Tandem Grating Spectroheliograph

I LOVE finding out about different ways to appreciate the Sun and light in general. Use this forum to post your info or questions about various outside the mainstream ways to appreciate our life giving star!
Post Reply
User avatar
p_zetner
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1644
Joined: Sun Mar 18, 2012 4:59 pm
Location: Winnipeg, Manitoba, Canada
Has thanked: 166 times
Been thanked: 719 times

Tandem Grating Spectroheliograph

Post by p_zetner »

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.


User avatar
swisswalter
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 17948
Joined: Thu Feb 06, 2014 4:28 am
Location: Switzerland
Been thanked: 1 time
Contact:

Re: Tandem Grating Spectroheliograph

Post by swisswalter »

Hi Peter

very, very interesting stuf. Thanks for sharing all that information


Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

from 7 am - 7 pm http://www.nanosys.ch

from 7.01 pm - 6.59 am http://www.wastronomiko.com some times vice versa ;)
Derek Klepp
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 12900
Joined: Wed Oct 19, 2011 10:02 am
Been thanked: 171 times

Re: Tandem Grating Spectroheliograph

Post by Derek Klepp »

Would you be able to use this on Nova or do you need a brighter light source?


User avatar
marktownley
Librarian
Librarian
Posts: 42274
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Has thanked: 20442 times
Been thanked: 10246 times
Contact:

Re: Tandem Grating Spectroheliograph

Post by marktownley »

very impressive Peter!


Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!
User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Tandem Grating Spectroheliograph

Post by Merlin66 »

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.


"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
User avatar
p_zetner
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 1644
Joined: Sun Mar 18, 2012 4:59 pm
Location: Winnipeg, Manitoba, Canada
Has thanked: 166 times
Been thanked: 719 times

Re: Tandem Grating Spectroheliograph

Post by p_zetner »

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.


User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Tandem Grating Spectroheliograph

Post by Merlin66 »

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)


"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
Post Reply