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MKV
Carpal Tunnel
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Reged: 01/20/11

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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MKV
Carpal Tunnel
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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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Mike Lockwood
Vendor, Lockwood Custom Optics
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Reged: 10/01/07

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Re: Ross (et al) Null testing parabolas [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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MKV
Carpal Tunnel
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Reged: 01/20/11

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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Brian Engel
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Reged: 08/25/09

Loc: Cincinnati,Oh
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
sage
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Reged: 12/03/07

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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MKV
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Reged: 01/20/11

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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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
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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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
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
super member


Reged: 08/25/09

Loc: Cincinnati,Oh
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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MKV
Carpal Tunnel
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Reged: 01/20/11

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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MKV
Carpal Tunnel
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Reged: 01/20/11

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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MKV
Carpal Tunnel
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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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Ajohn
sage
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Reged: 12/03/07

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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Ajohn
sage
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Reged: 12/03/07

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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MKV
Carpal Tunnel
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Reged: 01/20/11

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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ccaissie
professor emeritus


Reged: 09/13/10

Loc: Whitefield, Maine
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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MKV
Carpal Tunnel
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Reged: 01/20/11

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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Ajohn
sage
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Reged: 12/03/07

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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Mike Lockwood
Vendor, Lockwood Custom Optics
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Reged: 10/01/07

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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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