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MKV
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Re: Ross (et al) Null testing parabolas [Re: Mike Lockwood]
      #5628712 - 01/17/13 07:29 PM

Mike, the caustic test was devised precisely because the professional community felt there was a need for a more accurate quantitative method. Since the 1930's the community has moved away form reliance on the Foucault test. In the process, it has also abandoned the caustic test because it is so time-consuming. Intrerferometry pretty much excludes the human bias factor, to which the Foucuat is not
immune. But that doesn't mean an experienced and well tooled individual can not produce superb mirrors with the Foucualt. So, yes the method is old, but it's not the tester that makes the mirror, the mirror maker does.

Mladen


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Ed Jones
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5830417 - 04/29/13 02:48 PM

This is an old thread but here is a video I did of a cheap, easy null test that might be of interest.

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Pinbout
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Re: Ross (et al) Null testing parabolas new [Re: Ed Jones]
      #5830521 - 04/29/13 03:30 PM

Great vids Ed!

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Ajohn
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Re: Ross (et al) Null testing parabolas new [Re: Pinbout]
      #5831604 - 04/30/13 05:47 AM

Thanks Ed. Another option.

I wonder if the Ross test can be re arranged to be set up like the Dall test. That is set with the knife on the centre ROC, easy to do to a fraction of a mm. The tester is then moved to bring the slit image to the same place - ie to null out the centre again or use an optical aid to achieve both settings. That then leaves the problem of measuring the distance from the mirror to knife edge and setting the test lens accordingly. To me this is a better option than a measuring stick. All depends on how accurately the mirror vertex rad is measured anyway.

Walland used Foucault to test fast mirrors having the advantage of knowing what he would see. I get the impression that this method was common augmented with a subsequent Hartman test as a final check. During final titivating he used a Ross type lens positioned to completely or partially null out SA, no measurements at all. No super precision lens either. He puts the method down to Dall and suggests a lens twice as fast as the mirror being tested.

There isn't much information about on modern methods but they seem to use nulling lenses and an interferometer. I wonder if that is a more sensible method of using Bath for figuring leaving the software for a last final check. The eye is much better at gauging flickering fringes than a camera.

On Bath I recently bought a double convex 4mm FL lens. While in theory it should be easily capable of putting out an F3+ beam from an ordinary laser diode set up it falls well short. All I can put this down to is a radial error right on the vertex of the lens. This wouldn't matter in normal use as it contributes little to the image. It looks to be a hot pressed lens and clearly shows variations in RI.

No one seems to come clean on exactly what a precision lens is for any of these uses, other than one Ross type which is too expensive in my view. Precision lenses seem to be shaped to 1/4 wave in red light but are these sufficiently precision? Dall for instance talks about standard ophthalmic crown lenses and measuring the FL so just how important is the precision of the lens for each type of testing.

John
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Edited by Ajohn (04/30/13 06:44 AM)


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Ed Jones
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Re: Ross (et al) Null testing parabolas new [Re: Ajohn]
      #5831710 - 04/30/13 07:58 AM

John,
I'm not sure I follow you, it sounds like the Dall set up.
I'm advocating this test for beginners or anyone because it's easy to set up, inexpensive ($40), insensitive to spacing and you can test any parabola even down in the F/3 speeds where the Ross starts to fail. I've looked at a number of Newport and OptoSigma lenses and find the middle area is reliably good enough. If you are really doubtful you could buy a plano concave lens and fringe test the convex curve and test the flat on a good flat. You only need to set the KE/lens space with only a little accuracy and the light source to the nearest foot.

Ed

PS It's also perfect for testing a purchased telescope without having to remove the mirror.

Edited by Ed Jones (04/30/13 08:00 AM)


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MKV
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Re: Ross (et al) Null testing parabolas new [Re: Ajohn]
      #5831870 - 04/30/13 09:49 AM

John, you can get a decent laser-quality lens by cannibalizing medium to higher price laser pointers.

Other than that, like Ed noted, I got a little lost as far as what you were trying to say, but be mindful of the fact that you can obtain an optical null even if the optic under test is not fully corrected (the "Hubble") unless your distance is within the required enevlope.

In the Ross null test, the essential parameter is the mirror to lens distance. Depending on how much wavefront error you're willing to tolerate that distance can vary as little as a fraction of a mm to dozen or so millimieters!

The conjugate null test Ed described is a litte more forgiving in that respect but it's down side is the requirement for the light source to be at a distance that makes the test subject to air currents.

Also, let's not forget that the conjugate null test is a single-pass test and not that much more sensitive than than Ronchi null-testing your scope using star light -- which is even simpler and cheaper to accomplish.


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Ed Jones
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5832028 - 04/30/13 11:17 AM

Mladen,

The big problem with the Ross is the cost of a qualified null lens, not something a beginner would use. BTW the critical airspace on the Ross is the KE/lens spacing.

The Dall conjugate is affected by air currents but there's a big difference between 100 feet in your back yard and looking through miles of atmosphere.


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DAVIDG
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Re: Ross (et al) Null testing parabolas new [Re: Ed Jones]
      #5832079 - 04/30/13 11:47 AM

Ed,
Actually the critical spacing on the Ross is the Lens to Mirror spacing and you just move the knife edge or Ronchi screen behind the Ross lens to find focus just like your doing a type Foucault or Ronchi test. Just like say in your video about your test many times only a small portion of the lens is used in the Ross Null so a typical 50mm off the self lens can also work well. As you say in your video many ATM's have problems measuring zones and a null test is better alternative. With a little bit of testing of the lens like you say in your video you can qualify the accuracy of a Ross lens as well. Places like Surplus Shed have "precision" lenses that are inexpensive so one can purchase a couple and test them to find the best one.
Having help teach mirror making classes for over 20 years now, using Null test methods ( double pass autocollimation and Ross Null) ATM first get hang up with accuracy of the test methods but soon find out that isn't the real issue. What is, is getting the mirror to show a clean null. A true 1/8 wave mirror that shows a clean null will perform better then 98% of the optics out there and one can have the confidence that they have achieved that accuracy with a little homework to prove out their test methods by checking the quality of the nulling lens they use and the accuracy in the setup for their spacing. I alway recommend double checking the results with other test methods as well. If they don't agree, then one needs to take the time to find out why and not simple ignore the results of one in favor of another. So depending on the diameter of the lens one could do a Ross Null and then your test as well using the same lens and then also a Foucault test and finally a star test. Then compare the results and see how well they agree.
Nice video of what I viewed so far. YouTube for some reason is only downloading about 3/4 of the full movie. So I don't know if you shows this or not but if you haven't it would be educational to show what a 1/4 wave of error looks like in the amount of bowing of the Ronchi lines to get a feel for the sensitity of your test under real world conditions. Maybe test a 6" f/8 sphere which is just 1/4 wave and typical of the size and F-ratio many ATM's make the as a first mirror to see how much bowing is visible compared to a parabolized 6" f/8 of known wavefront.

All the Best,
- Dave


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Ed Jones
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Re: Ross (et al) Null testing parabolas new [Re: DAVIDG]
      #5832333 - 04/30/13 01:22 PM

Hi Dave,

No the KE/lens air space is the more critical airspace in the Ross. For example a 20 inch f/4 set up with a Ceravolo null lens has .015 wave nominal wavefront test error. If you would make a 0.05 inch error in the mirror/lens airspace and find focus with the KE then the wavefront error increases to .23 waves. However a 0.05 inch error in the KE/lens airspace results in a .54 wave front error. Not sure how you can test the convex side of a Ross lens reliably without a test plate or inteferometer.

Not sure why you're not getting the whole download, I've downloaded it on 2 computers here? Yea it would be nice to have shown an uncorrected mirror. I didn't have many Newts to pick from (mostly Chiefs) and of course it had a good mirror . Later I can do one on parabolizing perhaps.


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MKV
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Re: Ross (et al) Null testing parabolas new [Re: Ed Jones]
      #5832374 - 04/30/13 01:43 PM

Quote:

No the KE/lens air space is the more critical airspace in the Ross. For example a 20 inch f/4 set up with a Ceravolo null lens has .015 wave nominal wavefront test error. If you would make a 0.05 inch error in the mirror/lens airspace and find focus with the KE then the wavefront error increases to .23 waves. However a 0.05 inch error in the KE/lens airspace results in a .54 wave front error.



I made a setup with precise spacing (using radius bars)



for both mirror-lens and lens-ke setting, and when I looked at the mirror I saw -- nothing! Of course, because the image was in focus! Moving the Ronchi screen in and out a little produced characteristic Ronchi bands, which in this case were straight. So, I am not exactly sure how can lens to k-e distance be critical, considering that in order to see the bands you may have to move your screen more than 0.05 inches in and out of focus.

Mladen

Edited by MKV (04/30/13 01:48 PM)


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Ed Jones
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5832571 - 04/30/13 03:17 PM

Well yes they both should both match. I don't think I made a mistake in my Zemax set up. I did make 2 typos. In the nominal test error, .15 not .015 waves error or about 1/6.5 waves and for an F/3 not an F/4 mirror.

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Ajohn
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5833030 - 04/30/13 07:12 PM Attachment (7 downloads)

Sorry if confusing - another go. Not sure which part caused confusion.

Ross test. I assume that it's essentially the same as the Dall null that uses and offset test unit and knife or could be except that the lens is on axis in Ross's test. On this basis rather than using a measuring stick a knife edge could be set to the centre of curvature of the mirror and the lens then introduced at the correct distance from the knife edge. As far as I am aware the only feasible way for most people to measure the radius of curvature is via a knife edge anyway. The distance is needed for the calculations and it's likely to vary as the mirror is figured. The distance from the lens to the knife edge is short and more easily measured. If the radius changes that needs changing as well. One way of taking knife edge to mirror distances quickly might be to build a steel tape into the set up.

Waland. Waland used a moving source Foucault tester. He gives detailed information on figuring 2 60in mirrors. A precision F4 and a F2.5 for IR work. He figured via shadow position using tooth picks at 1in intervals. To finally titivate a precision mirror he introduced a plano convex lens into the beam from the tester in much the same way as it's used in the Ross test and moved it around to control shadow density and spot minor zonal problems etc. Used this way the precise characteristics of the lens aren't important. I would hazard a guess that the Foucault tester would be set at the vertex radius of curvature. At some distance from the tester the lens will achieve as complete a null as it's capable of producing so it's essentially a sensitive test. As he suggest a lens that's twice as fast as the mirror under test like most he seems to be relying on the centre of the lens being good. Maybe some work with a hard pitch lap would ensure that it is.

I like shadows especially via a mask. I suspect most peoples problems relate to too coarse a control of knife edge into the beam. Maybe too narrow a slit too. I've struggled with the usual M6 tilt bolt with it's normal head. It really needs a 50mm dia head at least to give sufficient control. That way the way the shadows behave as the knife cuts into the beam can be observed. I'm not a fan of fuzzy ronchi bands I'm afraid.

Lens precision. I'm in the UK and can only buy easily from sources like this one. Shudder to think what research grade would cost

http://www.galvoptics.fsnet.co.uk/lenses.htm

Other sources give similar tolerances and state things like grade A glass and max surface errors of 1/4 wave.

There are a couple of other tests that deserve some interest. One uses a camera to read the shadows. There are 3 separate software packages available for this. The other is the caustic wire test with a loupe some way behind the wire. The wire can be centred in the fringes very quickly but the X readings need to be taken with a high degree of precision. A lashed up a mod to my tester to try it. It's basically 2 Texereau type sliding tables one on top of the other. My next version will be a more refined version of that. Once I have made it any of the usual testers can be place on it. The Bath people have finally produced a more complete write up on what is needed so if I ever build one the same x-y stage will be useful. 1/20 wave beam splitter if possible though. I am inclined to conclude that this test gets more and more complicated as one gets into it. It looks quick, cheap and simple to do but ........ Using it to figure means processing photo's at each stage or a very stable environment. I do wonder if adding a null lenses and working for straight projected fringes might be a good way of taking a tester as far as it can reasonably go. Then using the software for a final analysis.

My tester has been in a sad place for several years. Needs a rebuild anyway really. It had a sliding stop to allow the mic head to be zero'd when the knife is set to the mirror. It was basically as per Texereau except I tilted the source to set it parallel to the knife edge. Less bits to make. It was made out of odd off cuts that were available for free at the time.

I can now add the conjugate test to the list. I'm likely to need a number of options on testing.

John
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MKV
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Re: Ross (et al) Null testing parabolas new [Re: Ed Jones]
      #5833353 - 04/30/13 10:33 PM

Ed, when you do a Ross null test and your lens to mirror distance is correct, your focus and your light source should be equal and you should observe a clean null with a k-e. With a Rnchi grating you don;t observe anything at the focus! To the contrary you will see the bands (straight if the resultatnt wavefront error is nulled) as you move your Ronchi screen in and out of focus. The shape of the bands will not change indicating the wavefront is n ot shifting; only the number of Ronchi bands will change as you move the screen along the axis.

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MKV
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Re: Ross (et al) Null testing parabolas new [Re: Ajohn]
      #5833416 - 04/30/13 11:00 PM

Quote:

As far as I am aware the only feasible way for most people to measure the radius of curvature is via a knife edge anyway



In my experience a spherometer and a radius bar provide for accuratee and repeatable setups.

For the Ross test, you need a couple of clarifications: (1) set the mirror lens distance accuratelty, (2) move your light source and ke tester away or towards the lens until you see the bands. This will happen at the correct lens to source distance.


The Bath is not something you want to use during figuring. For aspheric mirrors, the Bath should be used with some type of null nulling test.

I highly recommend an optical flat. Either buy or make one slightly larger than the largest scope you think you'll ever own.

If that's not an option, then stick to the good old Foucault and Ronchi tests with Ed's conjugate null or a simple Polaris null with a Ronchi eyepiece.

Edited by MKV (05/01/13 08:50 AM)


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Ajohn
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5833803 - 05/01/13 08:08 AM

I tried to put some one off on another group from using a ronchi screen for figuring via matching patterns against screen movement. He had read it in a book so it must be good as I often say. Matched 6 patterns that I assume are shown in the book, why 6 pass. He then took the mirror for coating and asked the coater to check. Seems it's at least 90% corrected, we hope a lot more. If he has managed 95% or greater which is doubtful he would probably have a mirror just within Rayleigh's limit. If it's close to 90% figured it's way way short of that.

Out of interest the knife edge on my rusty tester can turned through 90 degrees so that the proper mic head can be used in the X direction. It reads to 0.0001in via a vernier and there were indications that finer readings than that could be taken. Biggest doubt is the Y measurements. Rather than following the method outlined in ATM III it may be better to note Y readings and then use trig to work back to the normal Foucault readings.

I do have a coated flat but have no idea how flat it actually is. Once I have a sphere it should be easy to find out. As I understand it smoothness is important and slight dish etc of the order of a 1/4 wave or so doesn't really matter. It's like testing against a star that is several miles away. This made flat backed achromats popular. The tool can be polished up and used as a flat to finish the lens the argument being that dishes of this order can just be detected with a home made spherometer zero'd on one and readings taken on the other surface. Mentioned in case some one fancies making a flat. Some people have simply ignored errors due to none figures of revolution errors in the flat.

I believe a few people have figured mirrors with Bath plus software but I suspect it's only really convenient with a web cam style set up. It all boils down to how many shots need to be taken, mirror rotated because it sags etc and how quickly they can be got into a PC for analysis.

I fancy giving Waland's method a try. I'm fairly convinced that the main problem with Foucault is the need for fine knife control both ways. I'm told deep shadows are a problem at faster ratio's and his use of the lens gets round that aspect without using an excessively wide slit. I don't need a Ronchi screen to test for a sphere as fringes can be generated with the knife edge as Texereau outlines. That allows it to be checked right up to the edge. Not disregarding advice. How well a test can perform is one of the interesting aspect to me and there is no harm in trying several. Figuring F3 is bound to be difficult. It seems spin polishing and a harder than usual lap might help with that. Something else to try as are semi flexible laps and ring laps plus anything else I can think of.

John
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Ed Jones
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5834064 - 05/01/13 10:45 AM

Mladen,
Note I said KE/lens airspace not Ronchi. Where there are no bands with a Ronchi is at focus and that's where you want the KE to be a null, then you can leave the lens fixed and put the Ronchi where you want. Perhaps I should have said lens to focus airspace. You can set it up either way but the focus/lens airspace has the tightest tolerance and the one I would want to know is most correct.

I don't use this test often but I have used it in the past. My preferred test is the Offner.


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MKV
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Re: Ross (et al) Null testing parabolas new [Re: Ed Jones]
      #5834278 - 05/01/13 12:31 PM Attachment (6 downloads)

Quote:

Mladen,
Note I said KE/lens airspace not Ronchi. Where there are no bands with a Ronchi is at focus and that's where you want the KE to be a null, then you can leave the lens fixed and put the Ronchi where you want. Perhaps I should have said lens to focus airspace. You can set it up either way but the focus/lens airspace has the tightest tolerance and the one I would want to know is most correct.

I don't use this test often but I have used it in the past. My preferred test is the Offner.



Thanks, Ed. I figured you meant lens-to-focus distance.

The Ronchi setup is easy and provides a very quick reference where your mirror's figure is, but I wouldn't attach too many numbers to it beyond 1/4 wave. A kinematic mirror stand allows for easy realignment of the mirror and the Ross lens and makes it practical enough to use the Ross setup during figuring (my mirror stand has conical support pegs)

Why Offner? More glass, more spacings...

Mladen


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Ed Jones
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5834385 - 05/01/13 01:19 PM

Quote:

Why Offner? More glass, more spacings..



Actually no, still just one airspace. Here is a description of it. When I has at 3M I fully certified the large reference/null lens (surfaces,bfl,centration) with their phase measuring interferometers. It works with the collimated output of my GH Zygo. It is used with a very weak field lens and the lens to lens airspace is the only one I need to set and I do that by zeroing off the cat's-eye focus on the field lens and then moving it forward or back depending on the mirror focal length. The tolerance on this airspace is much looser than say the Ross because the field lens is so weak. I align the mirror center dot to a dot on the null lens, easy. I get spoiled using this set up especially in figuring because interpretation is instantaneous. It easy to see straight parallel fringes, like testing an optical flat.


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MKV
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Re: Ross (et al) Null testing parabolas new [Re: Ed Jones]
      #5834559 - 05/01/13 02:39 PM

Thanks Ed. I believe the certification of the reference lens is where the rubber meets the road for most amateurs. Such a lens might be a tad bit out of their league, as is Zygo. Professional community, no doubt, has instrument options ATMs can only dream about. The beauty of products such as OSLO, Foucault, Ronchi, Bath, Ross and autocllimation is that they are affordable is that they make it possible for ATMs to design and make decent if not superb optics.

Mladen


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Ajohn
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5834928 - 05/01/13 06:07 PM

Seeing mention of ray tracing test set ups I wonder if some one could post an example oslo file. So far I haven't found a method of introducing a near point source with or without a lens in the light path.

Hopefully Thanks
John
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