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Surface contrast in CaK for various filter bandpasses.

Posted: Fri Sep 14, 2018 8:14 pm
by p_zetner
Hello Everyone.

I recently posted an animated spectral sweep through the CaK assembled from images taken with my spectroheliograph on Aug08.

viewtopic.php?f=8&t=25028

It brought up some interesting questions from Valery and Christian that I thought I'd look into. The basic question was: What filter bandpass would be required to see filaments in a CaK image? The bandpass of my SHG (in the configuration used) was about 180 milliAngstroms. This is narrow enough to clearly see filaments with very good contrast. Here's a line centre image:
092854_pipp_flat-div_siz_Hflat-div_1020_line-ctr_crop_siz.png
092854_pipp_flat-div_siz_Hflat-div_1020_line-ctr_crop_siz.png (757.6 KiB) Viewed 924 times

To answer the question above, I simulated the performance of a filter of given bandpass by averaging over a specific number of frames used in the spectral sweep animation. Here is a montage of 25 out of 40 of these frames around line centre.
Montage092854_pipp_flat-div_siz_Hflat-div_1008-1032_proc_lbls-frm_siz.png
Montage092854_pipp_flat-div_siz_Hflat-div_1008-1032_proc_lbls-frm_siz.png (1.03 MiB) Viewed 924 times
A large size version of this image with frames labelled in wavelength offsets can be found here:
https://www.astrobin.com/full/366718/0/?nc=user


The wavelength separation between frames is 95.8 milliAngstroms and I've taken line centre to be located at frame 20. To give a reasonable simulation of filter performance, I used a weighted average of frames, weighted in such a way to mimic a Gaussian filter profile. For example, here is a set of weighting coefficients for a filter fwhm bandpass of about 1.1 Angstroms, shown in graphical form. To make my job simpler, I stopped averaging frames whose contribution to the average was determined by 1% or less (weighting coefficient of 0.01 or less). The image frame number corresponds to the number shown in the montage above and the corresponding wavelength offsets from line centre are also shown in the plot.
Gauss coefficients 1113mA fwhm siz.png
Gauss coefficients 1113mA fwhm siz.png (30.27 KiB) Viewed 924 times
Here are some results, showing simulated CaK images for fwhm filter bandpasses of 1113, 860, 593, 500, 400 and 300 milliAngstroms. I'll leave it to you to judge at what point filament contrast becomes good. To me, it looks like a fwhm of less than 400 milliAngstroms would be very desirable. By the way, there was a bit of processing applied: levels and sharpening. Note that the Lunt CaK module has a bandpass 0f ~2400 milliAngstroms, making filaments essentially invisible.
CaK_fwhm-study_montage_proc_siz.png
CaK_fwhm-study_montage_proc_siz.png (2.66 MiB) Viewed 924 times
There is a fine point to the analysis I used. To take into account the native bandpass of the SHG, I used the following equation to relate the desired filter bandpass to the SHG bandpass and the fwhm of the Gaussian curve used to give the weighting coefficients for the average ("SIM" in the equation):
fwhm eqn.png
fwhm eqn.png (20.22 KiB) Viewed 924 times
This equation basically relates the bandpass of the modelled filter, the SHG and the simulation curve (SIM) through convolution. So, for example, a filter bandpass of 500 milliAngstroms involved a "SIM" fwhm of 466.5 milliAngstroms convoluted with the 180 milliAngstrom SHG bandpass.

So, the conclusion is that a CaK filter bandpass of less than 0.4 Angstroms would do a very good job, giving very contrasty views of the Ca network and filaments. I don't know much about filter manufacturing fine points, but perhaps this is not too difficult to achieve?

Cheers.
Peter

Re: Surface contrast in CaK for various filter bandpasses.

Posted: Fri Sep 14, 2018 9:39 pm
by Bob Yoesle
Hi Peter,

Thanks for this very informative and interesting information.

Not sure what it would take either, but so far at least no one has made such a filter...

Re: Surface contrast in CaK for various filter bandpasses.

Posted: Fri Sep 14, 2018 10:12 pm
by pedro
Very interesting info

Re: Surface contrast in CaK for various filter bandpasses.

Posted: Fri Sep 14, 2018 11:23 pm
by christian viladrich
You nailed it Peter. This is a wonderful analysis. Many thanks !
So it takes something like 0.3 to 0.5 A FWHM to show the filaments in Ca K (depending on the expected contrast).

I think Mark W told once that narrow band Ca K etalons are more difficult to do than Ha etalons ? I am not 100% sure.

Re: Surface contrast in CaK for various filter bandpasses.

Posted: Sat Sep 15, 2018 8:38 am
by Carbon60
Excellent work, Peter. This is invaluable information when considering CaK system design optimisation.

You've got a very fine instrument there!

Stu.

Re: Surface contrast in CaK for various filter bandpasses.

Posted: Sat Sep 15, 2018 3:58 pm
by p_zetner
Thanks for the comments everyone.

One thing I wanted to note was that my simulated filter (spectral) transmission curve is Gaussian while an etalon filter (single stack) has a Lorentzian profile. The Lorentzian curve has broader shoulders (viewtopic.php?f=8&t=22830) so the Gaussian model might be slightly optimistic with regard to minimum required bandpass (but still not too far off). I’m guessing a sufficiently narrow band filter would be multiple stacks anyway, with steeper shoulders.

Bob and Christian: Not sure what the issue would be with uv etalon manufacture. There seem to be good, high reflectance coatings available. Newport claims R>99% on their dielectric, excimer laser mirrors (~ 350nm). A bandpass of 0.4 Angstroms with 10 Angstroms free spectral range requires a coating reflectance of 0.94.

Cheers.
Peter

Re: Surface contrast in CaK for various filter bandpasses.

Posted: Sat Sep 15, 2018 5:37 pm
by eroel
Peter:
Thanks for the interesting information, I always wanted to make one of Veio´s instruments.
Very nice comparo´s.
Best regards,
Eric.

Re: Surface contrast in CaK for various filter bandpasses.

Posted: Sat Sep 15, 2018 11:13 pm
by Merlin66
Eric,
The digital spectroheliograph (SHG) that Peter (and others) use is not a visual spectrohelioscope (SHS) as used by Fred. Think of it as a second generation SHG making use of software to separate and re-combine the target spectral line to form a detailed image. The basic design is well within the capabilities of a competent DIY amateur.
My book "Imaging Sunlight using a digital Spectroheliograph" gives all the necessary details...and more!

Re: Surface contrast in CaK for various filter bandpasses.

Posted: Sun Sep 16, 2018 1:46 am
by eroel
Ken:
Thanks for the explanation, I will get your book from Springer. BTW, I still have Fred´s book on SHG.
Best regards,
Eric.

Re: Surface contrast in CaK for various filter bandpasses.

Posted: Sun Sep 16, 2018 11:43 am
by george9
This is really excellent, Peter. Thanks so much.

Another approach is to compare your CaK sweep to your H-alpha sweep. The filament at top seems to be visible in a range that is at best 2 times narrower than H-alpha but not quite 3 times narrower. So maybe 2-2.5 times narrower. You can see filaments in H-alpha at 0.8A. At 1.0A, it gets harder. That would imply 0.5A being near an upper limit, corroborating your convolutions and the other estimates.

There are other dark linear structures that are visible in a much wider band. In the active region at left, there is a horizontal band running over the top and another diagonal to the left. And another horizontal line at 2 o-clock far above the right active region. They seem to be more than just an optical illusion, and they are visible through the entire series. So some subtle dark linear features should be visible in the Lunt or PST.

And obviously, the brighter areas of the active region (also called plage?) are visible all the way through, plus is it probably easier to detect a narrow bright feature than a narrow dark feature. And that corroborates the Lunt and PST's good performance there.

George

Re: Surface contrast in CaK for various filter bandpasses.

Posted: Sun Sep 16, 2018 12:26 pm
by marktownley
Very interesting results Peter!

Re: Surface contrast in CaK for various filter bandpasses.

Posted: Sun Sep 16, 2018 6:19 pm
by p_zetner
Thanks for your comments Eric, George and Mark.

George, I really appreciate your feedback on this. I am planning to do a similar filter simulation for the H alpha sweep. A direct comparison of the CaK and H alpha simulations will, I'm sure, confirm what you have concluded in your comparison of the sweeps. In the meantime, I can post this comparison of absorption spectra of the filaments in H alpha and CaK. I've marked, with a red arrow, the location in the filament where the spectra were measured.
Montage_Ha_CaK_Fil-spec_comparison.png
Montage_Ha_CaK_Fil-spec_comparison.png (458.35 KiB) Viewed 704 times
You can clearly see a huge difference! These spectra were normalized to the spectrum of the underlying chromosphere in the two cases (hence, the maximum intensity of 1.00). Squares represent the actual measured intensities while curves represent a fitting model based on some elementary radiation transport theory. The fwhm of the filament absorption line in H alpha is about 970 milliAngstroms while, for CaK, it is only about 260 milliAngstroms. Not only that, you can see that the H alpha absorption is much greater, dropping the intensity to around 0.7 near line centre. More technically, the optical depth of the absorption can be extracted from the modelled spectrum and it was found to be 9.8 for H alpha and 1.3 for CaK. An optical depth of 1.0 usually serves as the dividing line between optically thick (> 1.0) and optically thin (< 1.0) absorbing media. The H alpha filament is clearly very optically thick and, hence shows up more easily in H alpha images.

As far as the dark structures in CaK, I've seen these in many of my CaK spectroheliograms and some of my, conventional, filtergrams taken with the Lunt CaK module. I originally thought they might be stacking artifacts but, since they appear in spectroheliograms, this doesn't seem to be the case. They are dimmer than CaK filaments and don't seem to correlate with H alpha features ... some peculiar aspect of the Ca landscape.

Regards.
Peter

Re: Surface contrast in CaK for various filter bandpasses.

Posted: Sun Sep 16, 2018 6:35 pm
by marktownley
Fascinating high brow solar science here Peter, i'm most impressed! This really does from an empirical way what we might realistically expect from our CaK devices.

Those dark structures are interesting though, i've regularly seen them too.

Re: Surface contrast in CaK for various filter bandpasses.

Posted: Mon Sep 17, 2018 3:24 am
by Merlin66
Peter,
I’ve sent you an email.
Ken

Re: Surface contrast in CaK for various filter bandpasses.

Posted: Mon Sep 17, 2018 11:16 am
by Montana
Very very interesting, this should be put in the library when the discussion is finished. I have also seen these dark channels in CaK and never really thought about them till now, I wonder what they are? we will have to call them ferrets :lol:

Alexandra

Re: Surface contrast in CaK for various filter bandpasses.

Posted: Mon Sep 17, 2018 4:42 pm
by krakatoa1883
Very interesting and informative discussion. I have been able to image some large and unusually dark filaments through a Quark calcium device, which is declared to have a BP around 5A, a quite large one. It occurred only twice since I purchased it a year and a half ago. Hope to be able to find those image, they were the first imaging experiments I did with my filter.

Re: Surface contrast in CaK for various filter bandpasses.

Posted: Tue Sep 25, 2018 8:37 am
by Derek Klepp
Thanks Peter very informative.