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Equipment Discussions >> ATM, Optics and DIY Forum

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Mike I. Jones
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Re: 9.25 inch refractor project new [Re: Gord]
      #5730616 - 03/13/13 04:14 PM

Mladen, curious, I wasn't aware you actually worked in optics shops. Where all have you worked?
Mike


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: MKV]
      #5730633 - 03/13/13 04:20 PM Attachment (47 downloads)

The star tests will be a confirmation of whether or not the objective is under or over-corrected with a look at the in and out focused star images. Initially the inside focus image showed a distinct hole in the center and the outside focus showed a disk full of light with no hole as a matter of fact it had a bright point in the center indicating long focus central rays and shorter edge ray focus ... an under corrected objective according to my books. NOW ...I have the opposite after using the convexing tools for some polishing spells. Now the inside focus rays leave a light filled disk and the outside focus rays have a distinct hole in the star test. The flat I was using shows no change in the appearance of the Foucault test. It still shows the central rays as being long in focus and the edge rays as shorter. That is why I suspect the flat to have a significant curve to it. The star test cannot be denied as it is the eating of the pudding so to speak.

As far as using the 12.5 inch mirror as a collimator for a single pass test, it has proven itself with the production of a really excellent 6 inch lens by myself so I know it works very well. Once it is set up it is like doing a Foucault test at the focus of the scope on a star AND you can look at the pinhole with a very high power eyepiece without looking through all the atmosphere. I was amazed at how much improved the pinhole image was at high power when the objective showed a really good null in the Foucault test! It was like looking at the small pinhole through a microscope - the detail just jumped right out. After that I looked at Mars with 500 X and saw the most awesome detail on a particularly steady night. The stars are textbook diffraction images.

I have a week trip coming up here and when I get back it will be using the reflector mirror for a collimation test. In the meantime I am still star testing the objective since we are having clear weather here now. It is set up for a session tonight after another brief spell of polishing this morning. I am closing in on the perfect star test images at any rate - working back towards where I was from an over corrected state at present.Here is a shot of the focalgram I took of just the crown using the green LED with the flat in autocollimation. The light source is on the right from the knife edge. The light marks (squiggly lines) in the mirror are defects on it - not the lens.


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jimegger
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5730637 - 03/13/13 04:21 PM Attachment (46 downloads)

Here is a shot of the 9.25 inch on a really undersized wobbly mount for star testing purposes.

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jimegger
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5730645 - 03/13/13 04:22 PM Attachment (40 downloads)

Here are the 6 inch and 9.25 inch scopes set up looking at the mountain for visual comparison purposes.

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jimegger
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5730914 - 03/13/13 07:00 PM Attachment (35 downloads)

Here are the tools I have been using as of late. The convexing tools in the back and the normal tools in the fore.

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Mark Harry
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Reged: 09/05/05

Loc: Northeast USA
Re: 9.25 inch refractor project new [Re: jimegger]
      #5730950 - 03/13/13 07:20 PM Attachment (30 downloads)

Here's what I use for polishers. It's a type we used to use on flats. I discovered they can aspherize surfaces just as well as those square types above, but far more predictably. Pitch on these was real old, perhaps 10 years or more, #73.
I never, never let mine get glazed- generally they're just as black and shiney as when first made. These 2 were for a 6" F/10 primary, the far one didn't work, the near one smoothed and corrected the mirror in 20 minutes. Minor differences, but those differences either made it work, or it didn't. This type of polisher can do down to around F/4.5 paraboloid without a problem.
M.

Edited by Mark Harry (03/13/13 07:23 PM)


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Mark Harry
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Re: 9.25 inch refractor project new [Re: MKV]
      #5730965 - 03/13/13 07:30 PM

I'm fairly CERTAIN the flint is not biconcave; but rather a negative meniscus.
M.


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MKV
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Re: 9.25 inch refractor project new [Re: Gord]
      #5731460 - 03/14/13 12:29 AM

Gord, I think the best and most practical way to test convex lens surfaces is by the use of test plate interference method. The last surface to be figured can be tested by DPAC test agains a flat mirror, with a fully assembled objective, or using a spearate test plate.

Jim also mentioned an interesting oil test , which requires vertical testing. The oil would have to have a refractive index very close to that of the glass used. The test would require raytrace analysis for a proper test setup. However, cleaning and refilling the oil in the exact volume would make this method highly impracticsl for figuring purposes.

And yes, R4 in Jim's case is convex rather than concave, so he has three unknowns.
Mladen


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jimegger
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Re: 9.25 inch refractor project new [Re: MKV]
      #5731763 - 03/14/13 08:28 AM

Mladen, Yes the objective design has 3 convex surfaces, R1 , R2 and R4. As to the oil or any liquid for that matter , all you need is for one surface of the lens to be covered so as to prevent a flat surface to the knife edge. It would not matter if it was just covering or a couple of inches deep although just covering it would be best and all you would need. All you want to do is see the backside curve with the front side negated. At the very least it would be an easy way to tell which surface is the bad or worst one so you know which one to work on. The interference method is good if you are doing production work but for a hobbyist doing many different lens designs you would need a plethora of tooling. With a single pass collimation system such as I have using the 12.5 inch mirror You can accurately test many different designs with one setup. Same for the DPAC.

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DAVIDG
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5731938 - 03/14/13 10:17 AM Attachment (26 downloads)

There is some misunderstanding on how the oil test is done. The oil test is done by having the lens enclosed in a tube that is as long as the radius of curvature of convex surface you want to test and the tube is filled with oil. The oil has a refractive index that matches that of the glass so what your doing is making a test tunnel that has replaced the air and filled it with oil to cancel out the glass of the lens. Your testing thru the lens and the convex surface now acts like a concave surface. If done this way then when the convex surface is spherical it will test just like a spherical concave mirror but your going to need enough volume of oil to fill the tube not just cover the lens.
If you just cover the one surface of the lens with enough oil to form a flat surface and by doing so make a plano convex lens and test thru the oil surface, you have to take into account the flat surface. If you use a fluid that has different refractive index then the glass you have to take that into account as well. So if you tested this way and figured the lens to show a null and assume that the convex surface is spherical you'll be way off. The convex surface won't be a sphere but a hyberbola.
Here are the results from OSLO showing what happens if only an enough oil is used to make a flat surface on one side of the lens and having a perfect sphere on the convex side. As one can see the spot diagram is very bad and the convex surface would look like it is strongly aspheric. So this is not a very good test for a convex spherical surface and a test plate using interference testing is much better. To do interference testing all you need to do is flash polish the concave tool used to grind the convex one and polish it spherical. When the interference fringes are straight then the convex surface is also spherical.

- Dave


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DAVIDG
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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5731958 - 03/14/13 10:27 AM Attachment (26 downloads)

Here is what happens if you assume wrongly that if you just use enough oil to make a flat surface on the one side of lens and test thru this and think that if you figure the convex surface to show a null it will be sphere. In reality what you have done is put a very strong hyberbola on that surface and when you assembled both elements of the achromat and test it you would be wondering why the image is not very good.
Here is the spot diagram showing the results. The spots are very good and the lens would test like a perfect spherical mirror but again what has happen is that the convex surface is now a hyberbolic with conic of about -1.2 and not spherical.

- Dave


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Mark Harry
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Re: 9.25 inch refractor project new [Re: DAVIDG]
      #5732496 - 03/14/13 04:14 PM

Had the same results when I tested a couple lenses. Though the convex is spherical, the test reveals an aspheric surface.
If you test the whole achro, (with a flat)you should get a null; which is what I did.
M.


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Mark Harry
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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5732499 - 03/14/13 04:16 PM

I'm almost thinking that there might be an error between R2 and R3- which can give an indication of over/undercorrection. With polishers like that, I would think the R1 surface likely has a huge hill in the center.
M.


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MKV
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Reged: 01/20/11

Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5732854 - 03/14/13 08:00 PM

Quote:

To do interference testing all you need to do is flash polish the concave tool used to grind the convex one and polish it spherical. When the interference fringes are straight then the convex surface is also spherical.



Dave, the problem today is that most people don't use solid glass tools but rather ceramic tile tools which, obviously, cannot be used for contact interference. Purchasing extra glass disks just to turn them into matching test plates makes a one-time project really costly and wasteful, so it is no surprise ATMs shun away from such techniques and seek alternative ways to get the job done.

That's why picking a well-thought-through design from the start, one that matches your skills and tooling, is so critical. For ATMs, refracting objectives or compound correctors with as many matching surfaces as possible is the most economical approach. One such example is the Houghton corrector, where the rear element is biconcave and the front element biconvex where R1 = -R3, and R2 = -R4.

Almost symmetrical telescope objectives are also possible, especially with modern glass melts. If one can devise an objective where three out of four surfaces are known to be spherical, either by Foucault or by contact interference of matching surfaces, then the system can be nulled by DPAC by working on the front surface of an otherwise finished objective. Also, configurations with very long, almost flat R4 can be advantageous.

Unfortunately, Jim here is working in the dark, except for R2 (which can be contact tested against R3). Short of making two additional test plates his engima will not be easily solved. Assuming he determines R2 is spherical, then it would be a matter of judgment whether R1 or R4 or both are the culprits.

I would guess it's probably R1 because its relative power (shorter radius of curvature) is more likely to affect the overall wavefront error, than the weaker R4 surface, and is morel like to distort the wavefront even with smaller figure errors than R4.

I don't have time to do this right now, but this can be illustrated easily using a simple raytrace analysis of wavefront variation over different conic constant values for for R1 and R2. Maybe later on tonight I can find some time to do this analysis.

Mladen

Edited by MKV (03/14/13 09:17 PM)


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jimegger
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Re: 9.25 inch refractor project new [Re: MKV]
      #5733393 - 03/15/13 01:41 AM

I do know the problem is in the crown because while working ONLY on the crown the objective went from under-corrected to over-corrected while using the convexing tools. I over shot the mark because I was trusting the flat to be giving me an honest reading which it isn't. Even though it is over shot I am now working it slowly back to where it should be and checking it with star testing. I have put 1 hour of polishing on the crown on both R1 and R2 while watching the star tests get better each time. When I get back from a trip in a week I'll be setting up the 12.5 inch mirror for the single pass collimation test and doing both Foucault and star testing in the shop so I won't have to wait for stars to come out. It should speed the whole process up considerably. R3 is right on a sphere and R4 is a long radius curve so it won't have as much effect on the image. It really doesn't matter as long as the objective nulls out in the Foucault test.

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DAVIDG
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5733701 - 03/15/13 09:30 AM

Jim,
Are you only testing the crown using double pass with the optical flat ?
- Dave


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MKV
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5733952 - 03/15/13 11:38 AM

Jim, in similar configurations to yours, the R2 will contribute a lot more spherical aberration then R1 if it is not spherical. This is just another reason why it would pay to determine if R2 is spherical or not, and the only practical method I can think of with your tooling is the contact interference test against R3.

In MIJ's 4-inch f/15 configuration, with R2 = -R3, the LA will change 2.2 times more if R2 is given cc = 0.3 (oblate), rather then if R1 is given the same conic. The R2 will give LA = 0.0585, whereas R1 in LA = 0.0262.

Mladen


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jimegger
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Re: 9.25 inch refractor project [Re: MKV]
      #5756435 - 03/25/13 03:31 PM

I have tested the crown only with the flat but also the whole objective assembly.

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DAVIDG
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Re: 9.25 inch refractor project [Re: jimegger]
      #5756583 - 03/25/13 04:51 PM

Quote:

I have tested the crown only with the flat but also the whole objective assembly.




Jim,
What is purpose of testing the crown in combination with autocollimation flat ?

- Dave


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jimegger
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Re: 9.25 inch refractor project [Re: DAVIDG]
      #5757075 - 03/25/13 09:46 PM

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

Edited by jimegger (03/25/13 09:49 PM)


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