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Author Topic: What is the Correct Focusing Distance for Spectroscopy?  (Read 3739 times)

xpatUSA

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What is the Correct Focusing Distance for Spectroscopy?
« on: November 05, 2013, 10:24:41 AM »

I made a spectroscope some time back, just a toy really:



At the other end, a cheap 500 l/mm diffraction grating which splits the light from the slit into a spectrum.



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):



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?
« Last Edit: November 05, 2013, 10:54:28 AM by xpatUSA »
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best regards,

Ted

BartvanderWolf

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Re: What is the Correct Focusing Distance for Spectroscopy?
« Reply #1 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? 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
« Last Edit: November 05, 2013, 12:26:17 PM by BartvanderWolf »
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== If you do what you did, you'll get what you got. ==

xpatUSA

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Re: What is the Correct Focusing Distance for Spectroscopy?
« Reply #2 on: November 05, 2013, 01:27:23 PM »

Hi Ted,

When you first sample outdoor sunlight, could you try and look for 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:



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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best regards,

Ted

xpatUSA

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Re: What is the Correct Focusing Distance for Spectroscopy?
« Reply #3 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,
« Last Edit: November 06, 2013, 11:23:06 AM by xpatUSA »
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best regards,

Ted
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