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  #1  
Old 13-05-2017, 11:17 PM
luka
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Tunable spectroscopic filters

Hi,

I have been looking around but have found only links to spectrometers using gratings. I have an opportunity to obtain an acousto-optic tunable filter (AOTF) which, I was hoping, could be used to make an astronomical spectrometer.

I am not quite sure where to start (no experience in astronomical spectroscopy) so here are few details and questions.

The AOTF has a very small entrance aperture (2x5 mm square) so the light from a star would need to be focused on this. How do people focus light from a particular star using gratings? I could not find details how to pick a starlight from one particular star and not from whatever else is in the field of view of the telescope.

In the AOTFs the wavelength is scanned using a computer and I was planing to use a photodiode or something similar to detect the light coming out of the filter. The resolution would be around 2nm, depending on the wavelength.


Any thoughts/hints/ideas/advice would be greatly appreciated,

Luka
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  #2  
Old 15-05-2017, 08:21 AM
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bojan
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To isolate single star, you need a mask in focal plane, with appropriate aperture.
Then, you need a lens, to form a more-or-less parallel beam (not wider than your grating).
See here:
http://www.astrosurf.com/buil/us/sta.../design_us.htm
http://www.astrosurf.com/buil/us/spe2/hresol.htm
http://www.astrosurf.com/buil/us/spectro1.htm


Your resolution (not scale!) will be limited with number of grating lines engaged with the beam... apart from other variables (the quality of optics, the atmospheric visibility, etc.)

Last edited by bojan; 15-05-2017 at 08:33 AM.
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  #3  
Old 15-05-2017, 10:49 AM
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Merlin66 (Ken)
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Luka,
I'm certainly not familiar with the AOTF concept, but reading up:
http://www.olympusmicro.com/primer/t...aotfintro.html

It sounds very difficult to envisage how this could be set-up and arranged to provide a useful astronomical spectrum....

Unless you have some prior working knowledge I'd put it in the too hard basket.....
Have you had such a device working for you???
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  #4  
Old 15-05-2017, 12:07 PM
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It look like the (surface?) acoustic wave creates a pattern that could act as diffraction grating.

One argument against using this device in astro-spectroscopy would be variable resolution... dependable on variable lines/mm.

A positive thing would be the lack of mechanical moving parts of such spectrograph....
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  #5  
Old 16-05-2017, 02:17 AM
robin_astro
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You could perhaps build an imaging spectrometer something like the NMSU Acousto-optic Imaging Camera (NAIC) used for planetary work ? No need to isolate a point in the field. Just use it like a tuneable filter, taking the whole field and imaging the output as the wavelength is swept to build up a data cube.

AOTF-Based Spectral Imaging for Balloon-Borne Platforms
Journal of Astronomical Instrumentation, Vol. 3, No. 2 (2014)
http://www.worldscientific.com/doi/p...51171714400054

Sounds an ambitious project though !

Robin
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  #6  
Old 16-05-2017, 12:31 PM
luka
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Thank you all for your replies.

Bojan, thank you for the links. This made quite a few things clearer.

Ken, I have used AOTF in the past, hence the idea to try using it as a part of an astronomical spectrometer.

Robin, I never thought of actually recording a full image. That may be the next step if I ever get this working, see my ideas below.

Some physics:
In AOTF a single wavelength is separated from a broadband light source by applying an acoustic (vibrational) wave at radio frequencies (RF) to a crystal. By varying the frequency of the RF, the wavelength of the filtered light can be varied.

When an acoustic wave is passed through an optical medium, the crystal lattice is alternately compressed and relaxed, i.e. a refractive index wave is generated that behaves like a sinusoidal grating. However, unlike a classical diffraction grating, only one specific wavelength of light is diffracted as the diffraction takes place over an extended volume, not just at a surface or plane, and that the diffraction pattern is moving in real time. An incident light beam of the right wavelength passing through this grating diffracts into several orders, with first order having the highest efficiency. Its angular position is linearly proportional to the acoustic frequency.

In very simple words, a single wavelength is separated from the incoming light and passed out at a particular angle α (about 6 degrees for this particular AOTF). The remainder of the incoming light is passed straight through.


So, my idea was to mount a detector (a photodiode initially or even a camera as done in the paper Robin linked) at this angle α, with adequate separation from the AOTF to ensure the remainder of the light going straight through is separated and blocked.The selected wavelength can be scanned by a computer and the intensity detected by the photodiode recorded (or images stored).
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  #7  
Old 17-05-2017, 08:37 PM
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Hi Luka,

Even for stellar spectroscopy you will probably find it easier to use a camera sensor (a 2D array) rather than a single channel photodiode. Although single channel photometers are occasionally still used in astro-photometry,
http://www.optecinc.com/astronomy/catalog/ssp/
It is pretty niche these days and CCD imaging sensors now prevail for astronomical photometry generally. Using an imager makes it much easier to see what is going on with focus, positioning etc and allows you to subtract the sky background without having to make a second set of measurements.

Cheers
Robin
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  #8  
Old 17-05-2017, 08:50 PM
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There is a significant limitation with scanning spectrometers for astro work though compared with imaging the full spectrum. Because you have to expose each waveband in turn, total exposure times are much longer (~100x longer, even for a resolution of ~5nm) and any changes in atmospheric extinction during the scan will distort the spectrum.

Robin
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  #9  
Old 19-05-2017, 11:36 AM
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ZeroID (Brent)
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Just an interested readers comments but could the fact it is tuneable be useful in searching for particular wavelengths in a targets spectrum ?

( I read all sorts of things on here )
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  #10  
Old 24-05-2017, 11:19 AM
luka
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Robin, I think you hit the nail on the head when you mentioned speed of scanning. Very good point, thank you!!!

Brent, the non-tunable spectrometers using gratings also allow looking for a particular wavelength in the target spectrum. You just "image" the whole spectrum at once instead of scanning wavelength by wavelength.
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