The Sun in Helium D3, He I D3
This really feels like standing on the shoulders of giants. I got my Sol’Ex spectroheliograph, and started using it with INTI and JSol’Ex (links below). Long story short, while the Sol’Ex in the plastic feels like a toy, it bends, sags and kinks and twists and vibrates and all, I find it an excellent introduction, and beyond, to solargraphy and spectroscopy. The software? It blows my mind.
I’ve been photographing the Sun for more than two years now almost each and every day, in a bit of a hackish way: using all kinds of filters beyond the 540nm continuum – hydrogen alpha – near UV calcium triad. However, as I advanced, I discovered that this knowledge is simply not common in the community, and not just that, it’s also a bit hard to come by while googling around. Also, to better explain double stacking, off banding and similar stuff, articles like this one Altair Astro’s 3nm CaK Ultra filter: testing, comparison, review are maybe rare.
So I put together an interactive tool to play around with spectra, in a way amateur astronomers find it interesting. The Spectral Composer.
Solar disk in Altair Astro’s 3nm CaK Ultra filter, enhanced contrast
I got a new filter to pudding-test: Altair Astro’s 3nm Calcium K Ultra, stated to have 3nm FWHM and a very high transmission. Let’s see how it performs.
Before we jump in, let me also present some information I compiled along the way, and some theory and links fellow amateur astronomers may find useful.
Important note: this article is not sponsored by the vendor, and the vendor has not influenced the conclusions presented below in any way.
With a front ERF, Altair Astro’s Triband, 115mm, Tecnosky 102/1100 refractor, Lunt’s CaK B1800 and the ASI 533MM camera, I obtained the following:
CaK, 2.4Å
CaK 2.4Å, colorized
With a Tecnosky 102/1100 refractor, Herschel prism, ND 0.96, a filterwheel and the ASI 533MM camera, I obtained the following:
Antlia 3nm hydrogen alpha. The disk is clearly continuum, but the prominences were obvious even on the live view
Altair Astro’s G-band filter, 2nm fwhm at around 430nm cwl
Using a fairly new setup, as they were in the light train: Altair Astro Triband D-ERF 115mm, Tecnosky 102/1100 ED refractor, Lunt CaK B1800, ASI 533MM, along with my EQ3 with the home built controller, I captured the following frames.
full disk, linear light
full disk, paraboloid gamma curve
detail from the full disk
same, colorized
A while back, well, in 2017, a friend mentioned rather publicly that I should put a donate button onto my website. So I put one into the sidebar of the main page, and into the photo planner. However, the software (the photo planner for example) and content (astrophotos, webcomic etc) I publish, they all remain as they were: free to use (unless fine print blah-blah).
Using the good old 150/750 tube, with a ND3.8 film in front, I obtained the following frames.
Antlia CaK 3nm:
For reference, a Lunt CaK image, colorized and mono
And the infrared cwl 850nm, fwhm 20nm filter showing the active regions
ASI 178MM (cooled), IR850nm 20nm fwhm, Skywatcher 72/420 ED-APO, ND5, mountpusher, EQ3-mod — I believe this image shows the Calcium Triplet
For comparison, here’s the Baader CaK 5nm image (tovább…)
EQ3-mod, mountpusher, Lunt LS60T Ha B600, ASI 178MM (cooled)
The Sun at around 850 nm infrared*, false color
With my usual setup, the EQ3-mod, mountpusher, Baader Astrosolar ND3.8 Film, TS-Optics 76/342 TS76EDPH, ?filters?, ASI 178MM (cooled), I though what if I got creative. I put in the IRpass 850 filter from ZWO, and added an additional ND1.3 filter, because the ND5 front filter was a bit out of reach. Afterthought: I don’t know the transmittance curve of the ND filter, so maybe what I photographed is actually undefined-ish.
With the above in mind, the transmittance curve of the ZWO IR pass filter, along with the response curve of the camera, gives us a gentle hill around the magic 850 nanometer region, which is where the Calcium II triplet resides, and should draw the active regions in a similar way the CaK does. No matter what filters I used, these active regions show up in all of them, from UV, through CaK, deep blue, even in oxygen III, solar continuum, and 7nm Hα.
Digging out the details did take a good while, since the granulation (I guess it is that) on the surface of the Sun causes a sort of pixel noise to appear that doesn’t fare well with my emphasizing solutions. This „pixelnoise” is also present in my CaK images, and looks exactly the same on the DSO resources I checked at 160 and 170 nanometers.
ND1.3 and telescope glass aside, this is the wavelength-intensity area that should be in the photo. Telluric absorption is not that interesting in this region, and is uniform across the solar disk anyway.
