I usually get complaints that my questions are too difficult but I'll give it a try in the next 24 hours.

That is a good point.

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

JOOC, The theory being tested was general relativity? So, Jerry Jensen said that general relativity was untestable? There's been historical tests of it, why would he say that?

In many astronomical instruments, a problem arises when the stellar target does not remain exactly centered in the field of view. The result is that the beam will move around in the instrument, perhaps becoming partially vignetted or otherwise not following exactly the desired path. A solution is to place a lens such that an important aperture (perhaps a grating or camera mirror) is at that lens' focal point. The lens is usually placed close behind the instrument's entrance aperture. The question is what astronomer is credited with first applying this optical element to astronomy (extra credit for explaining just how the lens does its job).

(I warned you folks :-) )

First clue: He was a frenchman.

Sounds like you're describing a double-focussing spectrometer? The objective is focussed on the slit , and the second lens, behind the slit, is likewise focussed on it. At this point, the second lens acts like the objective lens of a microscope, staying accurately focussed on the slit image, whether or not the main objective image moves around a bit.The slit image is always centered and in focus.

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The Devil offered me power. I told him I preferred aperture.

Not quite it ... if a star is focused on the slit and it moves up or down the length of the slit (as it can if the seeing disk is smaller than the slit length) then the image formed by the second lens (as described by you) will move up and down. Note: I said "an important aperture (perhaps a grating or camera mirror) is at that lens' focal point" so is this lens "focused on the slit"?

Hint #2: the astronomer in question also made important contributions to our knowledge of a layer of the Earth's atmosphere

I always have a hard time visualizing these things without a pitcher. Not your fault; my shortcomings.

What I was describing is just the spectroscope my son made last year, a double amici prism type with a converging lens in front of the prisms, focussed on the slit. While the image of a star might wander up and down or back and forth, the image of the slit itself doesn't move (it might be partially illuminated as the star image moves, but the slit image doesn't). Again, apologies; when someone gets it, a source of an illustration would help me.

As far as the scientist, Auguste Piccard comes to mind.

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The Devil offered me power. I told him I preferred aperture.

The problem with a stellar spectrograph is that the slit is not illuminated except where the star hits it. Thus there is actually no 'image of the slit' in the slit-lengthwise direction. If the star moves up or down the slit (as we did back in the days of spectrocopic plates (before CCDs) to widen the spectrum, the whole beam moves and may move off the edge of the collimator, camera and/or grating resulting in vignetting.

Something to think about: If this lens is just behind the slit, and if its focal length is much shorter than the telescope focal length, an image of what will show up at the lens' focal plane?

Clue # 3: the astronomer in question was responsible for numerous advances in spectroscopy and was co-inventor of a particularly interesting high resolution device.

This is terrible. I'm having such a problem visualizing this. Part of my mind says that you would see an image of the objective, more or less evenly lit, much like looking through a Foucalt mirror test apparatus. Another part says, go on, you're completely missing it.

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The Devil offered me power. I told him I preferred aperture.

Absolutely correct ... this lens images the entrance aperture of the telescope (nearly always the objective) on its target ... since the onjective does not move around, neither does the beam. Of course, this means we are not talking about imaging instruments in the case I have described.

Hint # 4: This lens is used in stellar spectrographs and old fashion photoelectric photometers (which probably has let the cat of of the bag).

Trust me, my bag is still tightly tied.

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The Devil offered me power. I told him I preferred aperture.

I am off to Orlando for two days so that will give you and others time to contemplate. A reminder of the hints/clues ...

First clue: He was a frenchman.

Hint #2: the astronomer in question also made important contributions to our knowledge of a layer of the Earth's atmosphere

Clue # 3: the astronomer in question was responsible for numerous advances in spectroscopy and was co-inventor of a particularly interesting high resolution device.

Hint # 4: This lens is used in stellar spectrographs and old fashion photoelectric photometers (which probably has let the cat of of the bag).

Quick bludgeoning...
Janssen
Fizeau
Pouillet
Fizeau

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

George, you are bludgeoning thin air - he is out of reach at the moment...

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Non sunt multiplicanda entia praeter necessitatem.

True, but by pounding his question it produces fodder that one of ya'll might find revealing.

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

Hey, George, y'all talking 'bout me?

Given your (HINT HINT) could be Jules Janssen.

Which is strange if true, because I ran across his name twice in the last year or so, once when learning about telluric lines and once when reading about the Franco-Prussian War. Go figure.

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The Devil offered me power. I told him I preferred aperture.

Well, yes, you and everyone because ain't "y'all" plural, or should I use my grammatically incorrect "ya'll" for plurality. Y'all've got to let me know these things.

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

Yup, y'all is the plural. My dad woulda whupped me for that.

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The Devil offered me power. I told him I preferred aperture.

There ya go. Since IMO is away.... I was in a BBQ place a while back and used their facility. In there they were playing a cd on how to talk Texan and was available for purchase at the cashier, no big surprise. This reminded me of a stroy about a friend I know who had just bought a new pair of boots for the owner of a billion dollar corp. out of Germany. After the boots were given, much to the delight of his guest, he then took this dignitary and his entourage to this great BBQ joint (The County Line in Austin). When my friend heard the cd while in the restroom, he bought it an presented it to his German guest and told him that if he was going to walk the walk, he should learn to talk the talk.

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Lighten up! This is a stellar board! Author: duh.

"The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the universe to do..." Author: Galileo supposedly.

'S funny, but for all the ribbing the southern accent sometimes gets, it HAS preserved a a distinct form of the second person plural pronoun.

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The Devil offered me power. I told him I preferred aperture.