Solar spectrum help required, please

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Carbon60
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Solar spectrum help required, please

Post by Carbon60 »

Hi Spectrum experts....

At the start of the year I built a solar spectrometer and recently revisited it with an upgrade to try to optimise its resolution (still work in progress and needs a better slit).

Today I managed to capture 'first light' after a few false starts. My problem (for the moment) is that I have no idea which part of the spectrum I've imaged. I've tried matching the absorption lines to reference spectra, but so far I have failed to get a match.
Doublet Spectrum2.jpg
Doublet Spectrum2.jpg (42.22 KiB) Viewed 2632 times
It's possible that this is 'upside down' depending on whether the blue end is on the left or the right.

The obvious candidates are calcium and sodium, but when I use 'RSPEC' to calibrate using these absorption bands, the remaining lines don't seem to match anything. I'd be grateful if someone could point me in the right direction on this.

Cheers

Stu.


H-alpha, WL and Ca II K imaging kit for various image scales.
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Re: Solar spectrum help required, please

Post by danpro1 »

One trick that I use to calibrate mine is to "carefully" shine a laser pointer on the slit as a reference: the red ones are around 650 nm and the green ones are about 532 nm.


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Re: Solar spectrum help required, please

Post by p_zetner »

Hi Stu.

I think it’s kind of difficult to say without knowing roughly where you are within the overall spectrum (blue, green, etc.) and roughly what your instrumental resolution is. I occasionally use an interference filter to help find a spectral region of interest when I image with my spectroheliograph.

There’s an online interactive solar spectrum available which might help you identify the lines in your image:

http://fermi.jhuapl.edu/liege/s00_0000.html

For more accurate, detailed information I use the BASS2000 site:

http://bass2000.obspm.fr/solar_spect.php

Hope this is helpful.
Peter

(Btw. You might be seeing the Ca H and K lines. Comparison of your spectrum with the Liege “level 2” spectrum near 3900 Angstroms suggests this but I am guessing.)


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Re: Solar spectrum help required, please

Post by Carbon60 »

p_zetner wrote: Tue Jun 02, 2020 2:12 am

Hope this is helpful.
Peter
Hi Peter,

Nice links. Very helpful indeed.

Thanks.

Stu.


H-alpha, WL and Ca II K imaging kit for various image scales.
Fluxgate Magnetometers (1s and 150s Cadence).
Radio meteor detector.
More images at http://www.flickr.com/photos/solarcarbon60/
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Re: Solar spectrum help required, please

Post by Merlin66 »

Stu,
What's the design of your spectrograph?
I use BASS Project (https://groups.io/g/BassSpectro) for all my spectral processing....


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Re: Solar spectrum help required, please

Post by Carbon60 »

Thanks for the link, Ken. I'll look into that.

As for the design, it's a pretty basic 'classical' design with a slit, collimating lens, refocusing lens and camera, with the addition of a mirror to fold the light path back on itself as per the photo below.
Spectrometer_Annotated2.jpg
Spectrometer_Annotated2.jpg (316.4 KiB) Viewed 2551 times
The focal ratio of the collimator doesn't match the scope (which is 80mm/~f10). Is that critical for spectroscopy?

The slit is a simple home made one using two razor blades. At a guess the slit width must be at least a couple of hundred microns. My intention is to purchase a slit from Thorlabs. Presumably for sharper lines I'm better with a narrower slit width.

The grating has 1200 l/mm and is used in 'second order' format.

Stu.


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Fluxgate Magnetometers (1s and 150s Cadence).
Radio meteor detector.
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Re: Solar spectrum help required, please

Post by Merlin66 »

Stu,
The collimator must match the incoming beam. This means your 50mm lens is working at f10.
This will probably suit your smaller 10mm grating...It’s also usual for the imaging lens to be the same or less than the collimator focal length.
Pencil sharpener blades with the bevel facing the collimator work better. Try to set the gap using a very bright lamp, as narrow and even as possible ( about 2 or three times the camera pixel size)
Use a fluoro lamp to calibrate the spectrograph and the adjustment of the grating angle.
It’s a good start.....but opportunity to improve.


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Re: Solar spectrum help required, please

Post by Carbon60 »

Thanks for the pointers, Ken.

I use a longer focal length imaging lens with the aim of increasing the resolution. I’m not looking to image the solar disk, just capture a reasonably high resolution spectrum. My logic is to use a very narrow slit and ‘zoom in’ on the grating to achieve this.

Cheers

Stu.


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Fluxgate Magnetometers (1s and 150s Cadence).
Radio meteor detector.
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Re: Solar spectrum help required, please

Post by Merlin66 »

Stu,
Understood.
The SimSpec Spreadsheet will help confirm your design and the probable resolution etc. Just enter your data....

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Re: Solar spectrum help required, please

Post by Merlin66 »

Stu,
Making some guesses about slit gap (20 micron) and spacing between the grating and the imaging lens (100mm), I've put your parameters into the Simspec spreadsheet.
SimSpec V4.3_Stu_solarchat.xls
(78.5 KiB) Downloaded 102 times


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Re: Solar spectrum help required, please

Post by Carbon60 »

Excellent, Ken. This is really helpful.

In summary then....
Theoretical performance:
Resolving power R 15409
Spectral resolution 0.36

It's interesting to see the effect of changing the various parameters on these performance figures.
I'll see what I can squeeze out of my current setup and then maybe further refine using the spreadsheet.

Cheers

Regards

Stuart


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Re: Solar spectrum help required, please

Post by Merlin66 »

Stu,
Using a 10mm grating in 2nd order is marginal. Most gratings used are 25mm or 30mm sq.
I’d recommend you try to calibrate a reference lamp to verify resolution and tune the grating rotation.


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Re: Solar spectrum help required, please

Post by p_zetner »

Hi Stu.

I thought I would have a go at analyzing your spectrometer based on the description and photograph. I prefer to calculate things by hand rather than spreadsheet (a little old school) and have based the calculations on some notes (attached) that I generated some time ago for my original spectroheliograph.

My starting points:
f_collimator = 50mm (= f in the notes).
f_camera = 120mm (= f’ in the notes).
Grating constant = 1200/mm (-> d = 1/1200 = 0.8 microns, in the notes).
Lambda = 500nm = 0.5 microns (for the sake of calculation).
From the notes: Angle GAMMA = 90 deg (-> gamma = 45 deg)

Surprisingly (to me) my calculations show that using the grating in either either 1st or 2nd order (at lambda=500nm) gives the same “reciprocal linear dispersion” (see notes) of 15 nm / mm in the focal plane of the camera lens.

The bandpass (and resolving power) is then determined by the slit width (w) and the above value of the reciprocal linear dispersion. For slit widths of 100 microns and 10 microns I get:

Bandpass = 1.5nm and R = 333 (@ 500nm) for w = 100 microns.
Bandpass = 0.15nm and R = 3333 (@ 500nm) for w = 10 microns.

If we assume your slit width is 100 microns (honestly, I think a value of this order is pretty likely for your razor blade arrangement) then rough analysis of your spectrum image gives a separation of about 3nm for the two absorption lines. (Assuming you are showing the entire width of 5.8mm for the DMK 1/3” sensor.) Interestingly, the Ca K and H lines are separated by roughly 3nm so these may definitely be the lines you see.

For your question about the collimator lens focal ratio, I would make the following comments. The 50mm lens and the 10mm grating width establish a focal ratio of about f/5 for the spectrometer input. To best couple light into the spectrometer requires a telescope of matching focal ratio. On the other hand, using a “slower” telescope like your f/10 does no harm...you’re just not illuminating the entire grating. Conceivably this limits the ultimate achievable resolving power of the instrument (by illuminating fewer grating grooves) but I don’t think you are near this limit (“Nm” in the notes). Using a faster telescope (than f/5) is problematic because you would be spraying light beyond the grating edge and contributing to stray light problems.

One thing you might take away from the notes is the usefulness of a long collimator focal length. This (along with narrower slit width) is helpful in achieving high spectral resolution.

Also, Ken's advice about investing in some type of spectral lamp for testing and calibration is almost critical.

I do have one question: What did you construct the housing from?

Cheers.
Peter
spectrometer resolution -pzetner notes.pdf
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Re: Solar spectrum help required, please

Post by Phil Rousselle »

Hi
are you able to post a known part of the spectrum taken with the same spectro, at the same scale (Ha, sodium doublet, Magnesium triplet, etc) ?
Phil


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Re: Solar spectrum help required, please

Post by Carbon60 »

Many thanks for your feedback and assistance, everyone. Still a lot for me to learn here.

Peter, in answer to your question, the box is made out of ~6mm plywood with a thicker (~12mm) particle board lining on the inside base to allow me to screw things down. There are some reinforcing wooded square and quadrants along the edges and corners.

Stu.


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Radio meteor detector.
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Re: Solar spectrum help required, please

Post by huwiler »

Hi

This is the sodium doublet...

Martin

Image

link to solar spectrum r=30'000:

http://o.garde.free.fr/astro/spectro/SunSpectraESP.jpg


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