Luminous Landscape Forum

Equipment & Techniques => Digital Cameras & Shooting Techniques => Topic started by: xpatUSA on November 05, 2013, 10:24:41 am

Title: What is the Correct Focusing Distance for Spectroscopy?
Post by: xpatUSA on November 05, 2013, 10:24:41 am
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?
Title: Re: What is the Correct Focusing Distance for Spectroscopy?
Post by: Bart_van_der_Wolf on November 05, 2013, 12:24:24 pm
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
Title: Re: What is the Correct Focusing Distance for Spectroscopy?
Post by: xpatUSA on November 05, 2013, 01:27:23 pm
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.

Title: Re: What is the Correct Focusing Distance for Spectroscopy?
Post by: xpatUSA on November 06, 2013, 11:21:33 am
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,