🚀 go-pugleaf

On Sat, 11 Aug 2012 22:56:20 -0700, Mark Borgerson
<mborgerson@comcast.net> wrote:

>In article <h6h528drc8h9e7772prit4j5r3drmrlc1p@4ax.com>, 
>jonk@infinitefactors.org says...
>> 
>> On Wed, 8 Aug 2012 05:03:53 -0700 (PDT), George Herold
>> <gherold@teachspin.com> wrote:
>> 
>> >><snip>
>> >> >Re spectrometer calibration:  We had this "black body" source. (from
>> >> >Ortec?)  That could be used for intensity calibration.  PITA though.
>> >>
>> >> The problem for students is that they cannot afford the $800
>> >> it takes to get a $8 tungsten bulb calibrated. (It can cost a
>> >> lot more than that, too.) Then they need to set up a careful
>> >> station for making the calibration. It's just plain too
>> >> expensive to do. And I don't know a way around it. Wavelength
>> >> calibration is CHEAP!! $12 buys a merc-argon calibration
>> >> lamp, which fires up with the Argon first (for about a minute
>> >> or two where you get some very nice Argon lines in the longer
>> >> wavelengths) and then the Mercury kicks in and dominates with
>> >> it's lines. A couple of well-timed snapshots and you've got
>> >> enough (with software) to completely calibrate your pixel
>> >> positions in a cheap ($10) megapixel camera.
>> >>
>> >> I'd LOVE to figure out a cheap intensity calibration setup,
>> >> though. So would a lot of other people, I suppose.
>> >>
>> >> >But maybe you could do something on the cheap with a tungsten light
>> >> >bulb.
>> >>
>> >> But that's the problem. I can get a cheap tungsten lamp. They
>> >> are VERY CHEAP. In fact, the EXACT SAME ONE I would buy,
>> >> calibrated, is dirt cheap uncalibrated. It is the calibration
>> >> data that comes with a newly calibrated lamp that costs so
>> >> darned much. It needs to be traceable to standards. And you
>> >> pay for that. Plus, you need an expensive power supply to
>> >> operate it (0.1% current control is standard.) And it ages,
>> >> too. Within 100 hours or so, it's drifted enough that it
>> >> needs recalibration.
>> >
>> >OK first a disclaimer, I know squat about the details of spectrometer
>> >calibration.
>> >
>> >It would seem to break down into two pieces.  Absolute sensitivity (at
>> >one wavelength) and then relative wavelength sensitivity.
>> 
>> The two calibrations I need are:
>> 
>> (1) Wavelength calibration which assigns a specific center
>> wavelength to each pixel in the array (or matrix in the case
>> of a camera.) Wavelength span is another issue, but it can be
>> finessed a bit with several different alignments of the
>> grating, if details are needed.
>> 
>> (2) Intensity calibration which assigns sensitivities to each
>> pixel (no two pixels have the same sensitivities and there
>> are other losses elsewhere, as well.) The whole system as a
>> unit needs to be calibrated for intensity on a per pixel
>> basis so that you know how much you are getting when you get
>> it. This is an absolute level calibration, not a relative
>> one.
>> 
>> Between the above two, you get accurate wavelength and
>> accurate intensity. Getting sources to help calibrate
>> wavelength is cheap. Getting sources (and setups) to help
>> calibrate intensity is not cheap.
>> 
>> >For the first part you could maybe do something with a laser diode
>> >into photodiode and then the same laser (though a bunch of
>> >attenuators) into the spectrometer.
>> 
>> I'd like to hear more about this and how I might make that
>> work for a megapixel camera over the entire CCD matrix. I'm
>> at a loss regarding absolute calibration here. And keep in
>> mind that it needs to account for the entire system's losses.
>> So I can't go tickering around with the optical path to move
>> the laser beam around because it will never be the same
>> again. (I don't think so, anyway.) Maybe that would work with
>> a monochromator? I don't have experience with those, though
>> in any case it's not what I'm caring about right now.
>> 
>> >For the second you have the students use the black body curve and the
>> >tungsten lamp.  Operate the lamp at two different temperatures.
>> >(Maybe build a nice stable DC supply for the lamp.)  It seems like you
>> >should be able to extract an approximate sensitvity curve from two
>> >black body curves, even if you don't know the exact temperatures.
>> >(But I'll leave that as an excersise for the students :^)
>> >
>> >How good a calibration do you need?
>> 
>> Good enough to do real work that is traceable to standards.
>> Otherwise, it's not repeatable across instruments.
>> 
>> For now, the students get something useable on wavelength
>> alone. And there is a lot of "science" that can get done
>> without more than that. Examining fluorescence wavelengths of
>> plant materials, for example. Stuff like that. They don't
>> have to have intensity to replicate some of the science
>> results they see in books.
>> 
>> But it would be very interesting to come up with a scheme
>> that is cheap for intensity calibration. But like temperature
>> calibration (freeze points, etc) it's not so cheap to do.
>> 
>Would good ol' Sol be an adequate calibration source?  You can
>probably look up the daily solar intensity somewhere on the net.
>Then you correct for local atmospheric effects.   Of course,
>if this project has to work in Oregon, it will probably have
>to be used only during summer school!   ;-)
>
>Oh,  and the spectral output is pretty well known and even
>includes a few good marker lines.

yeah. See:
  http://www.google.com/patents/US5835267

My intent is to only use parts that a student can afford to
buy and OWN at home. Not school equipment. Not access to
expensive tools. Plain, simple, stuff. I have worked out a
folding piece of cardstock paper that makes the box and has
all the right angles set up for them, with baffles (also out
of paper.) The DVD-RW is cheap and is a precision diffraction
grating. The camera is cheap (blister-pack, $10 or less, at
common grocery stores.) The merc-argon bulb is $8 to $12,
depending. That's the ballpark I'm talking about. They build
everything themselves, take it home, and own all of it.

If you come up with a scheme that can align with the sun in a
consistent fashion (without burning their poor eyes out),
get's good data on the sky filtering and effects due to angle
of incidence, uses a stop and baffles in the optical path
that children make by hand using glue and scissors and a
ruler, let me know.

Think 12-year old and up.

Jon