34 replies to this topic

[patta]

A setup I've built last year

 

A spectrometer relay; the color separation is done radially by lateral chromatic aberration.

Normally the slit of a spectrograph is a line; here instead it is a circle.

 

Can be used to monitor the Sun corona, or for spectral imaging.

 

Lateral chromatic aberration, which normally we don't like: unbalancing the color correction around the aperture stop - overcorrected on one side, undercorrected on the other, the colors separate.

Similar to a prism spectrometer. The overcorrected/undercorrected lenses do the same job as the dispersing prisms, but do it radially around the axis.

It works well in the visible range, but limited in the UV because of how color dispersion work in glasses.

 

 

Some researchers want a similar setup for a small coronagraph telescope to be sent in orbit. But using a circular diffraction grating instead of the glass dispersion.

CISS - Circular Slit Spectrometer - Landini - Frassetto INAF

https://indico.ict.i...derico_CISS.pdf

The circular diffraction grating has more dispersion than the glass and works ok into the UV.

But is more difficult to fabricate, has haze, many orders of diffraction, unchartered aberrations...

 

 

My setup:

 

Achromat objective - can be a telescope or a camera objective. I've used a standard 50mm f/1.4 camera objective that was around.

 

It forms image on a plane, where you put the circular slit

I've made the circular slit by gluing aluminium foil on the flat of a flat-convex lens, then cutting the slit on a lathe with a knife.  

After n attempts it come out decent, about 0.03 mm thick.

The circular slit can be otherwise ordered litographed for some 100$

 

The flat-convex lens works also as field lens, collecting the rays.

 

As dispersion lenses, I had around some overcorrected triplets, likely from Ortho eyepieces. Use two of them.

 

Aperture stop, very small

 

One undercorrected singlet lens

 

Sensor

 

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Rough sketch of the setup

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Raytracing of the relay; the telescope objective should be on the left; this raytracing start with the intermediate image and slit on the left, then the field lens, then the two triplets, stop, singlet, sensor. 

Axial chromatic aberration is corrected.

Lateral chromatic aberration is instead very strong, about 1 mm.

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Because of stock/ pillaged lenses, it had to work with small aperture to get decent imaging (ca. f/20 on sensor). The lateral chromatic is visible in the raytracing::

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Edited by patta, 04 February 2025 - 03:40 AM.

[patta]

The components were mounted into M42 barrels. 

In this photo, from left to right:

Aperture stop; Ortho triplet 1; Ortho triplet 2; circular slit on lens

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All screwed up, objective, slit, relay lenses and camera with APS-C sensor.

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The circular slit, my pride and joy. Outer ring diameter 30mm, inner 24mm.

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The slit under microscope

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Edited by patta, 03 February 2025 - 08:03 PM.

[patta]

Results:

without slit, the objective + relay is useful for fancy selfies, with horrible lateral chromatic aberration.

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with the (two) circular slits, can be used as imaging spectrometer.

here it is looking at the color bands of a Neon lamp

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Spectral resolution was moderate (ca. 20 Nanometer) but anyway.

For better resolution, needs a thinner slit (1-5 micron), more color separation and sharper relay optics.

Edited by patta, 03 February 2025 - 07:54 PM.

[Phil Perry]

Interesting. Can you share a typical image using this? I'm curious as to what it looks like and how useful it is (I don't do spectroscopic work). The slit looks a little irregular on its edges, which could degrade the results. Something lithographed onto glass might work better.

 

If I understand this rig, you're harvesting chromatic abberation from a simple lens rather than a using diffraction grating. That's an interesting approach, but necessitates a circular slit. I suppose that as long as it produces a good spectrum (comparable to a grating), it could be useful.

[patta]

Last post #3 are real images taken through the relay; second image is working as spectrograph.
Yes the slits weren't perfect (under the microscope), but a lot of fun to make!

Compared to grating, it has small color separation and UV cutoff.
And color spacing is nonlinear, more separation in the blue than in the red.

Normal gratings are straight lines - for circular spectrometer, one would need circular gratings - they exist,but rare.

Examples of circular gratings:
Circle filter for lasers
Diffraction lens on Canon telephotos.

Edited by patta, 03 February 2025 - 03:53 PM.

[walt r]

Yes the slits weren't perfect (under the microscope), but a lot of fun to make!

Just how did you make the slits?

[patta]

Just how did you make the slits?

glue aluminium foil on a round glass plate

fix the plate into lathe chuck

cut the slit with a knife blade tip mounted on lathe toolpost (Olfa black blade further sharpened)

 

With this method very thin and uniform slits can be made - if the aluminium foil is well flat and the blade sharp.

Some glue remains in the cut and scatter light around - in this application didn't matter as the slit is on the focal plane.

 

I'd bet there are 100 other methods to DIY a circular slit...  burn a DVD with only one turn engraved?

I tried with normal laser cutter but couldn't cut thinner than about 0.3mm (300 micron).

 

Daniel of DIY-optics offered to do the job with his  laser engraver, he cuts slits to about 25 micron width and maybe less, on copper foil.

 

While the guys at Graticules-Apertures UK do those things as daily job and quoted me something like 300£ to make the "master mask", from which  then print super thin and sharp slits chrome-on-glass.

 

Or one can order the custom filters for the "Gobo" projector, 10$ each,  conveniently mounted on glass disk and maybe sharp enough for the slit job.

 

 

 

The foil cut wasn't perfect but good enough for the moderate resolution of the relay, so had to to.

Edited by patta, 03 February 2025 - 07:32 PM.

[walt r]

Thanks for description.

[patta]

1) the optical design are published in the design thread

https://www.cloudyni...ead/?p=13954532

in particular the setup at post #416 is like the one I've built but better, feasible DIY (lenses are ortho triplets + stock singlets), for imaging the spectrum of Sun prominence and maybe corona.

 

2) Circular slit 2

Before I had no idea what a coronagraph actually was.. and how much heat the Sun produces.

Below the new slit:

A cone made of brass to block the Sun image - like standard occulters

An aperture stop around, slightly larger than the cone, so only a ring of light can pass

The two pieces fit together well centered, and opening is cut leaving some spider braces.

 

The thing is feasible with lathe. Below the current prototype with diameter 9.5mm, slit width ~0.1mm (100 micron) quite nice and centered.

With some patience one should manage to make the slit accurate to 10 micron (1/100 mm) which is enough for spectrum measurements.

 

Ideal design in section, with a 10 micron slit

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The  assembled ring slit, with the larger 3D printed prototype

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The two pieces of brass I've cut with lathe: outer ring and the cone

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the two diameters, slit ca. 100 micron

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Edited by patta, 07 February 2025 - 07:08 AM.

[R Botero]

Really interesting project

 

Roberto

[patta]

For the relay, is it possible to use gratings instead of the undercorrected-overcorrected lenses?

Since it is a spectrometer, one expects to use a grating, not lenses.

 

Landini and Frassetto plan indeed to use a grating for their satellite-based Corona spectrometer CISS, although I don't know with what exact optical configuration.

 

We have a circular slit, then we need a circular grating; such gratings is basically a Fresnel lens with very near tracks; so near that locally it works as a linear diffraction grating.

I'd assume they are "blazed" and send all light in one direction mode; here an example from Heidelberg Instruments:

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The spacing between the lines can be adjusted to make a focusing lens, in the same way of the Fresnel. Due to the diffraction, the power of the lens depends on wavelength (plus other aberrations).  The color dispersion is very strong.

Some optical softwares, for example Quadoa, manage to simulate/model circular gratings; here below the raytracing of a positive diffractive lens; see the very strong chromatic overcorrection, with Red focused much nearer than Blue:

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we can use the diffractive lens to build a spectrometer, by sending rays on the periphery through an off-center slit and/or putting an aperture stop spaced from the lens; it works more or less in the same way as a normal linear grating; different color are separated.

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From the image above, see already one issue - the colors separate before the stop. (Magic?)

Other trouble with larger aperture; the difference in focal length for different colors become evident, so the circular grating alone can separate the colors but doesn't manage to focus them on the same plane. One color is in focus, while the other are so much out of focus to be unusable:

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So, problems with a transmission circular gratings:

 

1) focal distance different for different color

2) pupil position moved for different colors (Lyot stop changes position)

 

how can we solve those issues?

Edited by patta, 10 February 2025 - 01:59 PM.

[patta]

My first instinct to solve axial chromatic aberration: Triplets!

 

Just like the designs with lenses only, we put two undercorrected triplets on the other side of the aperture stop; et voila', axial chromatic aberration is corrected and the lateral spectrum dispersion is enhanced.

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Performance is about the same as the most basic lens-only relay; also the spectrum is limited in the UV at about 390 nm by the absorption of the flint glass in the undercorrected triplets.

Basically, the circular diffraction grating substitutes the overcorrected groups in the all-lens design like at first post in this thread or optimized designs like https://www.cloudyni...ead/?p=13950979

 

Another funny issue, the diffraction grating has partial dispersion very different than glass: "refraction" goes to 0 for short wavelengths, while it become very large when the wavelength approaches the grating pitch.

So chromatic correction will suffer from secondary spectrum. While for the lens-only relay, secondary spectrum is moderate to start with and can be further mitigated  with APO combinations of glasses.

 

Partial dispersion of glasses vs. diffraction grating (qualitative)

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Edited by patta, 10 February 2025 - 10:17 AM.

[patta]

The cemented triplets were fashionable one century ago; today, let's instead use another annular grating to patch up the axial chromatic aberration.

Metasurfaces! That's cool stuff.

Canada Balsam and Lead glasses, Booo!

 

The second grating must be of negative focal power, so it has opposite wavelength behavior than the positive grating.

(positive grating: overcorrected for chromatic aberration;  negative grating:  - (overcorrected) = undercorrected ).

Such grating with high diffraction efficiency in the negative mode can be built by blazing the grooves like a negative Fresnel lens - opposite than the other grating, blazed as a positive Fresnel lens.

To make a 4f relay the negative grating is aided by a couple of positive lenses; they can be made of Fluorite or fused Silica, that allow transmission in the UV down to about 280 nm, and give very little contribution to color errors.

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Nice, this relay has axial color correction, no secondary spectrum issues, and works down to 280 nm.

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If "money is no issue" one can ask for blazed grating lenses of exquisite construction like those of Canon DOE: the grooves are sandwiched between two glass lenses to reduce haze & scattering; and the grating is bent on a spherical surface to balance out aberrations:

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Here an example of the usual 4f relay now with two buried diffractive lenses Canon-style, one positive and one negative, on Fused Silica support; grating parameters and lens curvatures optimized for max sharpness on each wavelength.

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Edited by patta, 10 February 2025 - 06:27 PM.

[patta]

A closer look at raytracing of the last setup, working on visible spectral band. Thanks Quadoa for the software free trial!

 

It has very strong color separation and allows a relatively fast relay at f/8 and diffraction limited at f/16 (image below at f/16; image in previous post at f/8). It can be made better with more time spent on optimization and more lenses.

The usable spectrum width depends on sensor size and relay length; which spectrum band, and how much wavelength dispersion, can be quite freely chosen by selecting the grating pitch.

 

Aberration correction, overall sharpness and spectral resolution are similar to the optimized "Lead glass & Canada Balsam" relay

https://www.cloudyni...ead/?p=13950979

However with the Lead Glass relay the spectrum is limited to visible, with somehow fixed dispersion, while the gratings allows more choices of central wavelength and dispersion.

 

The relay illustrated below has colors spread out on 6 mm, with resolution 20 micron, thus should distinguish 300 spectral bands; with range 460-620 nm, this translates into 0.5nm =  5Å spectral resolution.

 

Take those results with a pinch of salt since I have no idea about how exactly the diffraction grating is modeled in the software; the ray's behavior looks very realistic to me, let's assume it is indeed. But skew rays like to bend in nasty ways...

Also other typical grating issues, like scattering and diffraction orders, are not modeled at all.  Could be done but it's going to be some months of work.

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Edited by patta, 10 February 2025 - 05:41 PM.

[patta]

Made another cone-slit with my friend's little precision lathe

Diameter 9.594 mm, computed to cover the Sun on my 910mm fl scope

 

Cleaner and better centered than before, about 25 micron width; hope (little) enough!

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Edited by patta, 12 February 2025 - 12:32 PM.

[patta]

A close look into a specific issue of those spectrographs, relevant for solar use:

the chromatic whacking of Lyot stop position, hinted in post #11.

 

Using the two-gratings relay of post #13; we put a Lyot stop in the middle of the relay. Where exactly is "middle"?

Between the slit and the Lyot stop, there is a grating, which does chromatic aberration. Therefore the image of objective stop changes depending on the wavelength; so the optimal Lyot stop position depends on wavelength. Welcome to the cauldron of "Pupil aberrations".

 

Here example of telescope + relay setup to show the chromatic shift of pupil; the objective stopped down at f/100:

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At larger aperture, like f/15, if we place the Lyot stop at a certain place, it will vignette the colors that are off-center.

With a real Lyot stop of diameter slightly smaller than the objective pupil, the vignetting is severe and only a narrow wavelength band passes:

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Which band, can be chosen by moving the stop longitudinally.

So, with a Lyot stop there is this band limitation.

That is not necessarily bad; the relay can be set up to work with high contrast and resolution on a specific band; the stop does the job of a (wide) bandpass filter.

Below example of 2-gratings relay, with Lyot stop, Lyot Spot and focus adjusted to work on Green around 556 nm.

f/16 if I remember well; passband by stop 545-575 nm; spectral resolution about 1 nm.

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Edited by patta, 13 February 2025 - 06:43 AM.

[Gleb1964]

I think that Lyot stop should be considered in a "white light" , before dispersive elements.

Edited by Gleb1964, 13 February 2025 - 08:15 AM.

[patta]

Indeed; if we don't want chromatic aberration at the stop, the lateral chromatic aberration should -and can - be produced only after.

 

Here an example:

After the slit we put a lens assembly with external pupil, corrected for chromatic aberration (both longitudinal and lateral)

That's called an eyepiece;  picked an Ortho just to confuse you.

 

The slit is at the focal plane of the eyepiece, while the Lyot stop at the exit pupil. Pretty natural!

After the stop,  an undercorrected lens and one overcorrected - being the overcorrected near to the stop, they realize lateral chromatic aberration.

 

Spectral separation is about 1/3 of the previous relays, and aberrations more difficult to correct.

But feasible construction:

1x Ortho eyepiece 20mm

1x triplet from Ortho eyepiece 8mm

3x COTS lenses for the undercorrected triplet (Air-spaced for DIY)

 

Similar setup can be done with diffraction grating instead of the last triplet.

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Edited by patta, 16 February 2025 - 07:07 AM.

[patta]

And that's another eyepiece relay, as from previous post; using a diffraction grating in reflection.

 

It has strong color separation but also "Chromatic field curvature" and astigmatism.

It should work decently at f/20 over a narrow bandwidth (say 620-660 nm)

Why the Barlow lens? Because of the CD! Will explain later.

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Edited by patta, 16 February 2025 - 09:39 AM.

[patta]

Meanwhile I've tried to build a spectrometer relay using a CD (Compact Disc...) on axis as circular diffraction grating in transmission.

 

Attempt failed as my usual build method of "3D printed M42 adapters + duct tape" went outside its strength and centering specs. I need a proper optical bench...

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Edited by patta, 25 February 2025 - 02:41 PM.

[patta]

The CD is made usable in imaging by a Corrector that fixes the astigmatism coming from CD grating.

Either in transmission (easy build, but low efficiency)

or in reflection (more delicate build, high efficiency)

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Edited by patta, 25 February 2025 - 02:39 PM.

[patta]

Made an astigmatism corrector as described above, with my stash of small lenses.

hope it works

Afterward I realized that it can be made with larger spacing and weaker lenses (thus easier, less sensitive to miscollimation etc); too late.

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Edited by patta, 25 February 2025 - 02:46 PM.

[whereIsIt]

Meanwhile I've tried to build a spectrometer relay using a CD (Compact Disc...) as circular diffraction grating in transmission.

 

I also made one from a CD that included interface to Camera/Video.

I used it many times for Gases and reflected light from Metal's and through Oil's (with metal fragments)

I categorized the images and used them in various investigation's.

 

I 3D-printed an interface to Telescope-Len's... a bit of fun but, not not useful to me...

 

ADDED: I made one of these a few yrs ago...

 

Example's and showing the CD inside...

Attached Thumbnails

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Edited by whereIsIt, 25 February 2025 - 02:19 PM.

[patta]

 

 

Example's and showing the CD inside...

Nice color separation! CDs pack quite a punch as diffraction gratings; what we're trying to do now is to use the whole round CD on-axis, as post #3, with (long-term dream) sharp image

Edited by patta, 25 February 2025 - 04:25 PM.

[patta]

Here how the spectrograph may look like with a custom circular grating on flat substrate, with the spacing between grooves optimized for the task. Think of it as a custom asphere. No pesky CDs in the dream world.

 

Built as post #18 and #21; it starts at the exit pupil of an eyepiece, so we can have a white light Lyot stop.

The designs with 2 gratings as post #13 are more powerful (sharper, faster etc) but here we have only half relay to fix all the issues.

The grating may also be in reflection, as post #19 (which was a crude ancestor of this design)

 

The front lens elements do the job of astigmatism and longitudinal chromatic correction.

The collector lens is no more needed, and size much reduced, because of the custom grating.

This design is limited by the secondary spectrum mismatch between glass and grating (see post #12)

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Edited by patta, 26 February 2025 - 08:16 AM.