Spectroheliograph imaging experiments at 656.3 and 854.2 nm

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
thesmiths
Almost There...
Almost There...
Posts: 1053
Joined: Mon Jun 04, 2012 8:13 pm
Location: London, England
Has thanked: 42 times
Been thanked: 1485 times

Spectroheliograph imaging experiments at 656.3 and 854.2 nm

Post by thesmiths »

After posting some high-resolution H-alpha SHG results (see viewtopic.php?t=37081, viewtopic.php?t=37088, viewtopic.php?t=37061), we were "challenged" by Valery to investigate imaging at 854.2 nm. This is the middle line of what is called the "Calcium triplet". To quote Wikipedia:
The infrared Ca II triplet, commonly known as the calcium triplet, is a triplet of three ionised calcium spectral lines at the wavelengths of 8498 Å, 8542 Å and 8662 Å. The triplet has a strong emission and is most prominently observed in the absorption of spectral type G, K and M stars.
As usual, Christian Buil has something to say on this topic http://www.astrosurf.com/buil/lhires_ir/obs_us.htm. He presents a nice mosaic of spectra taken of various classes of stars. (The Sun, of course, is an example of a G-type main-sequence star.)

Sequence of stellar spectra near the Calcium triplet. The temperature of the star drops from top to bottom.
Sequence of stellar spectra near the Calcium triplet. The temperature of the star drops from top to bottom.
classif.png (45.24 KiB) Viewed 916 times

I used this mosaic in order to identify the correct line. Christian Buil also recommended placing a red filter in front of the camera: this filter, known as an "order filter", is to remove the blue part of the 2nd-order spectrum which would ordinarily overlap the 1st-order infrared spectrum.

We used our 80mm f6 APO refractor setup viewtopic.php?t=36961, which is very flexible and allows quickly changing the diffraction grating. For a first attempt at 854.2 nm, we used a 1200 l/mm grating, which gives lower dispersion but more range. We were able to quickly identify the Calcium triplet region:

Calcium triplet using our SHG, 1200 l/mm ruled grating, ZWO 178MM camera, Wratten 25A red "order filter". Exposure of 1.6ms with a gain of zero, 1x1 binning.
Calcium triplet using our SHG, 1200 l/mm ruled grating, ZWO 178MM camera, Wratten 25A red "order filter". Exposure of 1.6ms with a gain of zero, 1x1 binning.
082458_calcium_triplet_small.jpg (33.05 KiB) Viewed 916 times

The image is extracted from a real video file from our SHG, oriented to match the mosaic above. The middle line is a little narrower than the H-alpha line, and while some structure is visible in the line, there is not nearly as much as in the H-alpha, Ca-K/H or even the H-beta line. Here is an SHG image reconstructed from a scan of the middle Calcium triplet line:

SHG image at 854.2 nm. 1200 l/mm grating, 9 micron wide slit. ZWO 178MM camera, 1x1 binning. Stack of 9 frames.
SHG image at 854.2 nm. 1200 l/mm grating, 9 micron wide slit. ZWO 178MM camera, 1x1 binning. Stack of 9 frames.
150722_Ca_480_1200_9-18.jpg (411.36 KiB) Viewed 916 times

For comparison, below is an image of the H-alpha line taken with the same setup, just before the one above (morning of July 15).

SHG image at 656.3 nm (H-alpha). 1200 l/mm grating used in 1st order. Size reduced by 50%. Stack of 14 frames.
SHG image at 656.3 nm (H-alpha). 1200 l/mm grating used in 1st order. Size reduced by 50%. Stack of 14 frames.
150722_Ha_480_1200_14-18.jpg (266.72 KiB) Viewed 916 times

We also took the opportunity to image the H-alpha line in 2nd order, equivalent to the dispersion of the 2400 l/mm grating that we normally use (the light throughput is much lower, however, so the image quality is slightly reduced).

SHG image at 656.3 nm (H-alpha). 1200 l/mm grating used in 2nd order. Size reduced by 50%. Stack of 8 frames.
SHG image at 656.3 nm (H-alpha). 1200 l/mm grating used in 2nd order. Size reduced by 50%. Stack of 8 frames.
150722_Ha_480_2400_8-26.jpg (290.05 KiB) Viewed 916 times

We can also compare the 854.2 nm image to a CaK image taken with a 1200 l/mm grating download/file.php?id=68989 and a CaH image taken with a 2400 l/mm grating download/file.php?id=69613.

I should mention that Peter Zetner has also done work on the Calcium triplet viewtopic.php?f=8&t=29064. He imaged the 849.8 nm line rather than the 854.2 nm line.

My conclusion: while an interesting academic exercise, I do not find the 854.2 nm line as interesting as some others. Qualitatively, the image is intermediate between those from the H-alpha and the Ca-H/K lines and shows features in common with them. It is not particularly difficult to image the 854.2 nm line, but the final result is not as exciting as the more "popular" lines. I will give it another attempt with a higher dispersion grating (1800 l/mm), but I do not expect the image to be significantly different than the result presented here.


User avatar
Montana
Librarian
Librarian
Posts: 34526
Joined: Mon Oct 17, 2011 5:25 pm
Location: Cheshire, UK
Has thanked: 17521 times
Been thanked: 8763 times

Re: Spectroheliograph imaging experiments at 656.3 and 854.2 nm

Post by Montana »

It looks like a cross between CaK and Halpha, very interesting though :bow :bow :bow

Can I book my Helium D3?

Alexandra


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: Spectroheliograph imaging experiments at 656.3 and 854.2 nm

Post by p_zetner »

Qualitatively, the image is intermediate between those from the H-alpha and the Ca-H/K lines and shows features in common with them. It is not particularly difficult to image the 854.2 nm line, but the final result is not as exciting as the more "popular" lines. I will give it another attempt with a higher dispersion grating (1800 l/mm), but I do not expect the image to be significantly different than the result presented here.
My findings were that, for an image scale corresponding to full disk acquisition, there was very little difference between the Ca ir line at 849.8nm (presumably quite similar to the 854.2 nm line), the Ca H line and the Ca K line. Apparently, though, according to results from very high (spatial) resolution imaging done by solar observatories, there is a noticeable difference between the Ca ir and uv images. In fact, as Alexandra has mentioned, there is a real H alpha / Ca K "hybrid" nature to these images at very high spatial resolution.

The reduced contrast of the chromospheric network that you see in your result is probably due to the low dispersion of the 1200 l/mm grating. I expect that this should be improved with the 1800 l/mm grating.


thesmiths
Almost There...
Almost There...
Posts: 1053
Joined: Mon Jun 04, 2012 8:13 pm
Location: London, England
Has thanked: 42 times
Been thanked: 1485 times

Re: Spectroheliograph imaging experiments at 656.3 and 854.2 nm

Post by thesmiths »

Montana wrote: Fri Jul 15, 2022 6:09 pm Can I book my Helium D3?
Dear Alexandra, I did make an attempt earlier in the week and learned a bit how to do it. I will get back to it soon!


thesmiths
Almost There...
Almost There...
Posts: 1053
Joined: Mon Jun 04, 2012 8:13 pm
Location: London, England
Has thanked: 42 times
Been thanked: 1485 times

Re: Spectroheliograph imaging experiments at 656.3 and 854.2 nm

Post by thesmiths »

p_zetner wrote: Fri Jul 15, 2022 8:59 pm My findings were that, for an image scale corresponding to full disk acquisition, there was very little difference between the Ca IR line at 849.8 nm (presumably quite similar to the 854.2 nm line), the Ca H line and the Ca K line. Apparently, according to results from very high (spatial) resolution imaging done by solar observatories, there is a noticeable difference between the Ca IR and UV images.
I think you need to be looking at the image scale of a single sunspot to see an interesting difference. I don't think the kind of SHG instruments we are using have enough spatial resolution to observe a distinction.


User avatar
solarchat
Site Admin
Site Admin
Posts: 4355
Joined: Sat Feb 01, 2014 3:10 pm
Location: Atlanta, GA USA
Has thanked: 83 times
Been thanked: 1304 times
Contact:

Re: Spectroheliograph imaging experiments at 656.3 and 854.2 nm

Post by solarchat »

I find it absolutely fascinating


Stephen W. Ramsden
Atlanta, GA USA
Founder/Director Charlie Bates Solar Astronomy Project
http://www.solarastronomy.org
User avatar
Valery
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 4059
Joined: Sun Nov 18, 2012 3:13 pm
Has thanked: 156 times
Been thanked: 893 times

Re: Spectroheliograph imaging experiments at 656.3 and 854.2 nm

Post by Valery »

thesmiths wrote: Fri Jul 15, 2022 10:24 pm
I think you need to be looking at the image scale of a single sunspot to see an interesting difference. I don't think the kind of SHG instruments we are using have enough spatial resolution to observe a distinction.
Douglas,

Thanks for your efforts and sharing your results. I believe that your limited success is due to several reasons:

1. Too wide FWHM and so, too high noise from photosphere = too low contrast of chromospheric features.
2. Too small spatial resolution = we can't see chromospheric features (in AR with sunspots) where they should be seen more easily than in "desert areas".

Gianna Cauzzi - the leading scientist in this field - has told me, that for good (pro level) results we need to get 0.2A FWHM with well suppressed skirt of the transmission curve. But even FWHM of 0.25A - 0.4A will be OK inIf the case of supressed skirt of the transmission curve.

Is this possible to use a narrower slit and higher dispersion (2400 l/mm )? And may be not to image a FD, but obtain more scans of a given AR with a larger sunspot(s)?


Valery


"Solar H alpha activity is the most dynamic and compelling thing you can see in a telescope, so spend accordingly." (c) Bob Yoesle.

Largest full size 185 - 356mm Dielectric Energy Rejection Filters (D-ERF) by ARIES Instruments.
thesmiths
Almost There...
Almost There...
Posts: 1053
Joined: Mon Jun 04, 2012 8:13 pm
Location: London, England
Has thanked: 42 times
Been thanked: 1485 times

Re: Spectroheliograph imaging experiments at 656.3 and 854.2 nm

Post by thesmiths »

Valery wrote: Sun Jul 17, 2022 12:42 am Is this possible to use a narrower slit and higher dispersion (2400 l/mm )? And may be not to image a FD, but obtain more scans of a given AR with a larger sunspot(s)?
It is not possible to use a 2400 l/mm grating in the IR but 1800 l/mm will work. In theory, a narrower slit could be used but I do not have access to one (mine are 9 micron, some like Fulvio Mete, have access to 5 micron).

With SHG, the size of the scan does not influence the spatial resolution; it is primarily determined by the focal length, the slit width and the camera pixel size. We could increase the focal length to around 900mm, so a doubling from this experiment (480mm).

Therefore, in theory, I could double both the spectral and spatial resolution from current levels, but I am not sure that is sufficient to visualise what you are looking for.


User avatar
Valery
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 4059
Joined: Sun Nov 18, 2012 3:13 pm
Has thanked: 156 times
Been thanked: 893 times

Re: Spectroheliograph imaging experiments at 656.3 and 854.2 nm

Post by Valery »

thesmiths wrote: Sun Jul 17, 2022 1:05 am
Valery wrote: Sun Jul 17, 2022 12:42 am Is this possible to use a narrower slit and higher dispersion (2400 l/mm )? And may be not to image a FD, but obtain more scans of a given AR with a larger sunspot(s)?
It is not possible to use a 2400 l/mm grating in the IR but 1800 l/mm will work. In theory, a narrower slit could be used but I do not have access to one (mine are 9 micron, some like Fulvio Mete, have access to 5 micron).

With SHG, the size of the scan does not influence the spatial resolution; it is primarily determined by the focal length, the slit width and the camera pixel size. We could increase the focal length to around 900mm, so a doubling from this experiment (480mm).

Therefore, in theory, I could double both the spectral and spatial resolution from current levels, but I am not sure that is sufficient to visualise what you are looking for.
The idea of a smaller area for imaging is for increasing of the obtained scans within a given cadence. The more scans will be stacked, the better.

Valery


"Solar H alpha activity is the most dynamic and compelling thing you can see in a telescope, so spend accordingly." (c) Bob Yoesle.

Largest full size 185 - 356mm Dielectric Energy Rejection Filters (D-ERF) by ARIES Instruments.
User avatar
Valery
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 4059
Joined: Sun Nov 18, 2012 3:13 pm
Has thanked: 156 times
Been thanked: 893 times

Re: Spectroheliograph imaging experiments at 656.3 and 854.2 nm

Post by Valery »

Look here: https://www.arcetri.inaf.it/science/solare/IBIS/

See the gif movie how the sunspot changes its face with slit scans the line profile. If our slit will be wider, say, 0.4-0.5A, but centered at the line core, we will see all the chromospheric features seen in this movie.

The upper left image shows what can be seen in this line. IMO, this is significantly more appealing than H-a.


Valery


"Solar H alpha activity is the most dynamic and compelling thing you can see in a telescope, so spend accordingly." (c) Bob Yoesle.

Largest full size 185 - 356mm Dielectric Energy Rejection Filters (D-ERF) by ARIES Instruments.
Post Reply