Luminous Landscape Forum
Equipment & Techniques => Digital Cameras & Shooting Techniques => Topic started by: xpatUSA on November 05, 2013, 10:24:41 am
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I made a spectroscope some time back, just a toy really:
(http://kronometric.org/phot/lighting/spectroscopy/prototype.jpg)
At the other end, a cheap 500 l/mm diffraction grating which splits the light from the slit into a spectrum.
(http://kronometric.org/phot/lighting/spectroscopy/grating.jpg)
Inside, a collimating lens takes the rays from the slit and makes them more-or-less parallel on their way to the grating.
Here's a sample pic (halogen lamp):
(http://kronometric.org/phot/lighting/spectroscopy/spectrumHALcrop.png)
AF is out of the question.
Twiddling the MF ring produces no obvious "in-focus" point, so I wonder what the correct setting would be in theory?
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Twiddling the MF ring produces no obvious "in-focus" point, so I wonder what the correct setting would be in theory?
Hi Ted,
When you first sample outdoor sunlight, could you try and look for Fraunhofer lines (http://en.wikipedia.org/wiki/Fraunhofer_lines)? If the size of the spectrum is large enough on the sensor, and the resolution of the grating high enough, you may find a focus position that could then be used for other light-sources.
Alternatively you could try and temporarily use a filter material in front of the slit with a spiky spectral transmission/absorption.
Cheers,
Bart
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Hi Ted,
When you first sample outdoor sunlight, could you try and look for Fraunhofer lines (http://en.wikipedia.org/wiki/Fraunhofer_lines)? If the size of the spectrum is large enough on the sensor, and the resolution of the grating high enough, you may find a focus position that could then be used for other light-sources.
Alternatively you could try and temporarily use a filter material in front of the slit with a spiky spectral transmission/absorption.
Cheers,
Bart
Thanks Bart,
Yes, I could try those methods of practical adjustment - indeed, I made this image a year or two ago:
(http://kronometric.org/phot/lighting/spectroscopy/CFLsd9vsSpectraFlip.jpg)
However my latest shots are of a LED flood which has quite a smooth spectrum and that is what got me thinking about the theoretical aspect.
Since posting, I did find this:
http://www.astrosurf.com/buil/us/stage/calcul/design_us.htm
It doesn't say explicitly but the ray diagrams seem to imply that infinite focus is correct.
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Just an update:
While playing with the focus on my home-made spectroscope, I noticed fungus growing on the collimator lens inside the box. So I took the lens out. This had a positive effect with reference to focusing. Although the spectral image was reduced in size, it now had a sharp edge at the top and bottom which was easily focused. Even the camera beeped (Sigma in-focus beep works while manual focusing with the button half-way) appropriately.
Thanks for looking,