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MKV
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Reged: 01/20/11

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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Ed Jones
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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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MKV
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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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Brian Engel
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Reged: 08/25/09

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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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Brian Engel
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Reged: 08/25/09

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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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Ajohn
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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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MKV
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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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MKV
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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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Reged: 08/25/09

Loc: Cincinnati,Oh
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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MKV
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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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MKV
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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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MKV
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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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Brian Engel
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Reged: 08/25/09

Loc: Cincinnati,Oh
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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MKV
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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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Brian Engel
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Reged: 08/25/09

Loc: Cincinnati,Oh
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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MKV
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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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MKV
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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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Brian Engel
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Reged: 08/25/09

Loc: Cincinnati,Oh
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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