Impact of Ca K filter band pass on the visibility of filaments / comparison with SHG

[christian viladrich]

Hi,

Thanks to some help from Doug, here is a test illustrating the effect of bandwidth on the visibility of filaments in Ca K :

http://astrosurf.com/viladrich/astro/so ... G-Doug.jpg

We have successively :
- Alluxa 0.37 nm Ca K filter (3-cavity filter),
- Alluxa Ca K filter 0.14 nm (single-cavity),
- two Alluxa 0.14 nm Ca K filters, therefore a double stack assembly,
- an image taken by Doug Smith with one of his spectroheliographs.

The only processing applied to images is the adjustment of vizualization thresholds. So there is no unsharp mask/wavelet/deconvolution/gamma or anything contrast reinforcement or limb correction. I found this to be the best solution for showing filaments, as well as displaying at a reduced size.

- 0.37 nm filter : once we have located the position of the filaments on the SHG image, we can locate the filament with the most contrast (to the east) on the image made with the 0.37 nm filter . But it's really very weak.

- 0.14 nm filter : switching to the 0.14 nm filter does not bring a huge gain in contrast on this type of image (but the gain is clearly more visible as soon as the image is enlarged, in particular on high resolution images which do not have nothing to do with the images obtained at 0.37 nm). We can detect the south polar filament at 6 o'clock.

- 0.14 + 0.14 nm combination : going double stack brings a significant gain in contrast. In fact, I had to burn the bright areas a little bit so as not to have a disc that was too dark. The two filaments to the east and south are "well" visible. We can guess others if we first spot them in Doug's SHG image.

- SHG : the SHG provides a very significant gain in contrast. On the full resolution image (which I will post later), it is difficult to know whether the difference in rendering of fine details results from the difference in spatial resolution, or from the difference in spectral resolution. Doug told me that the version of SHG used here was not the one providing the highest resolution.

In conclusion, I would say that an SHG is maybe equivalent to a 0.14 nm triple-stack (or more) ;-)

I will post the full resolution images a later.

PS 1: thanks to Doug for providing the SGH image ! Do not hesitate to include additional information about the SHG and observing condition.

PS 2: the tilt of the 0.37 nm and 0.14 nm filters was set with a spectroscope. They were centered right on the centre of Ca K. Seeing conditions were fine. Sky was transparent.

PS 3: I measured the FWHM of the 0.14 nm double-stack combination. I will indicate the result later, once I have analyzed the measurement.

PS 4 : Ca K filaments are not all the same. Some are very bright / dark / very contrasty. Others are very faint.

PS 5 : for some reason I don't understand, it is easier to see the filaments on the computer screen during acquisition than on the processed images.

Clear skies !

[thesmiths]

That is a very interesting, and I think quite unique, comparison of using narrowband filters and diffraction grating techniques. Just to add some details about the SHG data: I was asked by Christian V. on the morning of Sept 7 if I could do an image in CaK that day (to coincide with his measurements). I used the instrument which I had set up at the time, my 80ED with 1800 l/mm grating -- i.e. neither the highest spatial resolution nor best spectral resolution. In addition, since I missed my early morning window of observation, I had to wait until the Sun was on the other side of my house chimney, so it was around 2:30pm local time, when seeing was not ideal.

Christian mentions that "I would say that an SHG is maybe equivalent to a 0.14 nm triple-stack (or more)". From geometric calculations, the grating I used should give a spectral resolution of around 0.30 angstrom, so quite a bit smaller than the 1.4 angstrom he references. The sampling factor at the sensor is actually quite high (around 3.3) and the grating I used was quite wide (50mm), so I think it is likely that the true spectral resolution is not much worse than the geometric value.

However, when we want to resolve fine structures like filaments, what we need is both an appropriately high spectral contrast (or no feature will be visible) and a sufficiently high spatial resolution (or there will be just an unresolved blur). This particular SHG image has a little worse spectral resolution than can normally be achieved (typically around 0.20 angstrom with a 2400 l/mm grating) but the poorer telescope optics and worse seeing conditions likely diminished the filament contrast from what could ideally be achieved.

I mention this because, like with planetary observation (or microscopy for that matter), the issues of spectral contrast and spatial resolution are intertwined with regard to what we visually call "resolution".

All that being said, much praise must go to Christian for organising a "joint observation" for instruments separated by many hundreds of kilometres (England to France).

Edit: I've imbedded Christian's mosaic into this post since apparently he's not able to.

7sept2023-CaK-compar-037-010SS-010DS-SHG-Doug.jpg (533.21 KiB) Viewed 4333 times

[christian viladrich]

Very good points Doug !

Indeed :
- the characteristic which determines the contrast of a filter (for the observation of the chromosphere), it is not the FWHM, but the bandwidth at 10% of the transmission peak,
- if we have this in mind, then we can see that the 10% bandwidth of the double stack combination of 0.14 nm is equal to the 10% bandwidth of a 0.09 nm filter (i.e. 0.9A),
- we are therefore still at a factor of 3 to 9x away from the spectral resolution of the SHG (between 0.1 to 0.3 A for a good, well-adjusted SGH?).

So my initial thought that a SGH would be approximately equivalent to a 0.14 nm triple stack filter combination is therefore very optimistic ;-)

To dig a little deeper, we would need to know what the profile of the transmission curve of an SHG is (Lorentz or Voigt curve?). For this, it would be necessary to have the photometric profile of the thinnest possible line around Ca K (in other words a line finer than the resolving power of the SHG).

PS : just for fun, the CHROMIS Ca K double-stack filter at the 1 m Swedish Solar Telescope has FWHM of 0.1 A. In line with amateurs' SHG ...

[allhoest]

Great work, very interesting.
I wonder the contrast increase by double stacking both 0.1nm.
It looks also to be a quite difficult test / observation.
I'm looking forward your further analysis / checks / high resolution pics.

CS
Alex

[Montana]

These are fantastic!! the only way to see the filaments is to have a more narrow band filter, similar to Halpha, the more narrow you go, the more the filaments stand out.

Is there any cheapish filter that can be added to a Lunt CaK wedge to narrow the band further from the 2.4A?

Alexandra

[marktownley]

Is there any cheapish filter that can be added to a Lunt CaK wedge to narrow the band further from the 2.4A?

The cheapest (and rarest) is probably the filter from a CaK PST, after that, another Lunt filter.

[DeepSolar64]

Alexandra,
I remember you using a Baader K-line in addition to the Lunt wedge. It's probably not narrowband enough.

James

[marktownley]

It's probably not narrowband enough.

Nowhere near!

[marktownley]

I'm looking forward to reading your analysis of the double stack 1.4nm

My results fit within what you have seen.

I am currently using 1 angstrom (probably 1.4 angstrom real world) Chroma Technology CaK filter double stacked with a 2.2a CaK PST filter which gives a result somewhere between your single 1.4a Alluxa and the double stack 1.4a Alluxa - here's saturdays disk: You can see the shadows of the filaments when you know where you are looking for them.

CaK FD 60mm f10 zwo183mm by Mark Townley, on Flickr

I'm about to take imminent delivery of a 1 angstrom (probably 1.4a real world) Alluxa, which, I intend to double stack with the Chroma Technology. The design of these filters is quite different, so I will be interested to see what the result is.

I might try triple stacking with a CaK PST filter, but, using the Chroma Technology and 2 CaK PST filters to triple stack I felt the gain very marginal compared to the loss in transmission. There will be a synergy between some combination, just need some sun to find it!

[christian viladrich]

I'm looking forward to reading your analysis of the double stack 1.4nm

My results fit within what you have seen.

I am currently using 1 angstrom (probably 1.4 angstrom real world) Chroma Technology CaK filter double stacked with a 2.2a CaK PST filter which gives a result somewhere between your single 1.4a Alluxa and the double stack 1.4a Alluxa - here's saturdays disk: You can see the shadows of the filaments when you know where you are looking for them.

CaK FD 60mm f10 zwo183mm by Mark Townley, on Flickr

I'm about to take imminent delivery of a 1 angstrom (probably 1.4a real world) Alluxa, which, I intend to double stack with the Chroma Technology. The design of these filters is quite different, so I will be interested to see what the result is.

I might try triple stacking with a CaK PST filter, but, using the Chroma Technology and 2 CaK PST filters to triple stack I felt the gain very marginal compared to the loss in transmission. There will be a synergy between some combination, just need some sun to find it!

Great shot Mark ! You can see for sure some filaments on your image. Going triple stack (Alluxa + Chroma + Ca K PST) could be very interesting. It is definetely worth trying.

It would be great if these 1 A Ca K filters were more readily available. Still, I understand that Lunt (or others) sell much less Ca K filters than Ha filters.

And we can be gratefull that some one decided to take the risk of having these 1 A filters made :-)

[LeoD]

Very interesting comparison, thanks! Hope to be continued.
regards
Leo

[DeepSolar64]

I bet the 1A CaK filters are quite pricey.

[marktownley]

I bet the 1A CaK filters are quite pricey.

Not hugely different to a Lunt CaK filter, but you need a suitable blocker to go with it. Unfortunately they're probably slightly more rare than CaK PST filters...

[Rob63]

Great work guys, very interesting thread.

[Brian3114611]

Hello everyone, never posted anything here before, it seems like even Christian and Valery way underestimates the spectral resolution of SHG, here I would like to share a comparison of Ca II K at spectral resolution of 0.12A(SST CHROMIS), 1A, 3A, and 10A.
Top row is quit-Sun area, middle row is region with plages and pores, and bottom row for sunspot with umbra and penumbra.
From Figure 5., M. Murabito et al 2023 ApJ 947 18, Investigating the Effect of Solar Ambient and Data Characteristics on Ca ii K Observations and Line Profile Measurements

Ca ii K comparison 0.12A 1A 3A 10A by Brian Liu, 於 Flickr

Hope this will be useful, possibly someone already shared this before.

Regards,

Brian

[Brian3114611]

Also from the same paper mentioned above:

"In particular, we
convolved the CHROMIS observations, which were taken with
a 0.12 Å spectral bandwidth, with 1D Gaussian functions with
a FWHM of [0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 1.0, 1.8, 2.5, 3.0, 5.0,
10.0] Å. These spectral widths (hereafter referred to as
bandwidths and spectral degradations) match the bandwidths
of most of the existing series of full-disk solar observations at
the Ca II K" (from 2.2. Data Reduct)

"As expected, smearing the original data with a spectral
kernel leads to the mixing of photospheric and chromospheric
emissions. The fibrils filling the original QS FoV (panel (A))
are faintly detectable with a spectral degradation of 0.6 Å (not
shown) and almost no longer seen with 1 Å bandwidth (panel
B), which allows PO and UM regions to manifest themselves
with spatial scales and intensities (panels (F), (J)) close to the
ones displayed by the same features in images acquired with a
spectral degradation of 3–10 Å (panels (G), (K) and panels (H),
(L))." (from 3.1. Effect of Spectral Band)

So for us to start seeing fibrils(faintly detectable) in quiet-Sun area with Ca II K, we will need a filter FWHM of 0.6A(with a Gaussian distribution bandpass profile), and these fibrils in the quiet -Sun area will not be shown(almost no longer seen) under 1A FWHM.

[Brian3114611]

Also here is relation of line core intensity contrast vs band width at different regions(Fig 12. in the paper)

line core intensity contrast C: C = I/IQS , where
I = Stokes-I intensities at the Ca II K line core over each image pixel
IQS = mean QS intensity averaged over the entire FoV analyzed

QM: quiet-Sun magnetized granulation
QG: quiet-Sun granulation
QS: quiet-Sun
PE: penumbral
UM: umbral


Fig12. Dependence of the estimaterd Ca II K line core intensity contrast on the spectral resolution by Brian Liu, 於 Flickr

[marktownley]

Welcome to the forum Brian, and thanks for the info.

I've attached the whole paper these are from as I know will be of interest to some members of the forum.

2303.17160.pdf

(19.95 MiB) Downloaded 112 times

[DeepSolar64]

Mark,
Maybe this should be saved on the forum as a valuable resource.

James

[christian viladrich]

Hello everyone, never posted anything here before, it seems like even Christian and Valery way underestimates the spectral resolution of SHG, here I would like to share a comparison of Ca II K at spectral resolution of 0.12A(SST CHROMIS), 1A, 3A, and 10A.
Top row is quit-Sun area, middle row is region with plages and pores, and bottom row for sunspot with umbra and penumbra.
From Figure 5., M. Murabito et al 2023 ApJ 947 18, Investigating the Effect of Solar Ambient and Data Characteristics on Ca ii K Observations and Line Profile Measurements

Ca ii K comparison 0.12A 1A 3A 10A by Brian Liu, 於 Flickr

Hope this will be useful, possibly someone already shared this before.

Regards,

Brian

Great and valuable info Brian ! Thanks for sharing.

It seems there is a huge gap between 0.1 A and 1 A, then another gap between 1 A and 3 A, then 3 A and 10 A looks relatively similar.

BTW, fibrils are visible at 1.4 A. Here is an example with the 300 mm Newtonian :

http://astrosurf.com/viladrich/astro/so ... CaK-1A.jpg

Or should I say, detectable rather than visible, since we are here at the very limit of resolution ;-)

There are is also an interesting paper describing the visibility of Ca K fibrils with ... the 1 m SST.

[Brian3114611]

Hello everyone, never posted anything here before, it seems like even Christian and Valery way underestimates the spectral resolution of SHG, here I would like to share a comparison of Ca II K at spectral resolution of 0.12A(SST CHROMIS), 1A, 3A, and 10A.
Top row is quit-Sun area, middle row is region with plages and pores, and bottom row for sunspot with umbra and penumbra.
From Figure 5., M. Murabito et al 2023 ApJ 947 18, Investigating the Effect of Solar Ambient and Data Characteristics on Ca ii K Observations and Line Profile Measurements

Ca ii K comparison 0.12A 1A 3A 10A by Brian Liu, 於 Flickr

Hope this will be useful, possibly someone already shared this before.

Regards,

Brian

Great and valuable info Brian ! Thanks for sharing.

It seems there is a huge gap between 0.1 A and 1 A, then another gap between 1 A and 3 A, then 3 A and 10 A looks relatively similar.

BTW, fibrils are visible at 1.4 A. Here is an example with the 300 mm Newtonian :

http://astrosurf.com/viladrich/astro/so ... CaK-1A.jpg

Or should I say, detectable rather than visible, since we are here at the very limit of resolution ;-)

There are is also an interesting paper describing the visibility of Ca K fibrils with ... the 1 m SST.

That's some of the best Ca II K images I've ever seen!

Yes, I agree the fibrils are visible at 1.4 A, but probably only around active regions(like your image showing fibrils around plages and sunspots), not quiet-Sun areas, according to the paper, (within QS FoV)fibrils are "almost no longer seen with 1 Å bandwidth".

It would require 0.6 Å(as I mentioned above) for fibrils to be faintly detectable around quiet-Sun area, so a 1 Å double stack will probably start showing fibrils all around quiet-Sun areas.

[marktownley]

http://astrosurf.com/viladrich/astro/so ... G-Doug.jpg

Christian, do you have a 'full size' link to the DS 1.4a disk you can share?

[christian viladrich]

Hello Mark,

Here are the full size images displayed with exactly the same processing.

- 0.37 nm Ca K filter :
http://astrosurf.com/viladrich/astro/so ... K037nm.jpg

- 0.14 nm Ca K filter :
http://astrosurf.com/viladrich/astro/so ... K014nm.jpg

- double-stack 0.14 + 0.14 nm filters :
http://astrosurf.com/viladrich/astro/so ... nm-lin.jpg

- Doug' SHG image (with same processing ) :
http://astrosurf.com/viladrich/astro/so ... HG-CaK.jpg

Alternate processing of the double-stack 0.14 + 0.14 nm in order to have a better view of the prominences :
http://astrosurf.com/viladrich/astro/so ... -TF-TC.jpg

[Dennis]

Hm.. now who can make us a temp controlled ca-k etalon that is narrower then 1A..
On the good side: it could be a very small etalon since only for imaging.

[marktownley]

Thanks Christian

[Astrochriss]

Hey guys,
I already read this thread a few weeks ago and was really surprised that it is possible to get a view of "only the chromosphere" as long as the bandwidth is small enough, like 0.12A in the paper. I always assumed the calcium concentration in the chromosphere would be too low, so that there is always a lot "photospehere leakage" no matter how small you go with the bandwidth. It is just stunning how similar the view of subframe A) of the paper is to a normal H-Alpha image, showing only the fibrils and no hint of the reverse granulation. I have never seen that before.

Even though we as amateurs probably never reach this crazy 0.12A bandwidth, I would suggest to shift this thread to the reference library. Very usefull stuff here, if someone builds a super-high-(spectral)-res SHG in the future.

Best
Chris

[thesmiths]

Even though we as amateurs probably never reach this crazy 0.12A bandwidth, I would suggest to shift this thread to the reference library. Very useful stuff here, if someone builds a super-high-(spectral)-res SHG in the future.

Actually, I've calculated it would be quite straightforward to achieve 0.09 angstrom spectral resolution at Ca-K using a 3600 l/mm grating. The bigger problem is that is difficult to achieve spatial resolution better than 1.5 arcseconds -- you need extremely good seeing conditions since the image data is acquired serially (and not by "lucky imaging").