The focus problem is due to the use of high dispersion gratings.
They spread the spectrum over a wide angle, so the middle of the
spectrum focuses farther back than the two ends do.
There are several ways to get around this.
First, you could simply eliminate focusing altogether, by having the optics
focus on infinity. The slit then looks like a point source at infinity, and is
always in focus.
Second, you could use a low dispersion grating, and simply back away
until the spectrum fills the imager. The angle between red and violet is
now so small that everything is in focus. If you don't want a meter-long
device, fold the optics with mirrors.
Third, you can convert a high dispersion grating to a low dispersion one
using a lens.
A cute fourth trick is probably not DIY, but a lens can be made that focuses
green light closer than red or violet.
Likewise, a lens or mirror can be made that focuses the center closer than the
edges (this is basically what the curved grating is trying to do). But the grating
itself does not need to be the active optical element -- if you have a lens do it,
you can use a cheap grating.
It is easy to have software accommodate various tilts and zooms. The software
I wrote for the spectrograph I did for MAKE magazine does that. But it cannot
fix focus problems without taking two or three shots at different foci and
combining them in something like CombineZP.
Making the spectrograph self-calibrating using a fluorescent light saves all kinds
of time and hassle, and allows for a lot of slop in assembly. It also gives you a good
feeling that your data is actually accurate between runs.
I would not worry about UV absorption. When you calibrate in the amplitude domain,
any absorption is accounted for. And it is completely silly to worry about absorption
in thin plastic films. If UV filtering were that easy, sunscreen SPFs would be in the
billions.
As with all optics, you will have to consider empty magnification. It won't matter how
many pixels you have if your optics can only resolve the spectrum to five or ten
nanometers.
With that in mind, consider very narrow slits, and compensate for the dimming by
integrating over time. You can also do HDR by taking several images and combining
them in software. If you jiggle the detector or the optics between shots, you can also
compensate for pixel sensitivity variations in the sensor. The software will do correlation
to match up the various images, and any pixel variation will average out.
Newport is a great company, and having local access to their experts is a big win.
If you are worried about quality control with any company, Chinese or not, just make
sure your contract allows returns of items that don't meet your specifications for cash
or full credit. Quality control is your job, you can't outsource that to your vendors.
Find out what your needs really are, and don't engineer a device to be more
expensive than it needs to be. Astrophysics might need extremely high resolutions,
but most biological applications might be satisfied with nanometer or even five nanometer
resolution in the frequency domain, and care more about dynamic range and reproducibility
in the amplitude domain. The time stability or calibration accuracy of your light source
might be much more important than resolving Doppler shifted spectral lines from one
side of a star to the other to determine rotation velocities (the astrophysics case for high
resolution).
Amplitude calibration will probably be done by comparing an empty cuvette to the one
with the sample. If the light source changes in that time, you have a problem. A design
that uses a tall slit, with the sample blocking only half of it, allows you to do the compare
in one shot. But then you would need a two dimensional sensor like a camera, or two
linear ones.
For protocols that specify growing a culture until it blocks n% of light at some wavelength,
you can go really sloppy. The bugs will keep growing after you measure, and ten percent
accuracy is probably overkill. A light meter and a few different colored LEDs will probably
do just fine, once you have set up some conversion tables between a lab spectrometer
and the DIY LED gadget.
-----
Get a free science project every week! "http://scitoys.com/newsletter.html"On Mon, Nov 21, 2011 at 1:29 AM, Nathan McCorkle <nmz787@gmail.com> wrote:
On Sun, Nov 20, 2011 at 11:07 PM, John Griessen <john@industromatic.com> wrote:I'm honestly not sure, as far as I could ascertain is that the
> On 11/20/2011 06:39 PM, Simon Quellen Field wrote:
>>
>> my earlier point about holograms is still relevant. A perfectly flat image
>> of a
>> grating and a lens will act just like the original grating and lens. So
>> there is no
>> point in making a holographic diffraction grating anything but flat. 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.
curvature with a holographic surface grating differs from a flat
holographic grating.
Since I'm off school this week, I'll see if I can meet with an optical
engineer up at the grating lab.
on alibaba I used "concave grating" "flat-field grating" "holographic grating"
>
> I do find many examples of reflective gratings by the gift wrap suppliers,
> but they probably can't make designs from custom requests for spectrometer
> use.
> this one is certainly showing off spectral divergence galore:
> http://www.alibaba.com/product-tp/105737634/Holographic_Films/showimage.html
>
> And all you need to buy is 100kg for $1000. :-)
>
> What are good search terms to find suppliers who would generate
> a dual lens and grating hologram film? I've looked at CGH on
I think only if its transmissive. UV diffraction is different than
> http://corticalcafe.com
> and it's a start, but... "Don't expect miracles, decent holography film has
> 5000 lines per mm (127000 lines per inch) but your laser printer has an
> anemic 600 lines" So go figure... Time to look for
> someone to hire to get a cylinder lens+non-linear-lens effect created.
>
> How about superimposing films -- do they need to be spaced apart some?
> Are film holograms all going to cutoff UV at some point related to their
> substrate material (polypropylene, vinyl, PET seem common) ?
absorbtion, but things really only start to get hairy below 200nm,
which is further down than I'd ever think of going.
The double slit will reduce aberration, but it will also reduce the
> Since that is probably so, getting all in one film and
> making that film very thin seems the good way.
>
> Here's a sketch about what we're talking about re: focusing on a line
> detector:
> http://ecosensory.com/diybio/spectrometer_focus_problem-1.gif
signal alot too. I don't think cooling the CCD has been discussed in
depth... not sure how you'd do that since the chip is DIP, that would
lower the dark current noise to get better SNR.
They make single fiber and bundle cables, single mode and multi-mode.
>
> And while thinking about unwanted UV cutoff, what kind of light gathering?
Most of the commodity cables are optimized for NIR, not sure what a
fused silica fiber costs, but I bet that's what's needed for doing UV.
We've got a bunch of boxes of networking fiber in the attic, and some
TOSlink cables and connectors we're going to be looking at soon.
$10 Xenon flash-bulb hot-shoe accessory???
> glass fiber? Make the whole spectrometer small and with a light source and
From http://www.openSpectrometer.com
> mirrors sticking out like a tuning fork you put your sample between?
> With a light source you call it six syllable spectrophotometer, right?
> Six syllables, wow!
>
> On 11/20/2011 09:45 PM, Nathan McCorkle 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...
>
> Nathan, I've asked you, and you don't say what your design goals are even.
> who knows what you want. I'm interested in low costs and about
"
We want to build a capable spectrometer that costs less than
comparable commercial units, and is easier to use. It's our hope that
an OpenSpectrometer is something that anyone can find a use for,
whether in the lab, the home, the classroom, or as part of another
project.
"
I personally want to build a spectrometer comparable to what I use in
my labs at school, with this I'll probably try and build a
microfluidic DNA sequencer.
With contributions from people with broad oversight like Simon,
cheaper designs can certainly be achieved.
Not sure what you mean here, the pixels in a CCD array are in the 5-15
> a focus spot size of 1 or 1.5 mm at first and worry later about higher res.
micron range... with the right grating/optic configuration you can
disperse a spectrum with sub-nanometer per pixel resolution.
The surface was actually described to me as having the consistency of clay.
> So, that might not need the latest, greatest, and commercially profitable
> reflective hard crystal substrate gratings.
They're made of a UV curable resin on top of a crystalline/hard substrate.
I just got a Chinese concave holographic aberration-corrected grating
on Friday. This type of grating is able to receive a divergent cone of
light emitted from a slit or fiber, and focus each component frequency
as closely as to an image plane (i.e. CCD).
>The difference in the foci of each frequency contributes to
> So, you want to talk aberration? OK. From what?
overlapping frequencies, or smearing. This depends on the
characteristics of the optics system in place. The tolerance to
smearing is application specific.
I never said that mega-corps could solve DIYbio problems, but as Simon
>
> Meanwhile, I think Simon's leads could turn up some nice usable spectrometer
> methods on the cheap. And no, I have little faith in mega-corps
> to solve diybio problems. Newport is a mega-corp that started with flat
> fiberglas with screw holes in it. Why get religious over that?
said, we've got to depend on suppliers for parts to build equipment,
unless we engineer photosynthetic organisms to create components for
us. You've been pushing Atmel chips lately, they're a mega-corp.
I never touched on religion. Newport manufactures gratings in
Rochester, where I live. I went and talked to them, told them about
the project, and they gave me an overstock grating for free. Do I
trust them and their quality, absolutely, I walked through their labs
at one of the two manufacturing locations. Do I trust the Chinese
grating less, yes, I'm worried about sending them hundreds or
thousands of dollars and being ripped off, with potentially little
fallback (I think PayPal is safe though), yes I found this company
through AliBaba:
http://www.alibaba.com/product-gs/361037953/Concave_Holographic_Gratings_for_Flat_Field.html
Newport retails for $550, Orienvac says $150
I can trust Newport to deliver scientific quality optics, Rochester is
known for its optical history (kodak, bausch and lomb). I need to
compare the Chinese grating to a Newport grating to trust it.
Then alternative, dirt cheap even, systems can be characterized in
relation to a standard.
Software can't correct everything, and to use software correction
>
> "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."
> But, what in the design goals needs correcting? Hint: Good design
> sidesteps hard problems that are mainstream. It just drops them
> in favor of another way. (Like SW correction, aiming at a certain
> accuracy and no more, etc. etc.)
properly you have to understand your system's physical/hardware
limits.
>
> John Griessen
>
--
Nathan McCorkle
Rochester Institute of Technology
College of Science, Biotechnology/Bioinformatics
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