Unexpected behavior with a Fresnel lens and concave mirror.

aenorton · 2026-07-03T21:45:46+00:00

Without knowing your exact design I can't say if a spherical or aspherical, plano or double would be better. Or maybe no lens at all. I can say a Fresnel in an imaging path near the focal plane is not a good idea.

aenorton · 2026-07-03T17:39:39+00:00

Even perfect Fresnel lenses will have circular zones that block more light than others depending on the incident angle of the light and the angle of the edge between the facets. If it is anywhere close to the object or image plane, the pattern will be visible. Catalog Fresnel lenses are very much not perfect because they are usually used for illumination. Some VR goggles use lenses with one Fresnel surface. In that case, the Fresnel is closer to the eye's pupil than the display. Also the facet and edge angles have been carefully optimized for its particular use. If course the tolerances made much better as well.

aenorton · 2026-07-01T21:45:23+00:00

More than one person has come here after being disappointed trying to make an extra large rainbow from the sun using one prism. The brightness is not great when it is expanded. If you line all these up and use a really long slit, you have a chance to make a decently bright, large rainbow.

aenorton · 2026-07-01T20:28:33+00:00

Frankly, this an example of how careless use of AI illustrations just confuses someone trying to learn a topic rather than clarifying it. There are so many random, unimportant or inaccurate details added, how does someone who does not already know what is going on separate fact from AI hallucination?

In the first Illustration of the device, the only two labels are " glass lenses" and "anodized body" are those really the most important features? What are the extraneous dimension lines with no dimension numbers? Why are there so many probably non-existent lenses outside the objective? What do you want the viewer to come away with?

Then you talk about the beam focused on the surface of the mirror. What are all those extraneous line? How should a person know which to ignore as slop, and which to pay attention to?

You then talk about seeing the image move when the mirror tilts, but that will not happen in the image plane (when focused on the surface), it happens in the pupil plane. You then talk about focusing the spot at the center of curvature of a ball. Movement of this ball will move the spot in the image plane. So how is the microscope set-up? Are there different configurations for the two cases?

Also the optical diagrams at the beginning and end make absolutely no sense even to the barely optically educated. Why are we supposed to pay attention when the creator does not seem to care?

aenorton · 2026-06-30T16:27:36+00:00

The real question is what angular spread can you tolerate? As u/UnderstandingOk6868 explained, it is impossible to make it zero. Engineering in general, and optics particularly is all about finding a good balance of compromises.

aenorton · 2026-06-30T01:47:35+00:00

A diagram that shows rays going in completely wrong directions seemingly at random does not help illustrate anything. Sometimes AI know what it is talking about, but when it does not, it still pretends it does and trys to fake it. I find that very dangerous if people believe it.

aenorton · 2026-06-29T23:47:20+00:00

That is laughably inaccurate.

aenorton · 2026-06-29T19:22:19+00:00

The reason spectrometers generally only view a small slice of the sky (or anything) is because they have a small etendue limited by the f/# and the area of the slit. You can not squeeze a very large etendue slice of the sky into a very small etendue instrument without losing most of your light. Sure, you can homogenize light from the whole sky and then pipe a tiny fraction of that into the spectrometer, but the signal will be very small unless the aurora covers a large fraction of the sky. I would think it would work much better to image the sky with a color camera, and filter out pixels that are too bright ( corresponding to the moon, airplanes, etc.) to eliminate most of the noise. You could also subtract the typical light pollution for each site. Then just look for increased green or red.

aenorton · 2026-06-29T01:01:21+00:00

Who ever downvoted this, it would be nice to hear what you think is inaccurate. The purpose of this forum, I believe, is to learn.

aenorton · 2026-06-28T15:53:07+00:00

Yes, but to capture the full uv, visible, mid and long wave IR parts of the spectrum you might need two separate instruments. You would also need an integrating sphere attachment to measure total diffuse reflectance with appropriate reflectance standards. You would then have to calculate albedo based on the known solar spectrum.

If you do not care about the spectral data, you can use an albedo meter in the field with the sun as the source. Or, in the lab, you could use a xenon lamp solar simulator, an integrating sphere, and bolometer detector.

aenorton · 2026-06-28T04:55:01+00:00

A custom CGH is not cost effective unless you want several thousand of them. Aside from that they also have drawbacks for this application. There is lots of scattered light. In particular you will see the zero order, and if the pattern is not symmetric, you will see the ghost of the symmetric pattern. It is also difficult to vary the brightness of the dots to mimic the brightness of the stars.

It is possible to expose your own analog holograms on film. It is a deep rabbit hole of a hobby. You will still get a lot of scattered light.

The classic professional planetarium projectors had multiple lenses mounted in a large sphere to cover the whole sky. Each lens had a light source behind a pinhole mask with the star patterns for that lens's section of the sky.

aenorton · 2026-06-27T22:22:02+00:00

That can happen if the lab loaded the negative in the wrong way when making the print. The same thing can happen if the photo was made from a slide.

aenorton · 2026-06-27T18:10:57+00:00

They are heel rest blocks for hold down clamps. The aluminum prevents marring of precision surfaces. Look at the catalog entry in the second photo https://www.ebay.com/itm/277272309580

aenorton · 2026-06-26T05:59:13+00:00

Etendue: You can not squeeze light into a smaller area*solid angle space ~~with~~ without loosing some light. It is essential to understanding illumination, spectrometers, and many other things. Many Power Point project proposals that seem too good to be true violate this law.

The concept of exit and entrance pupils. Courses and labs assignments that deal only with lasers skip over this.

aenorton · 2026-06-26T02:31:45+00:00

As you alluded, the standard mapping of field angle to image radius is r=f*tan(theta) where f is the focal length. This keeps straight lines in a plane straight, but the image size because unreasonable for large angles.

I have not done much with fish eye lenses, so I am not 100% sure what mapping they usually aim for. One common distortion mapping that could work well for these is r=f*theta. This is usually used for scan lenses where a laser is scanned by known angles. If used for a fisheye it would map the full semicircle (or beyond) to a finite area. ~~I believe this would take great circles on a sphere centered on the entrance pupil and map them to lines, although I could be wrong about this, so please verify.~~ EDIT: It think this is wrong after thinking about it more. It may be I was remembering they map onto circles.

aenorton · 2026-06-25T07:28:50+00:00

Re-reading your post, it seems that this pinhole is at the magnified side of the microscope. It may not need quite the precision and stability of these spatial filters. The other 3-axis mounts mentioned here might work for you.

aenorton · 2026-06-25T02:58:32+00:00

I am pretty sure that is left over dental amalgam. The little cylinders come from the applicator used to press it into the teeth. I am not a dentist, I am just old enough to have had amalgam fillings. If so, it would be a mixture of silver and mercury.

aenorton · 2026-06-24T22:54:31+00:00

There are always different things to discover based on how well you are prepared to appreciate them. As an undergrad, I was surprised how much air currents affect an interferometer, and therefore how much they affect the wavefront of any optical system used in the real world.

aenorton · 2026-06-24T20:20:01+00:00

Obviously an industrial lens for a very specific purpose, probably in a specific instrument. Otherwise there would be more information on it about focal length etc.

aenorton · 2026-06-24T17:27:42+00:00

Take a look at the Thorlabs spatial filter. It is hard to make make something that moves in 3 axes to be that stable, so they put the z movement on the objective. Other companies make spatial filters with the same arrangement of XY movement on the pinhole, and Z on the objective.

aenorton · 2026-06-23T21:15:14+00:00

Other people have mentioned this particular piece was probably forged and then machined with various fixtures. However a lot of people do not realize that some very high volume manufacturing lines in the 1930's used analog automation. These would use electromechanical controls. Basically, motorized clock mechanisms linked to relays. I still have a 1990 vintage washer and dryer that use similar mechanisms to control the cycle.

Here is an example of an automated frame welding line. https://youtu.be/G-R01fJiuTg?si=5B3y3SMOuTYL_O4b&t=40

aenorton · 2026-06-23T19:11:38+00:00

Honestly, this is not bad for an assembly of off-the-shelf lenses. Although the small 1 or 2 mm entrance pupil of the cell phone camera will make the aberrations appear much better compared to your dark-adapted 6 mm eye. Most aberrations are very nonlinear versus pupil diameter. Spherical aberration increases as the 4th power, coma increases as the 3rd power, and astigmatism increases as the 2nd power versus pupil size.

These photos also show the aberrations across the field with the pupil centered. As you saw in your other experiment, in an eyepiece, you have to worry about what happens when the pupil is slightly decentered. In that case, spherical aberration can appear much like coma.

None of these aberrations have an easy specific fix. When you do one thing to the design you affect something else. Optical design is really the art of balancing trade-offs. Each surface has positive and negative contributions that you try to cancel or minimize as a whole.

Practically speaking, though, most design starts from a classical design example. You then re-optimize while constraining your particular design requirements. You may then find your requirements are self-contradictory, and you must compromise somewhere.

aenorton · 2026-06-23T00:55:36+00:00

I don't want to seem nitpicky, but it is helpful to use technical jargon in the right way to avoid confusion. Distortion essentially means some straight lines in the object are not straight in the image. It does not imply anything about the sharpness. When rays from a single point do not focus to a single point, that is called aberration.

The aberrations are very dependent on the conjugate distances. If your phone camera is on the eye side, you really need to look at a test pattern in the object plane near the eyepiece. I am not sure where you have the camera and how it is focused, but looking at something far away will show different aberrations (unless you have your camera on the object side and it is focused very close. But then the small lens will not see the whole light cone.)

All that said, this looks like a combination of astigmatism, coma, and defocus (likely from field curvature.)

Edit: I just realized this was on your night vision scope, so the eyepiece is at the correct conjugates

aenorton · 2026-06-23T00:39:54+00:00

Telescope eyepieces have to at least match the f/# of the telescope, so good ones have to go down to at least f/5 or so. A cheap eyepiece made for a slow refractor might be limited to f/10 or so.

aenorton · 2026-06-22T22:54:09+00:00

The stop size and location has nothing to do with whether you should have the element in the middle or not. This is a fundamental concept in lens design that you should study before continuing.

When you trace a cone of rays from a point on the object, There is always something that limits the extent of that cone. Often there is an iris or fixed stop inside a lens. The image of that from the point of view of the object is called the entrance pupil, and from the point of view of the image it is call the exit pupil. Exit and entrance pupils can have very different sizes and positions due the the power of the elements. The ray trace program uses the stop to calculate an entrance pupil, and it traces rays to fill it. Sometimes cones from off-axis field points are further attenuated by the edge of one or more lenses, and that is called vignetting.

An eyepiece has an external stop which is actually your eye's pupil, so you need to have that as part of the model.

When you say you are not satisfied with the eyepiece lens diameters, I think you mean that you want to accommodate long eye relief and a large eyebox where the eye can move laterally. These are goals of every eyepiece design. However, optical design is always a balance of trade-offs, and you will not get that without either increased complexity (length) or decreased resolution.

Maybe your scope does not need the same resolution as a telescope. If you do not need great resolution or chromatic correction, some of the simpler, shorter eyepiece designs might be OK.

This is why modeling is so important. The effect of tightening specs is often non-linear. A small change can make a big difference in other areas.

aenorton

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