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Brian Engel
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Reged: 08/25/09

Loc: Cincinnati,Oh
Ross (et al) Null testing parabolas
      #5610903 - 01/07/13 09:03 PM

Been a long time since I've posted but now after buying a new house and having a baby I am able to get back into mirror making once again :-).

So this has me thinking of building a null tester (again) and finish up work on my 20" f/4 (well, the hard part - figuring - has yet to begin).

I've read the rather long and interesting thread on Ed Jones' modified conjugate null test and just don't think that is going to work as I don't want to build the telescope 1st. So that leads me back to a Ross null tester.

Before I do, I have one nagging thing I want to make sure I understand (well, many things really :-). I had this discussion in a thread several years ago but.....

So much concern is made for very accurately measuring the "object" to lens and lens to mirror spacing. Measuring the object to lens distance very, very accurately is a piece of cake.... as you can get within several thousandths easily. So this is of no concern....

My question is why must the lens to mirror spacing be measured at all? Other than of course having a rough idea where to place your tester?

If you build a Ross Null tester that has a "stage" where by the source to lens spacing is measured and fixed; couldn't you just move that entire stage back and forth from the mirror until you get the best possible null (the object -lens spacing would NOT change)?

e.g. I would use the nifty "Ross Null Finder" software, plug in my lens data, mirror info etc... get my object to lens spacing, set that on my movable stage (as described above) and then just use a tape measure to measure my lens to mirror distance within 1/2". Then I would look across the KE or ronchi, and then precisely move the "fixed stage" back and forth until I get the best possible null. If I don't and see a raised zone etc... I would go back rework that area, then repeat.

As I understand it (which may be wrong), given a conic constant (-1 in this case) and using the object to lens spacing calculated by the software; there is only ONE conic I will null at and I can only get there when I have a parabola at the distance the software calculates.

To look at this the opposite way. For my understanding to be wrong, given that I set the object to lens distance the program tells me for a -1 conic, I can possibly achieve "false" nulls at other conics (hyperbolas etc...) by moving the "fixed" object/lens stage back and forth.... and thus end up with horrible under/over correction even though I see a perfect null through the Ross Null tester.


To make it more formulaic.... Object to lens distance (X) + Lens to mirror distance (Y) = -1 (Parabola).

If X is constant (as determined by the software), there is no Y value that will give me a null conic other than -1.

...and of course, I would be using my trusty Foucault tester as well and not rely solely on this.

Thoughts? Am I totally off base here?

Thanks...


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GlennLeDrew
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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5611287 - 01/08/13 01:25 AM

The mirror-to-null lens spacing is absolutely fixed for any given combination of mirror radius and null lens form. One cannot naively 'jiggle' things about, for then a Hubble-like incorrect figure will be imparted.

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MKV
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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5611310 - 01/08/13 02:15 AM

Brian, the Ross null test doesn't work the way you describe it. For accurate distance, rather than a measuring tape or a ruler, you may consider using a radius bar, like the one pictured below. The bar has nylon pan head screws for a single point contact at each end, and its total length is easily measured with calipers to within ±0.001".



You will also need some sort of leveling meachanism for the sled. My sled (above) rides on another sled which has 3 leveling screws. Both sleds use t-slots for anchoring.

As for the test itself, you must set both distances (light source-to-lens and lens-to-mirror) correctly, then (1) lock the sled, and (2) unlock the light source in order to be able to pull back and forth a little (in and out of focus) in order to observe the fringes/shadows, since the calculated distances are for the exact focal position. If you have a true paraboloid, you's see a null. Otherwise you will see bowed Ronchi bands or and k-e "doughnut" shadows.

The idea that you can just slide the sled back and forth until you see a null, and then check if the lens to mirror distance is correct is not how it works.

If things worked the way you describe it, there would indeed be no reason to measure the lens-to-mirror distance. The only way you can be certain you have a true paraboloid is if both distances are correct (within the tolerance. chosen).

One more thing: I checked the spacings necessary for a 22 inch f/4 mirror, and 1/16 wave tolerance on the mirror (1/8 on the wavefront), and for that precision both of your distances must not vary by more than ±1.2 mm (0.047 inches), which is less than 1/20". You will need a rigid radius bar and a good measuring tape (preferably in mm) for the lens to mirror separation as well.

Also, since the Ross lens has trenmendous amount of chromatic ebrration, the test must be done either with lasers or by narrow band filters. Care must be taken to interpret the final data correctly in terms of the wavelength closest to the sensitivity of the eye close to 550 nm.

In the initial stages of figuring, this may not be as crucial as it is the final stage. Thus an optical null in the red (say 632.8 nm) will not be an optical null in the green (547 nm)., as can be seen in this quality Rohr example

http://rohr.aiax.de/ts-lomo03.jpg

Mladen


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Ajohn
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5611396 - 01/08/13 06:18 AM

I haven't made a mirror for a long time but used the Dall null test which is similar. Here the mirror rad is measured and the tester set up. It's then positioned so that the image of the pin hole is exactly on the centre of curvature of the mirror. No need for measurements just a case of accurately locating where it is and then moving the tester not the knife edge. Dall suggested a type of red filter which I couldn't find so I substituted a red led.

The colour problems are down to the wavelength the set up is designed for. I suppose it could be ray traced for blue really but red was used because dispersion of optical crown is low in that area.

I followed Texereau to the letter to start off with but found the shadows in the mask hard to read. His idea for testing a sphere for turned edge are really good. It's just a case of slit size and where the knife goes.

The Dall test had many curious artefacts in the view but eventually everything looked ok. I then went on to use a version of the Hartman test using a wire and 1in dia holes in the mask to get an accurate idea of what I had. I used a 10x loupe instead of an eyepiece. Hard to make an x-y set up at the time but with the availability of cheap Chinese lathe compound slides (top slides in the USA?) very easy to set up now.

Also tried Ronchi. Bits not available in the UK so made one by carefully winding 2 strands of fine wire and round a metal plate with a hole in it and then removing one strand. Made me wonder why people use it.

Given the quality of some of the Chines lathe parts I might use the same from Taig or Peatol in the UK. Depends on how much movement is needed.

John

Edited by Ajohn (01/08/13 06:26 AM)


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DAVIDG
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Re: Ross (et al) Null testing parabolas new [Re: Ajohn]
      #5611670 - 01/08/13 10:25 AM

We use the Ross Null as one of the main tests along with Double pass autocollimation at the Delmarva Mirror Making class which we have been doing for 12 years now. http://www.delmarvastargazers.org/archive/mwhome.html
Over at least the last five years, one or more mirrors made at our class has won an award at Stellafane in the optical judging. I'm not bragging just saying that this is an independent verification of the test methods and proceduces we use and we have many years of experience in using this test to make quality optics.
You don't need to worry about the lens to object distance, what is critical is the lens to mirror distance. When this is set you do the test just like a Foucault or Ronchi test and move the knife edge or grating inside or outside of the exact focus position to examine the figure on the mirror. I recommend using a Ronchi screen with 100 to 133 LPI. Also if you use the Ross Null program it will give you the tolerance that you need to set the this distance for mirror to lens spacing.
Don't get hung up on the tolerance of setting the spacing. What your going to find is that it is MUCH more difficult to achieve a figure that shows a perfect null then being sure that the spacing is good to a few thousands of inch. A simple radius rod made from some pine works very well. You also need to know the radii on the lens and radii of the mirror to far degree of accuracy and you need to test in monochrome light of a known wavelength. Your tester needs to have the knife edge/grating coplanar with the light source.
Once you achieve a clean null, you can double check the results using other test methods and I HIGHLY recommend that you do use other methods to check the results. What your going to find is that each method has some uncertainty associated with it, but each test method should agree with the others to within the error tolerance of the test. If they don't, you need to determine why before you can having confidence in the results.
What a null test is going to show you is exactly how difficult it is going to be to polish a true 1/8 wave wavefront on a mirror of this size and F-ratio. As I said achieving anything close to a null is were the work is going to be and not having the spacing set exactly.

All the Best,
- Dave


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Brian Engel
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5611676 - 01/08/13 10:26 AM

Quote:


If things worked the way you describe it, there would indeed be no reason to measure the lens-to-mirror distance. The only way you can be certain you have a true paraboloid is if both distances are correct (within the tolerance. chosen).

One more thing: I checked the spacings necessary for a 22 inch f/4 mirror, and 1/16 wave tolerance on the mirror (1/8 on the wavefront), and for that precision both of your distances must not vary by more than ±1.2 mm (0.047 inches), which is less than 1/20". You will need a rigid radius bar and a good measuring tape (preferably in mm) for the lens to mirror separation as well.





If the object to lens distance remains fixed, can you get a "incorrect" null if you don't have the tester within the lens to mirror spacing the Ross Null program puts out?


Given a particular lens, and a particular spacing (as calculated by the Ross Null software), I thought there is only one condition where you will get a null.... 1) the surface is a parabola and 2) the spacing lens to mirror is exactly as calculated by the ross null software.

It is a 20" f/4 and as you have surmised, the lens to mirror spacing is quite tight....which is why I am concerned about it :-).


I guess what I am getting to is I am wondering what the consequences would be if I had my object to lens distance perfect but had my lens to mirror spacing off by, say, 10mm, then figured the mirror to a perfect null (if possible).

Also, BTW, I was planning to use a green LED for the source.


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Pinbout
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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5611688 - 01/08/13 10:38 AM Attachment (22 downloads)

if you could make it in march, you should attend the Delmarva Mirror Making Seminar, hurry up and sign up.

Steve Swayze uses the rossnull to final figure the mirror. you'll see it first hand and get experience with it.

He uses red led's.


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DAVIDG
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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5611691 - 01/08/13 10:40 AM

Quote:

Quote:


I guess what I am getting to is I am wondering what the consequences would be if I had my object to lens distance perfect but had my lens to mirror spacing off by, say, 10mm, then figured the mirror to a perfect null (if possible).

Also, BTW, I was planning to use a green LED for the source.




You don't want to set the lens to knife edge or ronchi distance. You need to be able to move the knife edge or Ronchi screen just like you do when your doing a typical Foucault test so you can see what the figure is. You set the lens to mirror distance and just do the test like your testing a spherical mirror.
If you use a green LED then your going to need to look thru a narrow band interference filter that has band pass of 10nm or less. This will define the wavelenth that your testing at and determine the spacing for that wavelength. You don't need the filter until you get close to a null, but once you get close you'll need to is sharpen up the Ronchi lines and/or improve the contrast of any faint zones you see with a knife edge.
I just use a Ronchi screen and position it so there are three lines showing and fairly quickly move it from inside to outside of focus and back again while looking for any slight bowing of the lines. When you can not detect any departure from dead straight, you have a good null.

- Dave


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Brian Engel
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Re: Ross (et al) Null testing parabolas new [Re: DAVIDG]
      #5611783 - 01/08/13 11:31 AM

Alright, I think I see what DAVIDG is saying....

As you described, the lens to mirror distance is fixed precisely. This has the effect of introducing aberration such that it allows a perfect parabola to focus at a single point. Then you vary the *KE (or ronchi) to lens spacing* to see the traditional shadow movements that you see with a standard foucault test. You move the knife edge back and forth through focus and make corrections as needed to the mirror surface to achive a null. So in effect, you really don't care about the KE to lens spacing. The "corrected" focus is where it is and you must move the KE to it.

What I am wondering is, why can't we fix the KE to lens distance and vary the lens to mirror spacing (by moving this stage back and forth)? Won't this have the same effect as above?

Edited by Brian Engel (01/08/13 11:32 AM)


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MKV
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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5611898 - 01/08/13 12:52 PM

Brian, the lens-to mirror distance is not critical for the Ronchi or Foucualt method. It is critical for interferometry because you you want to be exactly in the mid-focus position (i.e. between the marginal and paraxial focal points) in order tom get useful fringes.

The sled is useful for keeping everything lined up when you move the light source. Move the sled to the correct lens-to-mirror distance, then lock it in place. Then unlock the light source carriage and move where needed.

For lens-to-mirror distance use stiff radius bars with adjustable nylon tipped ends (a 1/4-20 screw is good enough). Tweak the ends to the exact distance needed and support the bar to avoid any sagging. Measure the length of the radius bar with a tape measure to the nearest mm.

Using autocllimation to check your Ross results is a good approach, but limited. At 22 inches, your mirror has pretty much exhausted that option unless you have access to a 24-inch flat. Foucault test is highly unreliable at f/4 and the Ronchi is not much more than "eyeing" the parabola - useless as far as determining the exact figure.

Your only other option is an interferometer, but again unless you have one that has a null lens built into it, such as the Zygo, you will have to depend on the Ross lens as your nulling lens, and therefore the correct lens-to-mirror distance, again, is the key factor. So, tailor your Ross test setup to meet the lens-to-mirror distance and the object-to-lens distance requirements to the "t".

Mladen

Edited by MKV (01/08/13 12:53 PM)


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MKV
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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5611935 - 01/08/13 01:15 PM Attachment (13 downloads)

Quote:

What I am wondering is, why can't we fix the KE to lens distance and vary the lens to mirror spacing (by moving this stage back and forth)? Won't this have the same effect as above?



You should be able to, provided you use an eyepiece which is focused on the plane of the light source, set at the correct source-to-lens distance. Then, by sliding the sled until the image of the source is in sharp focus, the lens-to mirror distance should correspond to the theoretical value within the margins of error allowed.

Mladen

Edited by MKV (01/08/13 01:44 PM)


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Ajohn
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5611997 - 01/08/13 01:55 PM

If a knife edge can produce a null with the ross test it must be possible to set a lens to knife edge distance and then move the lot as a unit.

The yahoo atm_free group is heavily into bath interferometers. They are common path and do not need expensive parts and the analysis software is free. These use a none polarising cubic beam splitter, a small mirror and a lens plus a red laser diode run below the lasing point to obtain a small source.

Making an xy stage may be expensive or a pain so I have put a microscope xy stage to one side.

This is the groups wiki

http://starryridge.com/mediawiki-1.9.1/index.php?title=Bath_Interferometer

They are a friendly lot on the yahoo group but following too many questions they will say best to build one and try it. There are plenty of reports by people who have used them on the group and for what you are doing it's likely to be a sensible way to go. Me too if I finish an F2.5 mirror.

John
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DAVIDG
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5612011 - 01/08/13 02:06 PM

Think of how you typically do a Foucault or Ronchi test by moving the knife edge/grating to different positions in the light path to examine different zones on the surface of the mirror. If the lens was moving along with the knife edge/grating, it would be varying the amount of spherical aberration and you again could find a position were it nulls but the conic is not what you want. So you couldn't examine a zone by looking a little inside or outside of focus. You really limit the usefulness of the test if you fixed the lens to knife edge distance.
This test is not difficult to do. One huge advantage is that you see how the whole mirror is departing from the parabola you want, just like one sees how a mirror is departing from a sphere. So a hole is a hole and a hill is hill and turned edge is turned edge but instead of sphere as the reference surface they are departing from it is a parabola. Even if the spacing isn't dead perfect and you achieve a null, the result is a slightly over or undercorrected mirror but that is smooth. That is much better then a mirror that is "zoney".
As I said, you'll discover that actually getting a mirror to show a clean null is going to be the real challenge. Once you get to that point you can start to worry about getting the spacing dead on and tweeking the figure. You'll also discover that the differences from a perfect null at very slight differences of lens spacing is not easy to detect. So it is going to take some practice to detect these small differences. Again I recommend using a Ronchi screen over a knife edge and by moving the screen from inside of focus to the outside and back again fairly quickly, you can detect very small chances in the straightness of the bands.
When you think you have a perfect null, move the lens spacing by 1/8" and do the test again and see if you can detect the slight bowing of the lines. When you have trained yourself to be able to detect the difference, then you can worry about getting the exact lens to mirror spacing.

All the Best,
- Dave


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Brian Engel
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5612030 - 01/08/13 02:14 PM

Quote:


You should be able to, provided you use an eyepiece which is focused on the plane of the light source, set at the correct source-to-lens distance. Then, by sliding the sled until the image of the source is in sharp focus, the lens-to mirror distance should correspond to the theoretical value within the margins of error allowed.





Why is the eyepiece needed? Guess I am missing something.... The source and the KE would be the same (slitless).

I am thinking that the same "effect" of looking across the KE with the the lens to mirror distance precisely fixed and varying the KE to lens difference would be exactly the same as fixing the source/KE to lens difference and varying the lens to mirror difference.

Guessing this is not the case? (obviously my understanding of lenses/optics is pretty limited :-) )


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MKV
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Re: Ross (et al) Null testing parabolas new [Re: Ajohn]
      #5612512 - 01/08/13 06:52 PM Attachment (11 downloads)

Quote:

If a knife edge can produce a null with the ross test it must be possible to set a lens to knife edge distance and then move the lot as a unit...The yahoo atm_free group is heavily into bath interferometers.... There are plenty of reports by people who have used them on the group and for what you are doing it's likely to be a sensible way to go. Me too if I finish an F2.5 mirror.



John, when I do the Ross test I set both distances. This way I know I am starting from the theoretical focus. To see the Ronchi bands, you have to move the source carriage a small distance until you see the bands. David G was only saying that for a Ronchi method you don't need a fixed object-lens distance, and he's right - because the Ronchi test doesn't work at the focus, but slightly in or out of the focal range.

Rather than using k-e at the focus,it's much simpler to use an eyepiece at the fixed source-to-lens distance. If the eyepiece shows a sharp image (a diffraction disc) of the object, but the lens-to-mirror distance is off, then your conic is off too. When both distances are correct and the image is in sharp focus, then the conic constant is correct within the tolerance limits.

For interferometry, you need to be in focus, so the source-to-lens distance needs to be fixed in addition to the lens-to-mirror distance.

Bath interferometers are very useful and easy to construct. I use them all the time. However, a Bath will not be of much use for an f/2.5 mirror. You're better of using an autocllimator or some other null test (such as conjugate null) with such short radius of curvature.

Mladen

Edited by MKV (01/09/13 02:00 AM)


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MKV
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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5612529 - 01/08/13 06:59 PM

John, for an eyepiece test you should use a pinhole, not a slitless source. Slitless sources are not as sensitive. Anyone who's done a Ritchey-Common test for flats knows this from experience. The closer the light source is to the resolution limit the more sensitive it appears.

For the Ronchi, test you must be inside or outside of focus. That's why you should be able to move your object source in and out of the focal position. You will immediately see if you are overcorrected, undercorrected or just right.

You can then apply the k-e and see if you get a null. If you don't have a null, the k-e test is of little value here because the TA is different than at ROC to determine how much you're off.

Mladen

Edited by MKV (01/08/13 09:31 PM)


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Ed Jones
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5612578 - 01/08/13 07:32 PM

Because the focal length of the lens is a lot shorter than the mirror then it's separation from the KE/source is by far more critical. This is assuming the lens focal length is known to a high degree of accuracy (which it should be) for the test wavelength and most likely the mirror focal length will be less accurately known. You should set the KE lens separation and use the lens/mirror separation as a check. Both should be good.

On the other hand if the lens is less accurately known and the mirror is known to high precision then it might make sense to do the opposite but then why would you be testing with a less qualified lens?


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MKV
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Re: Ross (et al) Null testing parabolas new [Re: Ed Jones]
      #5612766 - 01/08/13 09:37 PM

Quote:

On the other hand if the lens is less accurately known and the mirror is known to high precision then it might make sense to do the opposite but then why would you be testing with a less qualified lens?



That's a good observation. Industrial lens standard is not that high, so are you saying that most, if not all, amateur Ross tests are done with lenses of inferior quality compared to the mirrors?

Mladen


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Ed Jones
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5612892 - 01/08/13 11:07 PM

Well they have a focal length tolerane of 1 or 2 percent. Off-the-shelf lenses may not be good enough and need to be checked.

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MKV
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Re: Ross (et al) Null testing parabolas new [Re: Ed Jones]
      #5613068 - 01/09/13 01:49 AM

Quote:

Well they have a focal length tolerane of 1 or 2 percent. Off-the-shelf lenses may not be good enough and need to be checked.



One to two percent focal length tolerance in a typical PCX lens (although it doesn't have to be PCX) used in the Ross is negligible. Try raytracing it and adjusting the bfl.

What's more important is the surface quality, but since you're typically using only the inner 30-40% of the CA, or even less in some cases, chances are your lens meets the 1/4 to 1/8 PV wave WFE, which, as DAVIDG observed, is not so easy to achieve, contrary to popular opinion, especially for big mirrors.

Perhaps if you're shooting for 1/16 wave WFE you may find yourself having to either make a better lens, or buy one for a hefty price, but surplus precision lenses, with documented data are not too expensive, and come in more than sufficient size needed for a Ross. For example SS lens PL1031 is 75 mm in diameter, fl = 185 mm, BK7, optimized at 546.1 nm (so it should be used with a correspodning NB filter), unused, and costs $8! Its data sheet number LPX 263 is from Melles Griot from which you can see the tolerances and precision, as well as other pertinenet data.

I am using a 115 mm diameter, precision positive meniscus from SS, and only its inner 35% central diameter is used.

Chances are such lenses meet the 1/4-1/16 WFE requirement within those parameters.

Mladen

Edited by MKV (01/09/13 01:52 AM)


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MKV
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Re: Ross (et al) Null testing parabolas new [Re: DAVIDG]
      #5613084 - 01/09/13 02:17 AM

Quote:

If the lens was moving along with the knife edge/grating, it would be varying the amount of spherical aberration and you again could find a position were it nulls but the conic is not what you want



Dave I don't know if you replied to me (my post is referenced in your reply), but I just wanted to say you're right. The only time the lens moves is to set the lens-to-mirror distance. From there on you can do a fixed object-to-lens distance "at focus test" (k-e looking for a null, or an eypeiece test looking for a sharp image of the pinhole), or you unlock the light source stage and move the light source and the Ronchi grating in an out to assess the correction, as you said.

The latter is absolutely the easiest Ross null method. With it, one can immediately see if the mirror is overcorrected or undercorrected, or "just right" (straight "jail bars").

A Ronchi null assures that the WFE is ~ 1/5 to 1/6 wave p-v, especially if using a single Ronchi bar and slowly panning it across. This by itself assures you that the mirror is good to 1/10-1/12 waves, and, in the absence of too many scratches, pits, hills, etc., a smooth mirror surface testing as an optical Null in a Ross will have an even more impressive RMS rating.

Mladen


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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5613261 - 01/09/13 08:14 AM

Most of the off-the-shelf lenses I've seen do not meet the 1/10 wave regularity requirement. Finding a good lens is one of the problems with the Ross test.

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Re: Ross (et al) Null testing parabolas new [Re: Ed Jones]
      #5613304 - 01/09/13 08:45 AM

Quote:

Most of the off-the-shelf lenses I've seen do not meet the 1/10 wave regularity requirement. Finding a good lens is one of the problems with the Ross test.



Just curious, how do you test a lens for that quality? Aren't lens requirements less stringent than for mirrors?

Most lenses may indeed not meet the required quality, but precision lenses, especially if large, and used only in the 1/3 inner diameter, are usually deemed adequate.

Mladen


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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5613369 - 01/09/13 09:20 AM

Quote:


Just curious, how do you test a lens for that quality? Aren't lens requirements less stringent than for mirrors?

Most lenses may indeed not meet the required quality, but precision lenses, especially if large, and used only in the 1/3 inner diameter, are usually deemed adequate.




Certainly hope this is the case... the ross null tester I am about to build is using about 1/2 the lens diameter.


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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5613396 - 01/09/13 09:34 AM

Just to circle back, I think I am getting it. Given lens X, the lens to mirror spacing defines the amount of "offsetting " spherical aberration that is introduced.... i.e. the conic that it will null at.

It seems to me the KE/source to lens distance is really irrelevant. The focus is where the focus is, you move the KE to it. That is, it behaves just like testing a sphere at its RoC. Now, theoretically if you have the lens to mirror spacing exactly right and you have a perfect null; then the KE to lens spacing should be what has been calculated.

Is this about right (I hope :-))?


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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5613750 - 01/09/13 01:41 PM

The Bath seems to have a diameter limit at shorter focal lengths. There is a brief thread on the subject here

http://tech.dir.groups.yahoo.com/group/atm_free/message/11257

In this case it's a 210mm mirror that when I measure it is more likely to be a bit over F2.5 rather than under.

Still not sure I get this fixed distance from lens to mirror. What ever that distance is the mirror when figured will only show a null when the knife edge is at a certain distance from the lens. I do see a problem though. So maybe I do get it. When glass is removed to create the conic it can be removed from several different places to create it. I assume that can have an effect on the final true focal length of the mirror. Dall seems to get round this by placing the knife edge at the mirrors centre of curvature but makes no mention of what zone should look high etc when the tester is in position.

One thought. Lenses made for red laser work may be suitable for making either test set up. Also I had wondered about stretching cling film over a lapped ring to make a pellicle for the Dall test.

John
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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5613769 - 01/09/13 01:49 PM

Quote:

Certainly hope this is the case... the ross null tester I am about to build is using about 1/2 the lens diameter.



Precision lens? I hope so.

Mladen


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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5613792 - 01/09/13 02:10 PM

Quote:

Just to circle back, I think I am getting it. Given lens X, the lens to mirror spacing defines the amount of "offsetting " spherical aberration that is introduced.... i.e. the conic that it will null at.

It seems to me the KE/source to lens distance is really irrelevant. The focus is where the focus is, you move the KE to it. That is, it behaves just like testing a sphere at its RoC. Now, theoretically if you have the lens to mirror spacing exactly right and you have a perfect null; then the KE to lens spacing should be what has been calculated.



Yes. If you get a null with KE, you better double check the lens to mirror as well as lens to light source distance. They should both match the the theoretical figures within tolerances given.

Use a higher frequency Ronchi instead of KE, as the Ronchi immediately tell you if you are under, over or jsut about spot on, and you can follow the progress as the bands become straighter. Much easier to interpret than KE shadows. Once you are close to a null then the KE can give you a lot more detail - provided the light source is small enoug.

Also, the light to lens distance is not irrelevant for interferometry, so you should still be able to check and set your light to lens distance accurately and repeatedly.

Remember, the correct lens to mirror distance must remain fixed at all times.

Mladen


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Brian Engel
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5613825 - 01/09/13 02:27 PM

Quote:

Quote:

Certainly hope this is the case... the ross null tester I am about to build is using about 1/2 the lens diameter.



Precision lens? I hope so.




Well, it is a 4.5 inch Jaeger lens from surplus shed.

As a side note, I find interesting is Surplus Shed sells SO many lens but they do not have to means (sphereometer) to measure the radius on them :-). So I am going to have to derive it from the index of refraction and focal length.


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Re: Ross (et al) Null testing parabolas new [Re: Ajohn]
      #5613859 - 01/09/13 02:53 PM Attachment (6 downloads)

Quote:

The Bath seems to have a diameter limit at shorter focal lengths. There is a brief thread on the subject here

http://tech.dir.groups.yahoo.com/group/atm_free/message/11257




John, thaty individual doesn't go into any details, and nothing he reports is conclusive. he's speculating. The problem with faster focal ratios than f/3 is full illumination of the mirror.

In theory the focal length of the lens needed is f = d*R/D, where f is the lens fl, d is the laser beam diameter and R is the mirror ROC and D the mirror diameter.

For an f/2.5 you need a lens with f = 0.59 inches (~15 mm). You can find surplus lenses even of 10 mm fl but they are not ideal for lasers.

Your best bet is to cannibalize some cheaper laser pointers and extract their lenses, but even these tend to produce elliptical, rather than circular, beams thus failing to illuminate evenly in both x and y direction, resulting in uneven illumination, as shown below.

I have an 8-inch f/3 sphere and I know that the illumination is a problem with most lenses. Your best bet is to purchase laser-quality, lenses but they are not cheap. Also, don't skimp on laser diodes. The higher end ones have perfect circular beams and 3 mm beam diameter.

The quality of your Bath device will depend on quality components. You get what you pay for.

Mladen

Edited by MKV (01/09/13 03:11 PM)


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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5613883 - 01/09/13 03:04 PM

Quote:

Well, it is a 4.5 inch Jaeger lens from surplus shed.



Is that a Jaegers achromat?


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Re: Ross (et al) Null testing parabolas new [Re: Ajohn]
      #5613893 - 01/09/13 03:07 PM

Quote:

Still not sure I get this fixed distance from lens to mirror



That determines which conic will be nulled.


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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5614091 - 01/09/13 05:00 PM

Quote:

Is that a Jaegers achromat?




Not an achromat.... it is this one actually.
http://www.surplusshed.com/pages/item/l10266.html

I am thinking it is crown glass, could be BK7. Perhaps part of an achromat set????

Edited by Brian Engel (01/09/13 06:44 PM)


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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5614741 - 01/10/13 12:02 AM Attachment (6 downloads)

Quote:

Not an achromat.... it is this one actually.
http://www.surplusshed.com/pages/item/l10266.html. I am thinking it is crown glass, could be BK7. Perhaps part of an achromat set????



Brian, the lens is BK7. When you plug in BK7 glass you get a fl = 274.999 mm. The radius of curvature of the lens is 142.12 mm.

Surplus Shed lists the lens as L10266, which means it's not a precision lens (otherwise it would be PL10266).

Besides the fact that the lens is not a precision lens, the Ross null setup for your 22 inch f/4 mirror with this lens gives distance tolerances of only 0.001 inches (or 0.025 mm, or 25 microns!) for a 1/8 wave maximum error. That's a heck of a positioning requirement.

Mladen


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Ajohn
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5614984 - 01/10/13 07:42 AM

Hi Maiden.
I did ask around this sort of thing on the yahoo group a while ago now. The mirror is a sort of on going problem. So far I have collected an edmunds laser diode and that's about it. My impression was that I would need a 10mm or maybe even smaller diameter biconvex with a focal length of 10mm plus a rather small cubic beam splitter. Everything needs to be small to keep the beam path separation small.

I suspect the oval illumination is probably down to the shape of the source which might be why they seem to specify shorter focal lengths lenses than should ideally be needed. It's interesting to note they even mention LED's now. It was suggested that the source size of those was too big when I asked.

It's interesting to hear from some one who uses them off group. I asked too many questions and eventually got the answers are in the posts on the group response which of course in my terms aren't. I spent a day going through them.

1st thing before I do any more on the mirror is a polishing machine. I used hard tiles for the tool and these don't give the same level of finish I could get with glass on glass. I do have the parts needed to make that now and a couple of ideas that may improve on the finish the tiles give.

John
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5615125 - 01/10/13 09:41 AM

Quote:


Surplus Shed lists the lens as L10266, which means it's not a precision lens (otherwise it would be PL10266).

Besides the fact that the lens is not a precision lens, the Ross null setup for your 22 inch f/4 mirror with this lens gives distance tolerances of only 0.001 inches (or 0.025 mm, or 25 microns!) for a 1/8 wave maximum error. That's a heck of a positioning requirement.





I never really got much clarification from them on what is "precision" and what is not. I'm only using the center of it, so hoping it is good (eyeball Ronchi test against a star doesn't show anything horrible with it).

Hmmm... The precision from Ross XP software I get is about 1 mm....not much better :-). But it is not so bad for < 1 wave.... I am using the Ross null as more of a qualitative than quantitative analysis of the mirror. I plan to rely on the Foucault test for that (the tolerances for that are pretty frightening as well :-) ).

In order to get more relaxed lens to mirror spacing (you can see my concern now :-) ), you have to go with a longer FL lens..... @ 20" f/4, that means you have to use more of the lens and thus the accuracy of the lens across its entire surface becomes more critical.

Honestly, I totally realize that using this lens (or others of its ilk) for a Ross test on my mirror is really "ambitious". I really can't plop down $500 for a certified one, so it is kind of a tinkering around thing. The Foucault test (which I have done many, many tests over the years on fast mirrors) is going to have to be my qualitative test.

Of course, if anyone has a better suggestion for a reasonably priced (<$100) lens, I would be glad to hear it!

I'll probably get the final,final test on a interferometer. I was hoping to hit Ed Jones up for that but I think he sold it (just kidding)....


I have probably 5 more hours of polishing to do, but so far my mirror is a good sphere (small hill in the middle) with no stig.....


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Re: Ross (et al) Null testing parabolas new [Re: Ajohn]
      #5615145 - 01/10/13 09:53 AM

Hello John! First, that would be MLADEN, not MAIDEN.

The Bath beam separation is important in faster mirrors because of relatively larger astigmatism caused by the separation. This astigmatism can be mathematically removed in fringe analysis, so it's not absolutely necessary to have a very small beam-to-beam separation to account for astigmatism.

The amount of astigmatism expected for a given beam separation (d) is given in terms of OPD waves as OPD = D^2*d^2/(16*w*R^3), where D = the diameter of the mirror, R = the radius of the mirrors, d = beam-to-beam separation, and w = wavelength used.

For example, for a given w = 0.00055 mm, and d = 10 mm, a 200 mm f/4 will produce astigmatism equivalent to 1/9 or 0.11 waves of optical path difference. By comaprison, a 200 mm f/8 mirror will have only 1/72 or 0.014 waves of OPD due to artificially induced beam-tobeam astigmatism.

You do need a biconvex lens of relatively short focal length. You can get them by cannibalizing inexpensive laser pointers. At least you'll know they are laser-quality, unlike some of the lenses you can buy second hand.

You can use super bright LEDs in conjunction with a narrow-band filter, a collimating lens and diverging lens. I don't see any reason to use LEDs in the Bath.

Regards,
Mladen


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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5615181 - 01/10/13 10:19 AM Attachment (10 downloads)

Brian "precision" refers to tolerances. For example, precision lenses guarantee that your lens focal length will not vary by more than 1 to 2%, your central thickness to 0.2 mm, etc. Non-precision lenses have much more relaxed tolerance range.

As for the Ross precision, I entered your 22 inch f/4 (in metric units) and your Jaegers lens as your Ross null lens, and the distance precision for 1/8 wave tolerance is 0.025 mm (0.001").

The Ross null using Ronchi or KE is always qualitative. If you use it in conjunction with an interferometer you'll get a qualitative result. The Foucault is unreliable when it comes to f/4! It's also undersampled.

Your lens as it is uses over 55% of the CA. You have to find a compromise between a "doable" distance tolerance (i.e. several mm) and the working lens diameter (a function of the focal length). In your case, if you went from 275 mm to 300 mm fl. your tolerance would change from 0.0025 mm (0.001 inch) to more than 3 mm (plus or minus 1/8 inch).

In your case it's the f/4 that's at the heart of your problem. That's why even some top mirror makers will not produce f/4 but will make mirrors no faster than f/4.5, even though they have test equipment.

You can look for precision surplus lenses on Surplus Shed of sufficient size and focal length. That will cost you a fraction of the $100 limit you have. Likewise, making a working (not necessarily top rated!) Bath interferometer will allow you quantitative analysis of your optics easily under $100.

Mladen

Edited by MKV (01/10/13 10:24 AM)


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Brian Engel
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5615333 - 01/10/13 11:39 AM

Quote:

22 inch f/4



It's a 20 inch f/4 (actually ~4.3 at the moment)


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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5615358 - 01/10/13 11:50 AM

Quote:

In your case, if you went from 275 mm to 300 mm fl. your tolerance would change from 0.0025 mm (0.001 inch) to more than 3 mm (plus or minus 1/8 inch).




I believe I do have a PL lens from surplus shed that is 85mm X 300mm FL. I went with the 117mm lens as, if I remember correctly, most of the 85mm lens would be used... and I am not sure I trust it enough. Maybe I will take another look it.

I have looked at their current inventory and they don't have what I really want... > 80mm with a FL between 300mm - 500mm.


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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5615389 - 01/10/13 12:03 PM

Okay, sorry about that. Your margin of error is 0.356 mm (0.014"), still a heck of tight requirement. You'd have to devise a radius bar with micrometric extensions at the end for that kind of accuracy.

Mladen


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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5615395 - 01/10/13 12:06 PM

Brian, with precision lenses you can use more of the lens precisely because it is a better corrected, more accurate surface. With ordinary lenses you want to keep the working diameter as small as possible.

Mladen


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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5616546 - 01/11/13 12:31 AM

Quote:

In your case it's the f/4 that's at the heart of your problem. That's why even some top mirror makers will not produce f/4 but will make mirrors no faster than f/4.5, even though they have test equipment.



f/4 is no problem.

Brian, if your mirror is fairly smooth and a good figure of revolution, then Foucault testing alone can be used to make a superb mirror.

If you have the telescope built, you have a decent figure of revolution test. A Foucault test will show the smoothness of the surface.

The only lens I would recommend buying is a small achromat to make a low-power telescope for your Foucault tester. That makes testing much more enjoyable.


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Re: Ross (et al) Null testing parabolas new [Re: Mike Lockwood]
      #5617197 - 01/11/13 12:31 PM

Quote:

f/4 is no problem...



Well, coming from someone who professionally specializes in under-f/4 mirrors of up to 60-inch diameters (and won't even touch anything smaller than 14-inch disks), I'd say that for your experience and tooling it's no problem for sure!

For most "mortal" ATMs, an f/4 paraboloid is, if not a problem, then a lot more work than anything even a wee bit slower. In fact, even some whose mirrors are their bread and butter, like Zambuto,will charge up to 20% more for an f/4 vs. f/4.5, simply because of the extra work required!

Anyway, the beauty of null tests is that you measure once and not dozens of times. There are no shadows to chase, and interpret, or estimate. The Foucault does give a nice picture of the surface quality and a KE null shows a great deal of detail. And, a Ross null can be used in conjunction with an interferometer for quantitative results.

On the subject of the Foucault, I am curious, since you deal with much larger optics than commonly encountered among amateurs, why do you think the professional community abandoned the Foucault way back in the 30's for their large observatory mirrors and bothered to devise alternative testing methods?

Regards,
Mladen


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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5617408 - 01/11/13 02:55 PM

No doubt a cheap, simple, easy to use null test for parabolids is the holy grail :-).

I would completely trust Mike ability/skill to Foucault test fast mirrors.... comes with extensive experience. The thing is, I doubt *mine* :-). Can I tell the difference in a zone null with the steep slopes of a f/4, within 1-2 thousandths?.... not so sure... :-).

I like the Ross null as a gut check and as a qualitative way to put the initial 5 or so waves(!) of correction in. In conjunction of course with the Foucault.

I think the Ross test is a fantastic tester for more "reasonable" ATM mirrors (e.g. 14 f/5 and such).

Edited by Brian Engel (01/11/13 03:46 PM)


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Ajohn
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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5617626 - 01/11/13 05:26 PM

I made a post earlier about a another null test that is probably more appropriate for a largish F4 mirror if real accuracy is needed. Seems to have got lost so briefly.

Ofner null, can be read about in Reflecting Telescope Optics II, manufacture, testing ......... on google books. It hardly more complicate than the Ross and just needs another lens but does account for higher order aberrations so is suitable for faster mirrors.

John
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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5617886 - 01/11/13 07:51 PM

Quote:

No doubt a cheap, simple, easy to use null test for parabolids is the holy grail :-).

I would completely trust Mike ability/skill to Foucault test fast mirrors.... comes with extensive experience. The thing is, I doubt *mine* :-). Can I tell the difference in a zone null with the steep slopes of a f/4, within 1-2 thousandths?.... not so sure... :-).

I like the Ross null as a gut check and as a qualitative way to put the initial 5 or so waves(!) of correction in. In conjunction of course with the Foucault.

I think the Ross test is a fantastic tester for more "reasonable" ATM mirrors (e.g. 14 f/5 and such).



First, Mike's reputation speaks volumes of his skills. There was never any doubt that whatever tests he uses he puts them to a good use. (heck, even my "tag line" is his - because it true!)

But for an inexperienced person (relatively speaking), i.e. someone who's more likely to produce a one-time 20-inch mirror, the Ross null is, as you cheap and best suited to tackle the delicate surface tolerances of an f/4 configuration.

The only requirement is to set the lens to mirro distance very, very accurately, and keep it that way. One measurement, versus hundreds for a Foucault test.

Ross null using a KE shows all the intricate minute detail normally seen when a good spehre is observed - as "flat", evenly illuminated disk with barely perceptible surface features of very low profile.

Straight Ronchi bands can also bi qualitatively analyzed using ordinary graphic tools - such as ruler or Photoshop. If the band remains straight across aperture in a null test chances are the mirror at least meets the Rayleigh's limit, and for a 20-inch that's darn good, given that atmospheric scintillation precludes seeing sharp diffraction images most of the time at that aperture, and that cooling issues are also very long unless you can produce an astigmatism-free wafer-thin large mirror, as Mike Lockwood does.

Autocollimation is out of the question for most amateurs at that size. I am not sure if Mike has a 22-inch flat but can make one. I can't. So, no matter how you turn it around, the Ross null test, considering the simplicity cost and no need for additional software or technology, is a blessing.

Mladen


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Re: Ross (et al) Null testing parabolas new [Re: Ajohn]
      #5617887 - 01/11/13 07:51 PM

Quote:

Ofner null, can be read about in Reflecting Telescope Optics II, manufacture, testing ......... on google books. It hardly more complicate than the Ross and just needs another lens but does account for higher order aberrations so is suitable for faster mirrors.



You bet. And it can be made all reflecting!
With a residual OPD of .009 waves, and that's for a goofy 1 meter RC primary, who wouldn't build one of these puppies?


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Re: Ross (et al) Null testing parabolas new [Re: ccaissie]
      #5617971 - 01/11/13 08:39 PM

Now that the construction and other issues of the Ross are handled, one of my favorite bench tests to analyze a null, is the bench star test, and that should be useful in the Ross. Anyone tried it? With a monochromatic pinpoint source, the nulled image can be observed with an ocular and analyzed as we would a star at infinity. Ref: Suiter.

this works good on spheres I've produced, and once the zones get to be hard to see, the star test faithfully shows any slight spherical abberration. In a shop, could be good to 1/60 wave. (Welford) But it is quite helpful in locating zones also, as the zones reliably show in the diffraction disk.

The point source I've most often used is a lensless laser pointer diode, which with lowered voltage does not "lase". I've also used a laser pointer aimed at a shined ball bearing next to the eyepiece to create a pointsource "glint" (Texereau). This latter source worked for me on a short focus concave, the light cone was ample for an autocollimated f/3, and yielded much info in the bench star test.


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Brian Engel
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5618160 - 01/11/13 10:42 PM

Quote:


The only requirement is to set the lens to mirror distance very, very accurately, and keep it that way. One measurement, versus hundreds for a Foucault test.




That is the "rub". But this too is problematic for ATM's that want to make large fast mirrors. As Ed has said over and over, getting a lens of sufficient quality is usually the biggest problem. You have to pay big bucks to get one :-). Otherwise you are left to find compromises and end up having to measure the mirrors to lens spacing within a few millimeters.

Given your average tape measure is lucky to have a tolerance of < 2mm, this presents a challenge. Which is what kicked off this thread :-). I was hoping to find a way "around" it by fixing the KE to lens distance and then move the whole stage back and forth to get the best null I can as I work the mirror (just like if it is was a sphere). Figuring the with a given lens and with the KE lens distance fixed at its calculated spacing for a parabola, the only conic it would null at is -1.... In my mind, it was kind of like "moving" the parabola through the sphere kinda like in figureXP.

It appears I misunderstood that and it will not work. So I am back to 1)find a more forgiving, large lens or 2) measure the distance with 1mm or so accuracy over that long distance.

I love the notion of a null test as you can look at it and instantly have a very good idea of what needs to be done to the mirror's surface - without tedious measurements and a "calculated" profile in figureXP (et al).

Guess I will just have to fool around with it and see how it works out :-).....


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Re: Ross (et al) Null testing parabolas new [Re: ccaissie]
      #5618429 - 01/12/13 03:41 AM

Quote:

Now that the construction and other issues of the Ross are handled, one of my favorite bench tests to analyze a null, is the bench star test, and that should be useful in the Ross. Anyone tried it?



Ever read the whole thread? Well, the short answer is yes it has been mentioned and illustrated on this thread already.

regards,
Mladen


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Re: Ross (et al) Null testing parabolas new [Re: ccaissie]
      #5618466 - 01/12/13 05:10 AM Attachment (19 downloads)

re: "The point source I've most often used is a lensless laser pointer diode" [ ccaissie]

Indeed, a nonlasing diode will have a tiny point but a true artificial star has to be equal to, or smaller than the Airy disc of a particular optical configuration.

The Airy disc diameter is given by d = 2.44*w*F#, where w = wavelength, and F# is the focal ratio (F/D). For an f/5 and w = 0.000633 mm (red laser), the Airy disk will be 0.0077 mm or 8 microns. I seriously doubt a lensless laser diode meets that requirement.

Texereau's method of using a steel ball can easily produce an artificial star source of accpetable size. The problem with Texereau's method is the quality of the steel ball, which would have to be optically perfect smooth and zone free, keeping in mind that on reflection any imperfection on the ball surface will be doubled. Perhaps unused precision ball bearings are smooth enough and meet astronomical surface requirements, but I doubt it.

Keep in mind that you'll get some kind of diffraciton pattern no matter what, it's just that it's not going to be equivalent to a star test because you will never know if the imperfections are due to the mirror tested or the light source.

An artificial star source must be optically corrected, and the best way to achieve that is by the use of quality microscope objectives and applying the lensmaker's principle in the setup. Microscope objective are specifically corrected in such a way that makes them ideal for artificial star source.

By placing the light source at the objective's focus a microscope objective will form a fully corrected image of that source some distance in front of it.

Say that you have an objective that was configured to examine specimens at a distance of 2 mm, and forms an image of the specimen at a distance of 200 mm. You have a 200/2 = 100X microscope objective. If your lensless laser source is 0.3 mm in diameter, the resulting artificial star will be 0.3/100 or 0.003 mm (3 microns), which will satisfy requirements for any system f/2 or slower.

regards,
Mladen


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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5618490 - 01/12/13 06:05 AM

Brian, you can measure the lens to mirror distance with sufficient accuracy to have a reliable Ross null test, but in your case the problem is the f/4 mirror. That configuration leaves you almost no room for error and makes it practically impossible to do a reliable test when your lens-to-mirror spacing must be within 0.001 inch with your Jaegers lens!

A slower lens mirror would work just fine. The working diameter would be only For example even an f/4.5 would leave you with +/- 2.8 mm margin, using the same lens lens, and that's plenty. A BK7 lens with R1 = 250 and f = 484 mm would give you almost +/- 5 mm of tolerance

Now, the real problem is the working diameter (WD) required. for a 20-inch mirror. In your case, this would not be eased even if you decided to go with a slower focal ratio. The WD of your lens would be a whopping 78 mm, which means you'll need a lens whose correction is better than 1/8 wave over at least that much - a toll order indeed.

Now, depending on how important this is to you, you may consider buying a larger lens, say a 6-inch PCX, and rework it to the radius and finish required for a 20-inch test.

Surplus Shed often gets new lenses and it's only a matter meticulous checking (along perhaps with eBay and similar sites) before you find a suitable lens. They sell large 6-inch lenses from time to time at bargain prices and it would not be too much work to have them reground, repolished and refigured to required surface standard over the WD area, keepong in mind that lens requirements are more relaxed than those for a mirror.

That's probably not what you were hoping to hear but there is a lot of good in it. For one, it will be a valuable learning experience with lenses, and, two, once you have a Ross lens that size it will always be useful. And don't forget that with the Ross you can NULL elliptical, as well as hyperbolpodial surfaces for Cassegrains and exotic telescopes of all kinds.

A null test is a dream compared to other tests. And, as you said, it gives you an instant idea where you stand correciton-wise, no tedious and numerous measurements needed, not squinting or guessing.

Mladen


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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5618504 - 01/12/13 06:27 AM

There is probably a very simple answer to all of this based round Dall's instructions. In fact given the vagaries of figuring it's probably the only way of doing it as the radius of curvature is very likely to change as the mirror is figured eg going too far in one zone and then playing with the middle.

Dall's instructions effectively mean placing the knife edge at the centre of curvature - easily done when the mirror is a sphere - and then adjusting the tester so that the image of the pinhole falls as close as possible to the knife edge. Not so easy when the mirror is not parabolic.

What is happening is that the base sphere of the mirror is being used as a reference sphere for the tester. It will always be possible to find the radius of that with the aid of a mask and the knife edge so that can always be placed in the correct position. As the parabolic form takes shape it will get easier and easier to position the tester so that the image of the pin hole falls in the same place. When that happens and there is a null it has to be a parabola providing the distance from the lens to pin hole is correct. Dall takes care of that aspect because that distance is fixed as part of the test and can be multiplied to obtain nulls on different conics. He doesn't mention correcting that distance according to changes in the central curvature of the mirror but if some one for instance wanted to form the conic by deepening the centre which starts getting attractive on very fast mirror that would have to be accounted for.

The question really is just what reference sphere the Ross test uses. I would say it's bound to be the same sphere. Then adapting this procedure to suite the Ross test.

When I used the Dall test I did this without really thinking about it as the mirror was already parabolic. Rather than making in a tubular holder for the lens and pin hole I put both in perspex holders that slid along a perspex bar. Much easier to do as suitable tube may not be easy to get hold of. Also easier to set up. I set the distance with vernier callipers - analogue ones. It was a long time ago

Perspex was a bad idea. Sufficient light came from the pin hole to illuminate the lens holder and give the mirror a sort square profile mostly removed by painting the front of the holder.

The pin hole was a slit using a rather small hole. Dall suggest no more than 0.030in long which is a size that is fairly easy to drill. The biggest problem with slits is the need to square them up to the knife edge.

On Ofner I'm fairly sure I just read something famous suggesting that a mirror can be figured to 1/20 wave with 1/12 wave error in the measuring system.

On suitable lenses I have seen mention of making a lap and repolishing them to a sphere without actually being able to check that. I can believe this can work out. I once had the spherical surface of a commercial lens tested on a Johansson coordinate measuring machine. The radii was all over the place and varied markedly across the surface of the lens. Dall's advice on lens focal length was to measure it as accurately as possible via a clear image within the aberration halo. I have seen mention of using this test with the convex side of the lens facing the mirror. My guess is that as he was a very bright cookie there will be very good reasons why he chose to use it the other way round.

John
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Re: Ross (et al) Null testing parabolas new [Re: Ajohn]
      #5618517 - 01/12/13 06:47 AM

Texerau mentions lapping a ball bearing to improve it's figure. Also rotating the ball to see if that is producing astigmatism. I also assumed from his comments that the set up is intended to give a "star" that is well below the diffraction limit of the test set up. Much like how certain lenses are tested with a pin hole that may or may not be round that is under 1/2 the size of the Airey disc.

I am heavily into microscope and I would be very cautious about exact calculations based round their parameters. They are all fully corrected with additional optics even if it is only the manufacturers eyepieces that are intended to be used with them. An additional factor of 2 wouldn't be a bad idea given how the NA seems to be specified. When checked they seldom live up to it. It would also be a good idea to use what is normally referred to as a metallurgical objective. These are intended to be used without a cover slip and even at an NA of 0.5 the absence of one on normal objectives has a noticeable effect. Some manufacturers now don't bother making 2 versions of those.

John

Edited by Ajohn (01/12/13 06:58 AM)


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Re: Ross (et al) Null testing parabolas new [Re: Ajohn]
      #5618600 - 01/12/13 08:35 AM

re: "Texerau mentions lapping a ball bearing to improve it's figure. Also rotating the ball to see if that is producing astigmatism. I also assumed from his comments that the set up is intended to give a "star" that is well below the diffraction limit of the test set up. Much like how certain lenses are tested with a pin hole that may or may not be round that is under 1/2 the size of the Airey disc"

How do you lap ball bearings? And why would a ball bearing produce astigmatism?

A ball bearing can easily produce an artificial star that's below the optical resolution of the optic being tested even if your light source is relatively large.

Let's say you use a 3 mm red LED, and a 4 mm diameter precision ball bearing. The ball bearing radius will be 2 mm and its focal length -1 mm. If you set the light source at a distance of 300 mm (~ one foot) from the ball bearing the image will be formed "inside" the ball bearing at a distance d2 = 1/f - 1/d2 or ~ 1 mm. The magnification ratio then will be d2/d1 = 0.00333*. Multiplying that with the original LED diameter of 3 mm you get an artificial star image that is 10 microns. Increase the distance d1 = 500 mm and the artificial star will be 6 microns, etc.

You can also create a more compact artificial star tester by using several short focus lenses and shorter seperations.

re: "I am heavily into microscope and I would be very cautious about exact calculations based round their parameters. They are all fully corrected with additional optics even if it is only the manufacturers eyepieces that are intended to be used with them"

That's a good point, John, especially for newer microscopes with ED lenses and complimentary proprietary eyepieces. Older microscopes objectives wre designed to give a corrected image at the focal plane and then magnify it withn an ordinary Huygenian and Kellner eyepiece. For artificial star images yu can be pretty safe with older microscope objectives.

They form textbook Airy discs with monochroamtic light source (usually a simple superbright LED and a narrow band filter of about 10 nm width).


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Re: Ross (et al) Null testing parabolas new [Re: Ajohn]
      #5618649 - 01/12/13 09:19 AM

Exactly that. I found the ball bearing had some microdefects on the surface so buffed it up, and much of the scatter etc. was cured.

LD chips can be pretty small and rectangular, like .3 x 5 microns.

Indeed, at high power bench star testing, like 800x, the image at focus does suggest the actual chip shape, but what is sought is a difference in the appearance of the out of focus rings, etc., which shows up well after the KE test gets difficult. Y'all must have seen that yourselves. I'm just touting that if we are looking at a correctly nulled point image, the star test protocols are useful in complement to KE and Ronchi.


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Re: Ross (et al) Null testing parabolas new [Re: Brian Engel]
      #5618657 - 01/12/13 09:26 AM

Quote:

I was hoping to find a way "around" it by fixing the KE to lens distance and then move the whole stage back and forth to get the best null I can as I work the mirror (just like if it is was a sphere).



Actually, even the lens thickness is a variable in figuring out the correct postions, and I am talking a fraction of a mm of lens thickness! So you have to be absolutely sure about the lens parameters, glass type, distances, mirror radius of curvature, and clear apertures (i.e. lens minus the bevel, etc.)

You can't change anything in the Ross null solution without changing all other variables proportionally. But I can totally relate to your thought process. I thought of the exact same thing because it seemed logical at the time, so I actually put it to a test! As they say, live and learn. When it became obvious I wasn't getting the expected results, I sat down to learn more about it, which is not easy because very little is written on how the equations were derived.


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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5618731 - 01/12/13 10:15 AM

Texereau mentions polishing a 1/4in ball bearing up in a lathe. An optician would do that by centring it in rosin the same way as lenses used to be mounted for centring. Me I would mount it on the end of stick and rotate by hand. Hopefully that could be arranged to cause the bearing to centre itself with a bit of thought.

He does mention rotating the ball bearing as a check that astigmatism if present is due to the mirror. He uses that as a check for the mirror and later uses the same idea to check the flat using the main mirror with a mask over it just leaving the central portion clear to approximate to a good sphere. The mirror ideally needs to be aluminised though.

On Huygenian eyepieces and microscopes it fairly generally accepted that these do not follow the usual prescription in order to compensate for objective aberrations.In my experience when pushing things to the limit it pays to get the correct eyepieces. The capabilities of Huygen types tend to vary over the years. Very early on they would be often used for all types of objectives, later additional variations may be available for plan eyepieces in particular or apo's etc. Nikon were the 1st to completely compensate for colour purely in their objectives. They come for 160 and 210mm tube lengths and at one stage where relatively cheap for what they are used but people started using them for direct projection colour photography so prices have rocketed. The 210 mm tube length ones are metallurgical types. With 160mm tube length objectives it's best to avoid Zeiss as there is no way of knowing how recent they are and they are known to have delamination problems up to some point in time. Leitz 170mm have to be rather old. Old japanese objectives are rather short. The 160mm tube ones aren't and I would say are good bet for a decent objective followed by 160mm Leitz. At the cheap end Vickers objectives are amazingly consistent and last well providing they don't have DIN marked on them but know one is entirely sure what tube length they used. The optical tube length on 160mm objectives is to the DIN standard which is 10mm down the mechanical tube so is 150mm. Vickers used a 160mm mechanical tube and people suspect the optical tube length was 17mm down that. Modern infinite tube length objectives are a different kettle of fish - amongst other problems the opticians have yet another piece of glass to play with in the microscopes tube. It is probably worth trying to project a near parallel beam of light down them but I would have doubts about them meeting their specs. The 160mm and green light would be the best bet as compensating eyepieces usually show an orange tint round the edge when just held up to the light. That can apply to Huygens types too.

Hope this all reads ok - I've been out of the house twice and eaten dinner while typing it.

John
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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5628615 - 01/17/13 06:23 PM

Quote:

Well, coming from someone who professionally specializes in under-f/4 mirrors of up to 60-inch diameters (and won't even touch anything smaller than 14-inch disks), I'd say that for your experience and tooling it's no problem for sure!



Just for clarity, I refigure mirrors of nearly any size, and occasionally make 10" and 12" mirrors from blanks that I have on hand. I just don't take orders for them. I also make smaller Cassegrain secondaries and elliptical flats.

So, yes I will touch smaller than a 14" disk.

Quote:

On the subject of the Foucault, I am curious, since you deal with much larger optics than commonly encountered among amateurs, why do you think the professional community abandoned the Foucault way back in the 30's for their large observatory mirrors and bothered to devise alternative testing methods?



Simple - they didn't, and your assertion is inaccurate.

Though you are complimentary of my work in later messages, and even if it wasn't your intention, the message above implies that I use outdated methods, which I assert is not true.

The 200" Hale telescope was mostly tested with Foucault, then a little bit of caustic testing, and finally Hartman mask testing in the telescope itself. Yes, a 200" f/3.3 mirror was made mostly with Foucault testing.

While the occasional parabola might have been done with autocollimation during the era you describe (extremely large flats just don't exist, as it turns out), as I have heard and it has been described to me by those who did serious optical work during those times and at major companies, most tests were done at the center of curvature.

I should point out that for Ritchey-Chretien telescopes, autocollimation is not a null test for the primary. Testing the system in autocollimation is possible, but tricky even with one mirror complete and coated, and thus two reflections off of an uncoated mirror. One has to work match wavefront intensities and get good fringes with an interferometer. Also, the larger the system, the more vibration becomes an issue due the physical size of the test setup.

Lasers (and thus interferometry) weren't even available to most shops until the mid to late 1960s, well after the date you mention.

So, the methods that I use are time-tested and have been used with great success for very large projects. They are still used today by other respected professionals, arguably producing better end products than other methods.

However, I must say that I highly recommend use of the more modern interferometer for checking figure of revolution of large mirrors. While other tests can show when there is a problem, the interferometer will quantify and locate it so that it can be remedied most efficiently and effectively.

Quote:

I would completely trust Mike ability/skill to Foucault test fast mirrors.... comes with extensive experience. The thing is, I doubt *mine* :-). Can I tell the difference in a zone null with the steep slopes of a f/4, within 1-2 thousandths?.... not so sure... :-).



Brian, give it a try. You might surprise yourself. One thing is for certain - if you never try it, you will never learn it.

While I'll try to keep reading this thread, clearly others who have been posting here have far more time to spend on posting than I do, and I don't have time to debate test methods.


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Re: Ross (et al) Null testing parabolas new [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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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Re: Ross (et al) Null testing parabolas new [Re: Ajohn]
      #5835365 - 05/01/13 10:44 PM

Quote:

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.



In the Surface Data spreadsheet for OBJect THICKNESS enter the the near point distance. It's in the instructions, you know.

Mladen


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Re: Ross (et al) Null testing parabolas new [Re: MKV]
      #5835754 - 05/02/13 08:00 AM

I didn't really ask the question very well MKV. Taking the problem as a whole I don't think oslo can cope with it other than being given precise source to lens and lens to mirror distances and or via sliders.

2 options on point sources - a perfect lens or a stop of pin hole diameter neither of which I believe involve changes to object distance. Not sure if the pin hole sized stop will work out or if it can be illuminated with a field angle close to 180 degrees.

John
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Re: Ross (et al) Null testing parabolas new [Re: Mike Lockwood]
      #5857141 - 05/13/13 10:14 AM

Quote:


The 200" Hale telescope was mostly tested with Foucault, then a little bit of caustic testing, and finally Hartman mask testing in the telescope itself. Yes, a 200" f/3.3 mirror was made mostly with Foucault testing.







The 200 inch mirror was figured using a three lens null corrector designed by Ross.


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