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Equipment Discussions >> ATM, Optics and DIY Forum

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DAVIDG
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Reged: 12/02/04

Loc: Hockessin, De
Re: 9.25 inch refractor project new [Re: jimegger]
      #5757238 - 03/25/13 11:29 PM

Jim,
The crown element by itself will show a huge amount of spherical aberration when tested by double or single pass autocollimation even if the surfaces were perfectly spherical. Only when the crown is combined with the flint will of the combination of the two shows a null when perfectly figured and you'll be able to determine if the surfaces are spherical. Just like the refractive index of the flint offsets the chromatic abberation of crown, the curves on the flint also offset the spherical aberation of the crown. It's the combination of both elements that correct for color and spherical aberration. Each by themselves has a large amount of both.
If you misuderstood this then this maybe the reason why you believe there is an issue with the flat. Since the crown element when tested by itself using double pass autocollimation would always show undercorrected spherical aberration but since it is a lens, all the errors are opposite of that of a mirror. So the picture you posted of the crown that looks like a parabolic mirror is not over correction but actually under correction. So if you believed that the test of crown was showing overcorrection and when you tested the assembled lens by a star test and it showed undercorrection, then that is were the confusion is coming from and misuderstanding that the quality of the flat maybe the cause.

- Dave


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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: 9.25 inch refractor project new [Re: jimegger]
      #5757293 - 03/26/13 12:19 AM

Quote:

To see if it is or has spherical surfaces. Look for turned edges and spherical aberration.



Jim, I am a little confused. Could you explain the rationale for testing a single biconvex lens by autocollimation?
Mladen


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5757298 - 03/26/13 12:27 AM

I did not realize that the crown could not be looked at alone to determine its aberrations. I have never read anywhere stating that the single lens elements could not be tested this way. One learns something new all the time ! Apart from that, I did also use the flat for both elements combined along with doing star testing. Originally the star testing showed under correction. After using the convexing laps it went to over correction according to the star tests. Now I am working on the crown to get it back gradually towards the proper amount of correction. I do understand the difference between mirror and lens Foucault appearance being essentially opposite and what needs to be done to correct it. Right now the lens is over corrected meaning the center has a shorter focal length than the edge so I am working to remove glass more from the center on the crown. It is gradually going towards the objective of equivalent star images on both sides of focus.

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DAVIDG
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5757746 - 03/26/13 09:18 AM Attachment (23 downloads)

Hi Jim,
Here is an OSLO plot for just the crown element of your lens being tested via Double Pass with perfectly spherical surfaces. The wave front error is close to 250 waves !

- Dave


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DAVIDG
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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5757756 - 03/26/13 09:27 AM Attachment (22 downloads)

Jim,
Here is a drawing that might make it easier to understand why the zones on a lens are opposite of that of mirror. The drawing shows both a mirror and a lens with a hill on the surface. In the case of the mirror, the light is reflected off the convex surface of the hill and focuses long. In the case of the lens the light is refracted thru the hill and focuses short. You would see what looks like a hole when you tested the lens but it is actually a hill on the surface.

- Dave


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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5758124 - 03/26/13 12:33 PM Attachment (16 downloads)

Dave, that is a very good pictorial example. The problem is that even a simple lens has two surfaces and both may have hills or holes. The illustration below shows a hypothetical situation where a hill on one surface and a whole on another results in complete correction!

But assume that the correction is not perfect. Trying to fix one surface will result in overcorreciton or undercorrection, so it's essential to know that all surfaces are spherical in order to know where exactly to work on. However that would require at least two tmeplates.


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DAVIDG
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Re: 9.25 inch refractor project new [Re: MKV]
      #5758253 - 03/26/13 01:28 PM

Mladen,
I agree that when both surfaces of the lens are unknown it is difficult tell which side or sides has the errors. As has been sugguested a good approach is to test the surfaces using interference against a test plate.

- Dave


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saemark30
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Reged: 02/21/12

Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5758524 - 03/26/13 03:55 PM

Great project. Can you tell me how you make the pitch laps?
Do you use curved ceramic tool or curved glass tool as base?
I am wondering if the base can be flat.


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jimegger
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Reged: 12/22/05

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Re: 9.25 inch refractor project new [Re: saemark30]
      #5758948 - 03/26/13 07:35 PM

So..... a simple lens will not be able to be tested via Foucault test even using a single monochromatic wavelength of light .... because..... why is that ?

Mladen, the test plates are of course one way to see which surfaces are spherical ... but there must be others as well. It can not be that there is only one way.

From what I have read in ATM books, an objective will work just fine if a defect on one surface is compensated by a correcting defect on another surface such as the hill hole scenario Mladen did above.

I have the glass tools to make test plates from if it becomes necessary. For now I am going to keep working towards a better star test and Foucault test by more work on the crown with blending strokes to try and get the center down for a longer focal length there. It is so close now I can taste it !


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DAVIDG
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5759072 - 03/26/13 08:21 PM

Quote:

So..... a simple lens will not be able to be tested via Foucault test even using a single monochromatic wavelength of light .... because..... why is that ?





Jim,
A singlet, using spherical curves can be designed to have no spherical aberration at only one wavelength but in your case the curves on the crown element were choosen to have spherical aberration so the crown with not null when tested by itself. In an achromat it is the choice of the curves and the index of refractive of the elements working together to both correct for color and spherical aberration at the same time. In reality your objective is only fully corrected for spherical aberration at only one wavelength as well, that being in the green at 546 nm. All the other wavelengths are either slightly over or under corrected. This is called spherochromatism ie spherical aberration as a function of wavelength.
The new book by Smith, Ceragiolli and Berry "Telescope, Eyepieces and Astrographs" has two excellent chapters on achromats and apochromats and it explains how they work and what each element is doing.
I agree that a lens will work well when a defect on one surface is corrected by another, but right now you have 3 convex surface that are unknown. You need to get them close to, if not perfectly spherical to have a chance of correcting the assembled lens. So I agree with Mladen, your best bet is to use test plates to test and figure the convex surfaces. Once they are well figured to spherical surface you can test the assembled lens by double pass autocollimation and tweek a surface (usually R4) until the lens nulls perfectly Now you'll have a very well figured lens that most likely show no spherical aberration when star tested and will give jaw dropping images.
If you believe your close then test the assembled lens by double pass autocollimation. There is no guessing and it is a far more sensitive and much less error prone test then all your others test methods. If your close then the lens should show only faint zones and look nearly spherical.

- Dave


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DAVIDG
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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5759081 - 03/26/13 08:24 PM Attachment (34 downloads)

Jim,
Here are couple of images of achromats being tested via double pass autocollimation. It takes me about 5 minutes to set them and I can see exactly what is wrong with them.
Here is a picture of 4" f/15 Jaegers lens. Note the Ronchi bands are bowing and also not the evenly spaced top to bottom. The lens is undercorrected and has some astigmatism.

- Dave


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DAVIDG
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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5759085 - 03/26/13 08:25 PM Attachment (29 downloads)

Here is another Jaegers lens, this one is 5" f/5. Not great as well.

- Dave


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DAVIDG
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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5759094 - 03/26/13 08:28 PM Attachment (29 downloads)

Here is a "Zeiss" Telementor lens. We're pretty sure someone swapped out the original lens and put this one in it's place. It well undercorrected.

- Dave


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DAVIDG
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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5759100 - 03/26/13 08:31 PM Attachment (29 downloads)

Here is a real Zeiss lens. It is of excellent quality, notice how straight the bands are.

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DAVIDG
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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5759114 - 03/26/13 08:34 PM Attachment (30 downloads)

Finally here is Selsi 80mm f/11 objective, also of excellent quality from the nice straight Ronchi bands.

- Dave


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Dick Parker
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Reged: 08/17/07

Loc: Tolland, CT and Chiefland, FL
Re: 9.25 inch refractor project new [Re: jimegger]
      #5759344 - 03/26/13 10:47 PM Attachment (21 downloads)

Jim -

I have been following this project because I, too, have built refractors. Testing refractors is different from testing mirrors because the design of a refractor objective corrects chromatic aberration (as well as an achromat can), spherical aberration, and coma. This relies on the interplay between both the elements, the surfaces, the thicknesses, refractive indices, and spacing. For best results, it is not a willy nilly affair fixing a hole in one surface with a hill in another.

For best results, the radii have to be close to the design intent as well as the surface figures spherical. The recommended tolerance for R2 and R3 should be within +/- 1/20th % of their design value. R1 about 1/10th % and R4 fairly loose. Keep in mind there are three aberrations being corrected.

Autocollimation testing ONLY verifies that the spherical aberration is corrected, but the color and coma is in the radii.

for best results, a spherometer is not adequate to validate that the radius is correct. There should be some more precise method. Often used is measuring R3 with a knife edge and carefully measured rod. The results from this method should agree with calculation based on a spherometer by only a few thousandths.

Attached is a picture showing measuring R3 of one of my flint elements with rods.

Dick Parker

Edited by Dick Parker (03/26/13 11:20 PM)


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Dick Parker
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Reged: 08/17/07

Loc: Tolland, CT and Chiefland, FL
Re: 9.25 inch refractor project new [Re: Dick Parker]
      #5759356 - 03/26/13 10:54 PM Attachment (17 downloads)

Then if your design has R2 = R3 you can, and should test the two together to establish that the radii match. R3 can be validated for exact radius and figure by the knife edge test that we all know. Interpreting interference patterns can be a bit tricky, but is part of the process of making a refractor. If you are lucky, when R3 and R2 match exactly, and you test them againt each other by interference the interference fringes should be straight and parallel as show in the picture of one of my objectives. If they don't match exactly, you will end up with a bull's eye and rings. As you press the side of the top element in the interference test the bull's eye will move from the center to the side. If R2 is exactly spherical, the bull's eye will stay perfectly round. If it turns into a "D" shape or pear shape or anything but round, then you have zones in the surface.

With this test you can validate that R2 is the correct radius and spherical figure.

Dick Parker


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Dick Parker
scholastic sledgehammer
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Reged: 08/17/07

Loc: Tolland, CT and Chiefland, FL
Re: 9.25 inch refractor project [Re: Dick Parker]
      #5759370 - 03/26/13 11:05 PM Attachment (19 downloads)

You can get by making a refractor objective with only one other test plate, that being for R1. R1 should test against its test plate with the same result as discussed before. If I could pass on any advice it would be to go make a test plate for R1 being careful to validate that the test plate is the exact radius and a perfect sphere.

With three surfaces out of four validated, you can deduce that any further errors of the lens tested assembled will be in R4 and you can figure that surface.

Then you can assemble the lens and test with autocollimation. Shown is the autocollimation test of my 6 inch f/15.

Again I say, the autocollimation test only validates spherical aberration, not color correction or coma.

One thing I don't see mentioned is that optical glass has horrible thermal properties and you need to wait a long time after you work on a surface before you can test it. A way to minimize the wait is to have a bucket of room temperature water handy and immerse the element in it for about 1/2 hour after working on it before testing. This is big time. Don't overlook it. Ask me how I know.

Dick Parker

Edited by Dick Parker (03/26/13 11:11 PM)


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Dick Parker
scholastic sledgehammer
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Reged: 08/17/07

Loc: Tolland, CT and Chiefland, FL
Re: 9.25 inch refractor project [Re: Dick Parker]
      #5759387 - 03/26/13 11:19 PM Attachment (16 downloads)

Back to R2 against R3 by interference. Shown is the bull's eye and rings I described when R2 and R3 are not a perfect match.

Dick Parker


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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: 9.25 inch refractor project [Re: Dick Parker]
      #5759607 - 03/27/13 05:53 AM Attachment (20 downloads)

Thanks DAVIDG and Dick Parker for your input. I believe this will at least give Jim some idea of what to do if his current attempts to correct the lens by trial and error don't work to his satisfaction.

On the measurement of radii, radius bars are the method of choice to for many optical applications. The picture below shows a setup where precise light source was to be positioned relative to a concave surface of a lens for a knife-edge test. The radius bar used consists of sections that screw into each other to form a rod. the ends have panhaead nylon screws that contact the glass surface and the light source at one point. The length of the rod is then measured either with calipers (I have 24 inch calipers) or on a lathe bed. Even a tape measure can determine the radius to within a mm. The nylon screws serve as fine adjustments for "tweaking" the radius bar length precisely to the desired distance.


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