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Equipment Discussions >> ATM, Optics and DIY Forum

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Ajohn
sage
*****

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
Postmaster
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Reged: 02/22/10

Loc: nj
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
sage
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Reged: 12/03/07

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
professor emeritus


Reged: 03/26/06

Loc: Oregon
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
Carpal Tunnel
*****

Reged: 01/20/11

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
Vendor
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Reged: 09/05/05

Loc: Northeast USA
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
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
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
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
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
Postmaster
*****

Reged: 02/22/10

Loc: nj
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
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
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
professor emeritus


Reged: 03/26/06

Loc: Oregon
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
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
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
Postmaster
*****

Reged: 02/22/10

Loc: nj
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
professor emeritus


Reged: 09/13/10

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

All's well that ends well.

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