Here is where the value of using a two dimensional imager comes into play.
Tilt the imager, so the top row of pixels is where the green lines focus, and
the bottom row of pixels is where the far red and far violet focus.
Now the software can just pick out the row that has the best focus for each
spectral line.
Of course you can also do this using a few linear sensors, but with all the digital
cameras around, I would have to think that two dimensional sensors are getting
pretty cheap. In 250 unit quantities, a 2048x1536 chip is under five bucks.
If you want 2560 pixels, tip the sensor and read from corner to corner diagonally.
Or tilt the slit and grating 45 degrees.
Or build the spectrograph as a lens attachment to your SLR camera, the way I built
the microscope into a lens attachment at the Science Hack Day.
When you aren't doing spectrography, you can be doing photography.
It becomes as simple as changing a lens on the camera.
And that gets you 5184 pixels across, or 6230 pixels if you tilt the slit.
-----
Get a free science project every week! "http://scitoys.com/newsletter.html"On Sun, Nov 20, 2011 at 7:45 PM, Nathan McCorkle <nmz787@gmail.com> wrote:
Maybe all the people buying products from the mega-corp Newport are
idiots, but I don't think so. Chromatic aberration is an issue,
because you're not imaging onto a slit, you're imaging onto a CCD
plane... so if all the spectra don't share the same focal length,
you're getting multiple frequencies falling onto the same pixel.
Here's a czerny-turner flat-field spectrograph, it uses two concave
mirrors plus a flat grating like Simon mentioned being easy to
produce.
http://images.pennnet.com/articles/lfw/thm/th_0711lfw03f4.gif
Here's a description of "Aberration Corrected (Flat Field Imaging)
Concave Grating", which is the simplest and highest resolution setup I
could think of (simple for someone to align, high enough resolution
and low enough stray light to do Raman)
http://www.ssioptics.com/definitions.html
In this sense I guess I was referring to astigmatism being corrected,
but I can't really tell what the difference is between that and
chromatic aberration, as the definitions seem quite similar.
> On 11/20/2011 05:41 PM, Nathan McCorkle wrote:
>>>
>>> You can make a hologram of a grating with a lens in front of it, and get
>>> > both
>>> > optical elements in the same simple, flat piece of film.
>>
>> As far as the optical engineer at Newport Corp that I talked to in
>> their gratings division here in Rochester said, its not just a
>> holographic grating, but its also concave to correct for chromatic
>> aberration
>
>
> I like the hologram lens/grating idea. The lens that would combine
> features is a cylinder lens for intensifying the light
> through the entrance slit, by making all of the slit height
> fall on a point, that you aim at your line CCD detector.
>
> Chromatic aberration
> seems the wrong concept to talk of in a spectrometer where we are
> wanting different wavelengths of light to diverge. Aberration
> in lenses is where wavelengths diverge in unwanted fashion.
> I think the problem curved gratings correct is that the light
> beam through the slit is going to be bent along paths of different
> lengths before hitting the flat line target of the detector surface.
> So you could have a lens focus the light spreading out from the slit
> to be a fine point at the target, but one simple lens could not do it
> for all paths to the line target -- the ends and middle could not be in
> perfect
> focus at the same time. This might be helped by Simon's idea of holographic
> lenses.
> a film hologram that does a nonuniform combo of cylinder lens plus
> a different amount of magnification at the center than at the ends
> could correct the path lengths problem.
>
> On 11/20/2011 06:39 PM, Simon Quellen Field wrote:
>> But chromatic aberration is kind of the point of a grating after all,
>
>> Any advantage
>> to be had from curvature could be done by changing the optics the hologram
>> is
>> an image of.
>>
>> And since these holograms are created by computers, and are not
>> photographs of
>> real objects, any optical manipulation you like is just a matter of
>> changing the
>> parameters to the computer program.
>
> Yes, Simon beat me to the comment about aberration in a spectromoer, and
> this sounds like a good approach. Who would you talk to about making such
> transmissive film holographs, and also is there a reflective version,
> or do you always use transmission with film holograms?
>
> John
>
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>
--
Nathan McCorkle
Rochester Institute of Technology
College of Science, Biotechnology/Bioinformatics
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