Hello Everyone.
In a quest to narrow the spectral bandpass of my SHG, I've begun to experiment with a tandem grating design. At the moment, I'm using an echelle grating in tandem with a conventional grating.
Here's a schematic of the spectrometer portion of the device:
Here are photographs of the side view and the top view:
The interaction of the two gratings makes tuning somewhat interesting. It will take some getting used to in the field.
Test measurements using a 316 g/mm echelle and 1800 g/mm blazed holographic grating gave me a reciprocal linear dispersion of 0.033 Angstrom/px and a bandpass of 0.10 Angstroms. Here's a sodium spectrum:
Test measurements using the 316 g/mm echelle and 2400 g/mm blazed holographic grating gave me a reciprocal linear dispersion of 0.0094 Angstrom/px and a bandpass (again) of 0.10 Angstroms. Here's the spectrum:
In the second case, there's somewhat of an imbalance between the very large dispersion and the bandpass. This might actually be useful for imaging to reduce the sensitivity of the result to the curvature of the spectral lines.
By the way, the test measurements were made using a 750mmfl collimator and 270mmfl camera lens with 25 micron slit. The Na light source was placed directly against the slit.
Of course, the tighter bandpass and increased dispersion come at a price - less intensity! It'll remain to try out some actual imaging to see whether the tandem design is worth the trouble.
Cheers.
Peter.
Tandem Grating Spectroheliograph
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Re: Tandem Grating Spectroheliograph
Hi Peter
very, very interesting stuf. Thanks for sharing all that information
very, very interesting stuf. Thanks for sharing all that information
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Re: Tandem Grating Spectroheliograph
Would you be able to use this on Nova or do you need a brighter light source?
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Re: Tandem Grating Spectroheliograph
very impressive Peter!
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Re: Tandem Grating Spectroheliograph
Peter,
Which spectral order are you using?
In the echelle arrangement the angles should be kept as low as possible (4-6deg).
A 150-300 l/mm is more usual for the cross-disperser.
I have a spreadsheet which allows calculation of the various orders, resolution and the possible/ probable wavelength coverage on your CCD.
email me for a copy.
Derek, it could certainly be used for the nova and any other stars - just needs a fibre optic connection and a guider system.
Which spectral order are you using?
In the echelle arrangement the angles should be kept as low as possible (4-6deg).
A 150-300 l/mm is more usual for the cross-disperser.
I have a spreadsheet which allows calculation of the various orders, resolution and the possible/ probable wavelength coverage on your CCD.
email me for a copy.
Derek, it could certainly be used for the nova and any other stars - just needs a fibre optic connection and a guider system.
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http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
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Re: Tandem Grating Spectroheliograph
Hi Derek.
At some point, I'll get serious about some stellar spectroscopy - including novae! The tandem arrangement I'm using is probably above and beyond what I intend for this type of work. I am completing a lighter, simpler spectrometer to attach to my C8. I plan to use the echelle in a slitless configuration.
Hi Ken.
Thanks for the post. I am keeping the echelle angle quite low, so operating near its highest order. I chose a 316 l/mm grating to give me the largest free spectral range (about 50nm in max order by my calc) but the dispersion here may be almost too large in the tandem arrangement! By the way, there is no cross dispersion in my layout. The echelle is being used as a pre-disperser. I intend to do some experiments with a 79 l/mm echelle shortly.
Update on the tandem spectrometer.
I'm becoming more familiar with the tuning of the device and am presenting some latest results with the 316 l/mm echelle in tandem with a 1800 l/mm holographic blazed grating. I'm pretty happy about achieving a bandpass of 0.036 A (yes, that's right: 36 milli-Angstroms, 3.6 picometers!) with a very high dispersion. The light throughput is reasonable with a 25 micron slit, 750mmfl collimator and 270mmfl camera lens. I could easily get away with 2x or 3x the present slit width to keep the bandpass near 0.100 A and boost the intensity considerably.
Here are some test results with a sodium lamp.
The 1st spectrum contains both Na D lines to establish the wavelength (/px) calibration and the dispersion. I couldn't get both lines sharply focused together, so I used a compromise focus here.
A spectrum of the Na D2 line, by itself, properly focused. With the calibration from the previous spectrum, I can determine the bandpass (line fwhm) from this.
An image of the sodium lamp (photographed at left with DSLR) taken through the spectrometer. A 200mmfl telephoto lens focused the lamp image on the slit and then the slit was removed. Only the D2 image is in good focus giving reasonably good quality. The D1 image is the blob on the right! I believe the distortion of the lamp image is due largely to the anamorphic magnification factor of the gratings. A real spectroheliograph image wouldn't suffer this particular problem (but other distortions will appear with construction of the shg image).
Cheers.
Peter.
At some point, I'll get serious about some stellar spectroscopy - including novae! The tandem arrangement I'm using is probably above and beyond what I intend for this type of work. I am completing a lighter, simpler spectrometer to attach to my C8. I plan to use the echelle in a slitless configuration.
Hi Ken.
Thanks for the post. I am keeping the echelle angle quite low, so operating near its highest order. I chose a 316 l/mm grating to give me the largest free spectral range (about 50nm in max order by my calc) but the dispersion here may be almost too large in the tandem arrangement! By the way, there is no cross dispersion in my layout. The echelle is being used as a pre-disperser. I intend to do some experiments with a 79 l/mm echelle shortly.
Update on the tandem spectrometer.
I'm becoming more familiar with the tuning of the device and am presenting some latest results with the 316 l/mm echelle in tandem with a 1800 l/mm holographic blazed grating. I'm pretty happy about achieving a bandpass of 0.036 A (yes, that's right: 36 milli-Angstroms, 3.6 picometers!) with a very high dispersion. The light throughput is reasonable with a 25 micron slit, 750mmfl collimator and 270mmfl camera lens. I could easily get away with 2x or 3x the present slit width to keep the bandpass near 0.100 A and boost the intensity considerably.
Here are some test results with a sodium lamp.
The 1st spectrum contains both Na D lines to establish the wavelength (/px) calibration and the dispersion. I couldn't get both lines sharply focused together, so I used a compromise focus here.
A spectrum of the Na D2 line, by itself, properly focused. With the calibration from the previous spectrum, I can determine the bandpass (line fwhm) from this.
An image of the sodium lamp (photographed at left with DSLR) taken through the spectrometer. A 200mmfl telephoto lens focused the lamp image on the slit and then the slit was removed. Only the D2 image is in good focus giving reasonably good quality. The D1 image is the blob on the right! I believe the distortion of the lamp image is due largely to the anamorphic magnification factor of the gratings. A real spectroheliograph image wouldn't suffer this particular problem (but other distortions will appear with construction of the shg image).
Cheers.
Peter.
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Re: Tandem Grating Spectroheliograph
Peter,
An interesting concept and arrangement. I had automatically assumed it was a traditional echelle layout.
Have you tried with a Neon lamp - closer to the Ha wavelengths (-for tuning trials)
An interesting concept and arrangement. I had automatically assumed it was a traditional echelle layout.
Have you tried with a Neon lamp - closer to the Ha wavelengths (-for tuning trials)
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
https://groups.io/g/astronomicalspectroscopy
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer