12 replies to this topic

[peleuba]

I had the opportunity to take a look at a Jaegers achromatic lens a friend had picked up and sent to me for testing. 

 

This is the longest focal length refractor lens I've ever tested and it was a bare lens-in-cell (no OTA) so alignment with my optical flat, Ronchi screen and LED's was slightly more time consuming at a ~90" focal length then if the lens was in the OTA.

 

In any event, I tested in Red, Green, Blue and White with green being the most useful for the Achromat.  Now, I don't have the breadth of ATM experience as some here -  I only test optics, not make'em - but my sense is the lens is decent with some overcorrection between 1/4 and 1/6 wave.  There is also a slightly rolled edge; a small central depression; and a narrow zone at about 90% radius that is not related to the edge anomaly.  

 

Sorry for the relatively poor quality of the photos.  I had to really compress the RAW images so they could be posted on CN.  But, they are good enough to give an indication of the overall correction.

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Edited by peleuba, 21 June 2022 - 07:48 PM.

[DAVIDG]

 The lens is over corrected  by about 1/4 to 1/6 wave like you stated but you also have to understand that spherical aberration in an achromat also affects chromatic aberration as well so they add . So the overall wavefront is worse then what it is in any one color.  The lens might also have astigmatism. Note in the images taken in the green that the ronchi pattern is slightly tilting in one direction in  the images on the left then tilting in the other direction in the images on the right. That is the classic sign of astigmatism when the pattern rotates from one side of focus to the other. 

   When it comes to aligning the lens to the flat, I use a laser.  Without the lens in place I adjust the flat so the laser reflected off  the flat falls right back on the laser. Next I put the lens in place so the laser is passing through the center of the lens. On the wall behind the laser you will see a couple of fainter laser spots. These are from the laser reflecting of the surfaces of the elements. Adjust the tip and tilt of the lens so all the spot merge into one with the beam still passing through the center of the lens.  Now the lens is aligned to the flat and can place your tester in place of the laser and you should have a symmetrical ronchi patern right to left and up and down.

 

                      - Dave  

[peleuba]

 The lens is over corrected  by about 1/4 to 1/6 wave like you stated but you also have to understand that spherical aberration in an achromat also affects chromatic aberration as well so they add . 

 

 

Hi Dave - thanks for the feedback!  Very glad you responded as I know you have a TON of experience and have tested some Jaegers lenses previously.  I think based on your previous comments in other messages, this one is probably represents the median - not the best, but neither is it the worst you've seen.   

 

Yes, I ultimately used a laser to fine tune the alignment.  

 

Regarding astigmatism, Your point is well taken.  Though, I'm not sure the lens has any appreciable astig.  The grating was placed in an eyepiece I had 3D printed that also contained the LED.  The eyepiece was placed in a series of extension tubes so that it could be placed in a v-block on my lab jack.  Its quite likely that I slightly rotated the eyepiece/grating combo in the extension tube.   So, I would not read too much into the orientation/tilt of the bands within the posted photos.  I did test for astig but saw none of the tell-tale signs such as the "chocking" of the bands as I transitioned slowly from inside of focus through best focus and finally to outside of focus.  I am pretty confident in this.

 

Regarding spherical correction at differing wavelengths - you're right and the point is not lost on me regarding correction at the various wavelengths.  These errors are additive.  Even so, I think this lens will provide reasonable images.  I don't often test longer focus achromats - mostly shorter ED doublets -  but I was surprised at the color error especially noticeable in the white light image.  I thought it would be more subtle/suppressed.  This preconceived notion is directly related to my lack of experience with these long focus lenses.

 

Also, it took a bit of time for the lens to thermally settle on my bench.  I started the alignment procedure when I took the lens out of the shipping box after it had been sitting in my den.  There is a only a minor delta in temp between the den and where my test bench is.  But it took ahwile for the lens to settle.  So I finally let it sit overnight on the test stand/lab jack.   

 

Interestingly, this lens is spaced with metal rings.  I'm used to seeing rubber rings, or thin foil spacers and not metal rings.  Are all Jaegers lenses spaced using metal rings that fit inside the cell separating each glass element?

 

Below are photos of my tester eyepiece. For this test, it was placed into a set of extension tubes so that assembly could be placed on a v-block on top of a lab jack.

 

 

Regards.

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Edited by peleuba, 22 June 2022 - 12:03 PM.

[DAVIDG]

  How thick is the metal ring spacer ? If it is  like 1/4" or so then it is of the Baker design . As you know 3 points define  a plane and when you have a solid ring as a spacer for the air gap you don't know were those three points are. So if the spacer ring is more like a few thousands of an inch thick  I would examine the lens under monochrome or semi monochrome light to see what the shape of the interference rings look like. If they are not round and centered then the elements are wedged and that will cause astigmatism and lateral color at high magnification.  I would then adjust  it to center up the pattern. 

   My tester is made on  an xyz stage so I can move the grating without it rotating.  I adjust the Ronchi screen so the pattern is vertical on the inside of focus and then move the grating to the outside of focus and see if  the pattern rotates at all. If it does it is from astigmatism. 

     In achromats, the radius, thickness, spacing and figure on the surface all interact to correct for both spherical and chromatic aberrations. Change one and they all change. 

 

                   - Dave 

[peleuba]

  How thick is the metal ring spacer ? If it is  like 1/4" or so then it is of the Baker design . 

 

 

I do not have possession of the lens any longer, I sent it back to its owner.  But the metal spacer is not quite ¼ inch thick.  Probably closer to 1/8 inch but perhaps somewhere between a quarter and an eighth.  I did not measure it.  But its no thicker then a quarter and no thinner then an eighth.  How's that for precision?!  

 

That is an excellent point - one does not know exactly where the 3 points are along the spacer ring.  What you describe using monochrome light (to do fringe analysis) is a Newton Interferometer.  I built one of these and use it to test flats.  It never crossed my mind to test this lens in that manner.   

 

Thanks again for all the helpful feedback - not just in this thread, but in many others.  I read it all.

[DAVIDG]

I do not have possession of the lens any longer, I sent it back to its owner.  But the metal spacer is not quite ¼ inch thick.  Probably closer to 1/8 inch but perhaps somewhere between a quarter and an eighth.  I did not measure it.  But its no thicker then a quarter and no thinner then an eighth.  How's that for precision?!  

 

That is an excellent point - one does not know exactly where the 3 points are along the spacer ring.  What you describe using monochrome light (to do fringe analysis) is a Newton Interferometer.  I built one of these and use it to test flats.  It never crossed my mind to test this lens in that manner.   

 

Thanks again for all the helpful feedback - not just in this thread, but in many others.  I read it all.

  To test  if the air gap is uniform and not wedged you don't need a Fizeau interferometer. You  just need to shine monochrome or semi-monochrome light on the lens to see them. The air gap needs to be only a few thousands thick thou. The 6" f/15 that you tested with about the 1/8"  air gap that would be too wide to see the fringes thou.

 

                  - Dave 

[peleuba]

  To test  if the air gap is uniform and not wedged you don't need a Fizeau interferometer. You  just need to shine monochrome or semi-monochrome light on the lens to see them. 

 

Understood and thanks! 

 

For those following at home,  Basically, a Newton interferometer (a type of Fizeau) is a monochromatic lightbox combined with an a master optical flat for testing other optical flats such as a Newtonian secondary.  If I remove the master flat I would have a monochromatic light box similar to what Dave describes above.  For the light source, I use a green compact fluorescent light bulb sitting behind a piece of frosted plastic to diffuse the light.    

 

Honestly, I am not sure why it never occurred to me to check the lens in this manner.

Edited by peleuba, 23 June 2022 - 08:53 AM.

[DAVIDG]

Understood and thanks! 

 

For those following at home,  Basically, a Newton interferometer (a type of Fizeau) is a monochromatic lightbox combined with an a master optical flat for testing other optical flats such as a Newtonian secondary.  If I remove the master flat I would have a monochromatic light box similar to what Dave describes above.  For the light source, I use a green compact fluorescent light bulb sitting behind a piece of frosted plastic to diffuse the light.    

 

Honestly, I am not sure why it never occurred to me to check the lens in this manner.

 Just to clarify a Fizeau Interferometer produces collimated light . A simple one to make either uses a convex lens or a concave mirror. The monochrome light source is placed at the focus of the lens or the mirror to produce a collimate bean  For the Newton version of a Fizeau, you mount a Newtonian vertically and pointing down at the optical surface you want to test. In the focuser you place a monochrome light source which is usually a neon bulb and slightly  off center  in the focuser and you look into the focuser to view the optics being tested. Why you may need to use one when testing flats is for another discussion. 

    A  light box to test flats is just a monochrome light source and it does not produce a collimated beam.  So to  see interference rings between the elements in an air spaced achromat all you need is a monochrome or semi-monochrome light source shining on the lens and try to view the lens from straight above.  A  common CFL light bulb is semi-monochrome enough to show the interference rings so nothing more fancy than that to see if  the air gap is uniform or not.

   If the air gap is not uniform that can cause astigmatism and lateral color i.e. the stars turn into short spectrum and people confuse that problem with atmospheric dispersion. 

 

                        - Dave 

Edited by DAVIDG, 23 June 2022 - 09:20 AM.

[peleuba]

 Just to clarify a Fizeau Interferometer produces collimated light . A simple one to make either uses a convex lens or a concave mirror. The monochrome light source is placed at the focus of the lens or the mirror to produce a collimate bean  For the Newton version of a Fizeau, you mount a Newtonian vertically and pointing down at the optical surface you want to test. In the focuser you place a monochrome light source which is usually a neon bulb and slightly  off center  in the focuser and you look into the focuser to view the optics being tested. Why you may need to use one when testing flats is for another discussion. 

 

 

Thanks again for the detailed response...  I don't have a photo of my homebrew Newton IF, here at the office, but I learned the concept years ago from Carl Zambuto and found one commercially that was relatively easy to duplicate - which I did.  And, actually you told me in a post a long while back that a CFL light would work fine - and it does.  I am glad you mentioned same above.

 

I can view the fringes from the bottom or the top.  Though, I almost always view from the top down as seen in the last photo I posted below.  When viewing from the bottom as seen in the first photo, an auxiliary mirror is required to reflect the image of the fringes at an angle that makes them easily readable.  See sketch.

 

The source for my Newton IF is www.surfacefinishes.com   Specifically their Cartrilamp product.  Mine is a virtual clone of it.  The photos attached are from a downloadable brochure I have on the IF.  Its also posted to the website.  

 

I have also at varying times - on eBay - have seen light boxes from Van Keuren.  Well made, but way too expensive. 

 

Thanks and best regards.  I've sort of exhausted everything I know about this lens.  Hopefully the owner will put it into an OTA and use it.

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Edited by peleuba, 23 June 2022 - 12:01 PM.

[DAVIDG]

 The pictures that you posted are of a  viewing box and not  a Fizeau interferometer since the light source is just diffused but not collimated. Because it is not collimated the air gap between the surface your testing has to be very small as in few thousands of inch or smaller or the fringes will be invisible. With a true Fizeau the air gap can  be much wider so one can use thicker shims so the surfaces never touch and reduce the risk of damage to the surfaces. Also with large gaps  the fringes will become distorted just from the angle that they are viewed.  So  a Fizeau forces the fringes to be viewed on axis. 

   Here is the design for Newton type Fizeau interferometer. The key is that it  uses either a lens or a mirror to collimate the light. Both the monochrome light source and the observers eye are  placed at the focus of the collimating optics to view the surface under test.  This forces the view to be on axis and the collimated light removes distortion of the shape of the fringes from being viewed on an angle.

 

                   - Dave 

Edited by DAVIDG, 23 June 2022 - 05:25 PM.

[ngc7319_20]

That diagram has me confused...  Maybe more like this:

 

 

[DAVIDG]

That diagram has me confused...  Maybe more like this:

 

newton interferometer.png

 The diagram is not of interferometer even thou that it might be labeled that way. It is a light box with a diffused light source and beam splitter window so you can view on axis. Light boxes work just fine when you place the  surface your testing in direct contact with the Master surface.  

   When you use shims to separate the surfaces so you don't scratch them you need a collimate light source and to also view on axis. If you increase the viewing angle from  on axis the fringes will become curved and you get a false reading. Increasing the air gap cause the angle distortion to become increased with the viewing angle. So to have a  true Newton Interferometer you need addition optics to collimate the light source.

    Now getting back to testing lens to check that the air gap  is uniform by viewing the interference rings, you don't need an interferometer , just monochrome or semi-monochrome light and look straight down on the lens.  The reason is that there is usually many  waves of difference between the surfaces. That is why the fringes are rings and not arc or close to straight lines so the distortion caused by not using interferometer is small and any error you see is from the non uniformity in the air gap.

    You only need to use interferometer when critically testing surfaces as in testing an optical flat and trying to determine if the error in flatness is less than 1/8 wave or better. 

 

              - Dave 

[davidc135]

Looking at the diagrams more closely isn't the post 11 arrangement the right one?  David

Edited by davidc135, 26 June 2022 - 04:03 PM.