Re: Spectroscopy

"Skywise" <into@xxxxxxxxxxxxxxxxxxxx> wrote in message
news:135mopm5usnsf6f@xxxxxxxxxxxxxxxxxxxxx

Thank you both Ioannis and AES for your replies. I have been
googling and found basically what you have described, so I
comfortable that I understand this in theory.

Now for the application. I am interested in the feasibility
for an amateur to try to detect specific chemicals/elements
in a cloud. Obviously, I am the amateur in question.

Having the cloud between me and the sun would be ideal as this
reduces things to my already understood case of transmission
absorption. That this can't always be arranged is what leads
me to my interest in reflection spectroscopy.

Here's my plan. Using a telescope (which I already have) to
gather light from the cloud of interest, at the focal point
having a fiber optic to collect light which is then taken to
the detector. I already have fiber. (plastic fiber like used
in start ceilings, etc...)

The next part then is the spectrometer or other detection
method. I'd love to get a "real" instrument, but obviously I
am on an amateurs budget.

My first idea is to take the fiber output and image it through
a reflection grating (I have one that's in OK bearable shape,
but I could buy a new one) and photograph it using a digital
camera (have a Nikon D200). I've had some success with a
similar approach when I made the spectrum of the HeNe bore
discharge that's on my website. Calibration can be done using
known light sources (lasers, street lights, etc...)

I also have a database of emission spectra of the elements
in both electronic form and in my CRC Handbook of Chem & Phys.
I'm sure I could find data for specific chemicals as well.

This could be fun.

Brian,

This would be unfeasible unless you have some characteristic
emission/absorption trait that's unique in the cloud material you want to
examine and this happens usually only when the medium is in a (semi-) excited
state.

Traits such as Raleigh scattering, absorption from diffusion, tar, smog, and
colorimetric absorption (such as that of paints) cannot be used easily because
they are not qualitative. In other words, you can have two different materials
which can absorb the same specific band in the spectrum.

In order to perform qualitative analyses, you need to have a characteristic
spectroscopic trait, such as atom excitation/de-excitation or molecular
rotation or vibration.

Even when you have molecular absorption, things can be really complicated
because of the many combinations.

For an easy exercise, try it with olive oil: It has a characteristic molecular
absorption spectrum, but it will be hell trying to determine constituents just
from its absorption spectrum.

Another famous example, the blue daylight sky. Although scattering occurs and
one has a characteristic absorption spectrum, you can't determine constituents
from the spectrum, until you start looking at specific element absorption
lines, in which case you fall back to examining the absorption spectrum of
daylight, anyway.

I am not sure what your cloud consists of, but for most practical purposes, if
it's a regular air mass on Earth, you'll simply get the absorption spectrum of
the sun minus some trace elements from the atmosphere, but usually the
see-through density of the sky for an Earth-bound observer is not enough to
give you strong absorption features for the constituents of the atmosphere.
You'd need a VERY sensitive spectrograph for that.

If you were on the moon, looking back at the blue Earth, it would be easier,
in the same way it is easy for us to determine constituents on, say, Neptune,
by analyzing its reflection absorption spectrum.

Brian
--
I.N. Galidakis --- http://ioannis.virtualcomposer2000.com/
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"There's ALWAYS a mistake somewhere"

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