be the same (for the figure) but might "read" differently. I also use 10 zones, but that wouldn't seem like an issue.

I'm just briging up a bath IF, well just got a DSLR, so may have some comments later on that. Surplus shed has small 10mm beam splitters at a reasonable price.

Note: a friend measured my mirror last summer with his bath IF. It and my foucault measurments agreed to about 2 decimal places... pure luck of course, but fun none the less. We were all shocked and it's unlikely to happen again.


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MKV
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Re: Caustic vs. Pinstick new [Re: gregj888]
      #5749022 - 03/22/13 03:38 AM Attachment (4 downloads)

Quote:

At F/3 the Bath might have a problem but it works on a 20" f/3.5 or seems to. You do need a reasonably good DSLR



If you have a diverging lens of sufficiently short f.l. you can test f/3 with a Bath. The trick is to evenly illuminate the whole surface at a short distance. Usually a 10 mm f.l. biconvex lens is sufficient. You need a f.l. that = d*2F#, or shorter, where d is the laser beam diameter, and F# is your mirror's focal ratio. Thus, for an f/3 mirror and a d = 3 mm, your lens should have f.l. = 18 mm or shorter.

Also you don't need super small beamsplitters. It's easy to use larger ones and keep the beam separation sufficiently small. I keep mine at 7 mm with a 20 mm beamsplitter.

For imaging your ifgrams, you don't need a DSLR, but it's a good idea to image them using a low-power Keplerian type relay telescope. Then you can easily use just about any camera, and you don't have to jam the lens right next to the beamsplitter. The last thing you want to do is disturb the delicate setup

If you use a DSLR you're better off using a DSLR that accepts old manual focus lenses suitable for film cameras.


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Mark Harry
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Re: Caustic vs. Pinstick new [Re: MKV]
      #5749164 - 03/22/13 07:58 AM

How many 20" F/3's haveyou made with yours?
M.


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ccaissie
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Re: Caustic vs. Pinstick new [Re: Pinbout]
      #5749270 - 03/22/13 09:30 AM

Quote:

nobody said build it, go have him check it.




Right. I'm joking. Can't go. My motto is, "I can therefore I must", meaning that I instinctively do it myself, causing all the repercussions of my life. Home made fuel, cars, equipment and of course, telescopes.


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ccaissie
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Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5749278 - 03/22/13 09:32 AM

Ccaissie, can you post a photo of the front of your foucault tester? I'm courious how far apart the source and slit are?
******
Slitless. About .5" vertical displacement.


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Pinbout
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Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5749295 - 03/22/13 09:40 AM

Quote:

"I can therefore I must", meaning that I instinctively do it myself, causing all the repercussions of my life. Home made fuel, cars, equipment and of course, telescopes.




you forgot beer...


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ccaissie
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Re: Caustic vs. Pinstick new [Re: Pinbout]
      #5749311 - 03/22/13 09:47 AM

Yeah, but interpretation of an asphere is where I'm having a problem. I'm considering touching up an old uncoated perforated Full Thickness Pyrex flat which is not particularly smooth. I've got a 12.5 perf and a 10 perf, but those are too small...again getting into complexity.

I've got an old scatterplate interferometer, but it's good for checking spheres or nulls.

Giving the whole thing a break. There's fresh snow to be ski'd on, a greenhouse to build a heating system for, and Spring/Ag season is starting. Have to work on this mirror in the evenings on CLOUDY NIGHTS.

Some issues just "go better in the morning", so, someday while I'm swingin' a hammer, I'll get the AHA!, and go back to the optical problem (sans hammer).


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gregj888
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Re: Caustic vs. Pinstick new [Re: MKV]
      #5752293 - 03/23/13 04:54 PM

I was thinking more about the number of fringes with a mirror that deep and no null.

I agree though, it should be OK id the sensor has enough pixels.

Ccaissie, you've run through my ideas... wish you were closer.

Edited by gregj888 (03/23/13 04:59 PM)


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ccaissie
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Re: Caustic vs. Pinstick new [Re: gregj888]
      #5771383 - 04/01/13 06:09 PM

Duh.

So the analysis programs take the readings you input, and fit it to the best parabola that it can find. Since I never told it that the center was set to a reading of ZERO, it merrily took the numbers and ran with it. My readings were .048 over, and nobody told me. Zone 1 should have read .022, not .070. So my center is .048 too deep.

If I had known that the program(s) did this, I would not have just guessed at the readings when I input a theoretically perfect paraboloid. I found out that the program did this by inputting a series of readings that were .100" OVER. Perfect mirror. AHA!

I had started with guesses, estimating the L.A. for a 16" f/4 or 5 would be about a quarter of an inch, and "corrected it" by altering the "readings" until I got a perfect mirror, then used those readings as targets.

The only problem with this is that my secondary does not quite come to my zone 1 radius, so my curve gets too steep just as it drops behind the secondary.

This is why the star test looked so good, except for a ghostly secondary shadow breakout that said OVERCORRECTED.

So, I can use these programs if I just create a zone inside my secondary shadow. I should be fine.

Or start with actual r^2/2R values instead of guessing at values and expecting the software to read my quirky mind.

"He who is clever is stupid"

So I need a 5" diagonal and I'll have a superb scope.

*Sigh*


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Pinbout
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Re: Caustic vs. Pinstick new [Re: ccaissie]
      #5771605 - 04/01/13 08:06 PM

thanks for getting back to us with on how you figured it out.

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ccaissie
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Re: Caustic vs. Pinstick new [Re: Pinbout]
      #5771872 - 04/01/13 10:30 PM

All's well that ends well.

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