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MKV
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Reged: 01/20/11

Re: 9.25 inch refractor project new [Re: jimegger]
      #5713873 - 03/05/13 05:24 AM

By the way Jim, you can also test the accuracy of your flat using the Ritchey-Common test. You will need a good sphere of a rather slow focal ratio (f/10 or so) and a bright pinhole a few microns in dimeter and a knife-edge. Make sure the mirror is a really good sphere.

You can also use a small stainless steel precision ball bearing in place of a pinhole. Just shine a super bright LED on it from a couple of feet. The image of the LED in the ball bearing should be close to or even less than the Airy disc size for the focal ratio. You want your light source small for maximum sensitivity of the test.

The other method is to use a small laser pointer < then 5 mW output, and a small lens (~ 10 mm fl) what will focus the 3 mm laser beam to a sharp point and onto a small pinhole. The waves emerging from the pinhole should be perfectly spherical and centered on the pinhole if the pinhole is small enough..

Also, it would be a good idea to measure your longitudinal spherical aberration in green light. From that parameter it is easy to figure out the wavefront error (optical path difference, OPD) and from that the surface error on R1 (provided R1 is the only culprit). Knowing how much surface needs to be removed would help you gauge how much polishing you need and not overcorrect.


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Mark Harry
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Re: 9.25 inch refractor project new [Re: MKV]
      #5713883 - 03/05/13 05:32 AM

In polishing the convex surface, TOT is the usual method. Size of the polisher should be kept down around 3/4 size; and in this way, nice long smooth strokes can be used, and it keeps the edge up. With full size, strokes can't be anywhere near as long and still maintain the same radius.
So it can be seen that with full size tool, the same effect on a convex surface can exist with long stroking, as short stroking relatively fast over the center. (!!!) Thought you might want to keep this in mind???

All the time I ever was in the mfr of optics, in contact testing, we never used shims. They were more of a headache than a solution. Just keep in mind- glass breaks and scratches, handle accordingly. With big pieces such as you have, I always tested by direct contact, but offset so that I had a good hunk of edge to grab/manipulate. Contact testing also teaches one how to -CLEAN- the glass to do the test correctly. Too bad you didn't have a concave testplate about 5" diameter- testing could be done in about 10 seconds.
Contact testing regardless, takes some time to get the hang of. But it's a very handy asset.
M.


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jimegger
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Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5714320 - 03/05/13 12:08 PM

Another way of testing to see which surface in the double convex crown is off spheroid is to put a collar around it and fill the surface with Acto oil which has a very close index of refraction to BK7 glass. Then you can do a direct Foucault test on the curve and see which one is bad. Of course you need to do the Foucault test either horizontally through a flat that is optically good or vertically looking down on the lens.
I am aware of the fringe testing but have never done it and am not sure I would be competent enough to evaluate the results accurately without more study. On my 6 inch objective I tried both sides and evaluated which one was giving me the results I wanted and it worked out. This lens will not suffer from more polishing action at this point and it has shown movement in the right direction.

On my six inch objective I turned my 12.5 inch mirror into a collimator and used a red pinhole to get a null as I had no optical flat at that time.

I am using the tool on top and have been all along , I was short stroking it all the time though and believe that is why I have an oblate spheroid as a result.


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jimegger
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5714649 - 03/05/13 03:00 PM

I just spent 1-1/2 hours polishing long narrow w strokes on the R2 curve and have seen significant improvement in the auto-collimation test. The figure is slowly working towards a sphere.

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Mike I. Jones
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5714894 - 03/05/13 05:00 PM

Would love to see DPAC Ronchigrams of your progress. Just put the camera on a firm mount with the lens behind the Ronchi grating, and focused on R4 of the lens.
Mike


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MKV
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Reged: 01/20/11

Re: 9.25 inch refractor project new [Re: jimegger]
      #5715470 - 03/05/13 10:04 PM Attachment (36 downloads)

Quote:

Another way of testing to see which surface in the double convex crown is off spheroid is to put a collar around it and fill the surface with Acto oil which has a very close index of refraction to BK7 glass. Then you can do a direct Foucault test on the curve and see which one is bad.



I never heard of such a test for convex surfaces, and don't really understand how it's supposed to work. Here is a diagram of what you describe.

Mladen

Edited by MKV (03/05/13 10:39 PM)


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jimegger
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Re: 9.25 inch refractor project new [Re: MKV]
      #5716306 - 03/06/13 12:32 PM

By doing that with the oil you have effectively eliminated the upper surface curve and now can do a Foucault test on the concave back surface same as when testing any concave.

Mike , I do not have a Ronchi screen. What would I need for one - matched to this lens focal length ? I know Willman Bell sells them and they go by focal length for the mirrors. Is not the knife edge a sort of Ronchi test in itself ?


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Mike I. Jones
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5716342 - 03/06/13 01:00 PM

I like and use a 1" square straight-line Ronchi grating with 100 lines/inch, like Edmund Optics sells, for wavefronts slower than f/8 at COC (f/4 at infinity focus). I find the diffraction sidelobes to be annoying much above 100-150 lines/inch.

Be careful to get a straight-line Ronchi grating from either Willman Bell or Edmunds, not a curved-line inverse(Mobsby) grating! Try the W-B first; it is 10X cheaper than the Edmund, but the lines may not be as sharply defined.

Yep, the slitless KE acts as one Ronchi bar, and to me is better for showing surface smoothness and faint zonal shading. I personally like Ronchi's test for looking at the overall figure, and especially for locating breaks in curve slope when TDE is present. But these are just my personal experiences, not necessarily what works best for you.

Mike


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MKV
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Reged: 01/20/11

Re: 9.25 inch refractor project new [Re: jimegger]
      #5716456 - 03/06/13 01:51 PM

Thanks Jim. I take it you have tried this test with oil. It is not as simple or practical as it sounds. On the other hand, the contact fringe method is a proven and easily repeatable method used in industry. When you are "tweaking" a surface, you need a method that is quick, clean and easily repeatable very frequently. There is also plenty of information on the Internet about the method.

The oil method resembles the way one can test a Cassegrain secondary. It requires ray raytracing to determine the setup parameters. If you use oil, it would require making a damn for oil and refilling it with oil after each figuring spell, which in the final phases can mean couple of times an hour, allowing the lens to cool down after handling for a short time and short figuring spells of about one minute each.

The vertical position of the lens is almost a must in this case, to keep the oil surface flat, which only complicates the setup procedure, and makes the oil method too impractical for production purposes.

Once you know R2, R3, R4 are spherical, the fourth surface is worked on under DPAC until you get a clean null. No guesswork involved.

Mladen


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jimegger
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Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: MKV]
      #5718715 - 03/07/13 03:58 PM

I have not done this test myself Mladen but it is discussed in one of Ingalls Telescope making books. It can be set up so you do it horizontally by using an optical flat at one end of a tube and the lens at the other end with the fluid filling the tube. The other way is to view the lens looking down on it with the fluid surface rendered flat by gravity. It is a good way to see which surface is the worst or bad then work on it from there.
I ordered a Ronchi screen from Willman Bell but in the meantime I am polishing with the convexing tool that has the center area pitch thinned out gradually towards the edge so as to have less action on the center and allow for it to come up faster as the outside gets worn down. It appears to be doing its job okay so far and the knife edge is being used to check the curve until my Ronchi screen gets here. The center flat spot is shrinking in size.


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MKV
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5718999 - 03/07/13 06:23 PM Attachment (46 downloads)

Tanks Jim. No doubt the oil test is possible for finished optics. But it's not practical for the final figuring phase of completing an objective. Covering a lens with oil over and over every half hour or so and testing the figure is tedious, imprecise, and unnecessary work and requires very exacting setup which can be determined by raytracing.

Contact interference tests are clean, quick and require no calculations. They are practical and accurate. The only thing that's required is a good cleaning method and very careful handling.

I think you will find the DPAC Ronchi test to meet the frequent testing requirements very handy for a "quick" assessment. I would make sure the glass has completely thermally stabilized (mabye 15-30 minutes for a 9-10 inch lens) after handling it, before assessing the results. You can also use the grating as a knife-edge test for greater wavefront surface detail. Just remember that theRonchi test is not as accurate as the knife edge shadow test. The Ronchi will give you a good idea where you stand overall with the wavefront.

Using a single Ronchi band test is quite sensitive if you slowly pan the band across the illuminated disk and observe any changes. At this stage, a 133 lpi can actually give a hint of very small wavefront surface errors. The picture below shows a 6-inch f/4 spherical mirror tested at ROC with 133 lpi grating and a small "hill" or bump seen as a "bite" against the edge of the band, as shown by the arrow. In a knife-edge test, the bump was clearly visible, and interferometry placed it at approximately 1/20 wave elevation.

Mladen.

Edited by MKV (03/07/13 06:29 PM)


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Mark Harry
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Re: 9.25 inch refractor project new [Re: MKV]
      #5719904 - 03/08/13 06:46 AM

One line and space, subtends about .008" in the above pic.
********
Nowhere near sensitive enough. (a grating is not an IF) There is an edge issue as well. I hope it isn't coated yet?
(what do I know?)
M.


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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: Mark Harry]
      #5722684 - 03/09/13 05:21 PM

After quite a bit more polishing with the convexing lap , there seems to be an obvious improvement in image quality while testing the scope on the mountain. I am suspicious of the auto collimation test due to uncertainties about the flat being up to snuff. After a few hours of polishing on both crown surfaces the Foucault test does not show as much change as I think it should. The star test showed under correction as I said earlier , but it wasn't bad. I am going to use my 12.5 inch as a collimator like was used with the six inch objective as I know it is excellent and gave me a top performing objective. That way I can double check the results from the flat as well.

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Mark Harry
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5722801 - 03/09/13 06:43 PM

Nice way for a double-check.
M.


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jimegger
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Reged: 12/22/05

Loc: Palmer,Alaska
Re: 9.25 inch refractor project new [Re: Mark Harry]
      #5728860 - 03/12/13 07:32 PM

So here's the skinny on all this polishing action up to date.
The objective is VERY polished now after many extra hours of figuring trying to get it right in the double pass auto-collimation test. It turns out my optical flat isn't so flat !! Star testing has proved that. I started out with an under-corrected objective according to the flat and star testing, after using the convexing tools on the crown the flat showed still- an under correction while the star test shows a definite switch to over-correction now. So now I am discarding the flat for testing and am going to use my 12.5 inch mirror as a single pass collimator tester. It worked beautifully for the 6 inch refractor I made. It's simple to do. Just put a pinhole at the focus of the mirror , in this case a Mini Maglite with a perforated foil cover for the pinhole. You can tell when it is in perfect focus by using another telescope focused to infinity to examine the pinhole reflection at high power. When the pinhole is sharply defined you have it exactly right distance giving parallel rays off the mirror. Now all one needs to do is set up the objective to be tested in its tube assembly looking into the main mirror. I use a knife edge made to insert into the 2 inch focuser to do a Foucault test then also examine the pinhole image at high power to check for sharpness. It is amazing how well this setup works ! With the 6 inch objective the sudden sharp focus and image were suddenly much better after some touching up in polishing/figuring as backed up by a really good Foucault test. For those who have no access to a good flat but have a good parabolic mirror available of the proper size, this works out just fine.

I am now working to correct the over corrected nature of the 9.25 objective and am closing in on the target.


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Mike I. Jones
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Re: 9.25 inch refractor project new [Re: jimegger]
      #5728992 - 03/12/13 09:04 PM

Jim,
See PM from me.
Mike


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DAVIDG
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Re: 9.25 inch refractor project [Re: Mike I. Jones]
      #5729948 - 03/13/13 09:30 AM

Jim,
The flat would have to be a 100 waves from flat to show an error in a double pass test. The problem I believe is that your reading the test like you would a mirror and for a lens every thing is backwards. What looks like hole on a mirror is a hill on a lens. So undercorrection is actually overcorrection when testing a lens. A double pass test has twice the sensitivity of a single pass test plus it has the added advantage that if the flat is smooth, the errors on it have little effect on the results but in a single pass test the error adds from the telescope producing the collimating beam, 1 to 1. A single pass test can also give you a false positive result. If your lens is 1/8 wave undercorrected and the telescope making the collimated beam is 1/8 overcorrected, they add and the test would show a perfect null. The same is true if both optical system were 1/8 under corrected, in a single pass test the result would be 1/4 undercorected. This doesn't happen in a double pass test.

- Dave


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kfrederick
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Re: 9.25 inch refractor project [Re: DAVIDG]
      #5730352 - 03/13/13 01:20 PM

Dave /Jim flat is one I got off the shed I think Garys test uses one like it .I have not tested it. It was sold as 1/10 wave .Maybe test your good refractor lens with it .

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MKV
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Reged: 01/20/11

Re: 9.25 inch refractor project [Re: DAVIDG]
      #5730518 - 03/13/13 03:01 PM

Quote:

Jim,
The flat would have to be a 100 waves from flat to show an error in a double pass test. The problem I believe is that your reading the test like you would a mirror and for a lens every thing is backwards. What looks like hole on a mirror is a hill on a lens. So undercorrection is actually overcorrection when testing a lens.



Initially, Jim reported the error to be such that the central focus was long and the edge or marginal focus was short, indicating the wavefront was oblate.

Since Jim also reported that his flint biconcave element had spherical surfaces, clearly, one or both sides of his biconvex crown element depart from the spherical shape.

I suggested that Jim test R2 against R3 by contact interference method in order to determine if R2 is spherical or not. The test is simple, quick and repeatable and easy to interpret and is quantitative. It is also used in industry all the time. A little reading about it, proper cleaning and handling avoids scratching the lenses.

Once he knows R2 through R4 are spherical, and the objective is still not corrected, the flaw is on R1. All he has to do is figure the front radius of curvature accordingly, and test the results by autcollimation until he reaches a clean null either by knife edge or by 100-133 lpi Ronchi grating. There is no guesswork in any of it.

As for his flat, he could have tested it by autocolliamting his 12 inch paraboloid, which according to him is good optics. If he got a clean null, the flat is good enough.

Mladen


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Gord
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Re: 9.25 inch refractor project [Re: MKV]
      #5730542 - 03/13/13 03:28 PM

Mladen,

A couple of very in-experienced questions:

1. How do you test R4 since it is a convex? Is there a way to test it from the front so it would appear concave? How would the differentiation be made between R3 and R4 then?

2. Once R2-4 are all good and the testing is being done on the remaining R1, do the elements have to be spaced properly (as per the design) during the test? Could any deviation from the design spec of any of the R2-4 or the spacing cause the reading of R1 to be incorrect or mistakenly interpreted?

Thanks,


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