Granulation study with various filters (540-395 nm)

[GreatAttractor]

On Saturday I decided to see if Meteoblue's enticing seeing forecast (~0.6") for the Flüela Pass in the Alps (2384 m/7822 ft) is true, and drove there with all the gear. TL;DR: it was very good indeed.

I was using the Intes M715 (180 mm f/15 RMCT) with Baader ND 3.8 solar film, Baader Solar Continuum and 3 other inexpensive bandpass filters I found on eBay:

F1. 450 nm CWL/40 nm width
F2. 410/20 nm
F3. 395/8.5 nm

(all with IR blocking to at least 1100 nm.)

The filters are 10x10 mm square or 15 mm round, just enough to cover the camera's sensor glass:

filters.jpg (61.74 KiB) Viewed 611 times

glass.jpg (63.15 KiB) Viewed 611 times

Provisional filter holder (goes into the 1.25" nosepiece):

holder1.jpg (105.85 KiB) Viewed 611 times

holder2.jpg (68.32 KiB) Viewed 611 times

M715 with the FeatherTouch 1.25" Crayford, which I got earlier for my Lunt 50:

focuser.jpg (172.31 KiB) Viewed 611 times

setup.jpg (504.36 KiB) Viewed 611 times

I was careful with my expectations, but the seeing was good even down to 395 nm; here's a raw video fragment (slowed down from 226 fps to 50 fps; exposure: 3 ms; there was some wind shaking):

8_5nm_raw.gif (8.43 MiB) Viewed 611 times

As for the processing, I settled on just a touch of unsharp mask. It may look a bit soft, but despite L-R deconvolution giving superficially "sharper" images, to me there was too much ringing around the intergranular lanes.

Keep in mind the material here should not be used for an objective comparison between the filters — I was modifying the camera's gamma setting slightly, and the final tone curves differ (next time I'll remember to capture some 16-bit videos at neutral gamma for a better comparison.) Also, any resolution differences (e.g., 395 nm should be 37% better than SC's 540 nm) are not immediately visible, since a 180 mm aperture only begins to resolve the granules (a 10" 'scope would be definitely better), and the impact of seeing probably also differed.

With that said, one can at least clearly see how the "chromospheric shimmer" (is there a proper name? I mean this fast-changing dark/bright pattern overlaid on the granulation) differs between wavelenghts. All animations were taken with 15-second intervals (10-second videos with 5-second breaks); additional histogram stretch was performed for granulation-only ones. Unfortunately, all there was on the Sun (not too far from the solar disc's center) were a few underdeveloped sunspots — I chose the southern part of AR 3045.

(note: MP4 videos can be looped via right-click)

Solar Continuum (20 min. total):
sunspot
granulation:

granu_540_nm.gif (5.42 MiB) Viewed 611 times

F1 (12 min. total):
sunspot
granulation:

granu_450_nm.gif (2.95 MiB) Viewed 611 times

F2 (10 min. total):
sunspot
granulation:

granu_20_nm.gif (2.77 MiB) Viewed 611 times

F3 (23 min. total):
sunspot
granulation:

granu_8_5_nm.gif (6.03 MiB) Viewed 611 times

Best frames:

Solar Continuum:

sc_34.png (340.68 KiB) Viewed 611 times

F1:

40nm_34.png (330.72 KiB) Viewed 611 times

F2:

20nm_38.png (236.15 KiB) Viewed 611 times

F3:

8_5nm_47.png (385.57 KiB) Viewed 611 times

(Everything shot with Blackfly S (IMX 273), processed with AS!3 and ImPPG.)

The "shimmer" is most distracting for F2. As far as granulation goes, I like the Solar Continuum's result best. I'm curious how the filters will do on a proper sunspot, with a wide, fibrous penumbra and thin light bridges.

[GreatAttractor]

If anyone's interested, here are the best raw stacks for each filter. Feel free to post your processing results, I'm curious to see what you can get out of it.

[RodAstro]

Hi
Very interesting.
I have been trying to image the granules with little success on both my 6" Zeiss refractor with Zeiss WL filters and my 10"SCT but only have 0.5 Baader film full aperture, also using continuum and OIII filters at the camera.
I can see the granules easy enough so it must be my camera settings or my seeing.

Anyway I have had a go with your images and find the continuum image gives the best granules but the 450nm-40nm image gives some lovely fine details around the spots and some interesting lines between the large spot groups to the single spot in the middle that's lost on the continuum image.
Setting were the same on both images apart from the tone curves.

Cheers Rod

[Montana]

What a location Filip!! have you recently moved there? wow, looks to be solar heaven

Great shots too

Alexandra

[robert]

Fabulous. I guess the secret ingredient is the location!
Robert

[Jordan Konisky]

Fabulous, interesting, and informative post. Exactly how do I download the attached zip file?

[Jordan Konisky]

Figured it out. Thanks.

[marktownley]

Nice results and location to observe from!

[GreatAttractor]

Thanks, everyone.

What a location Filip!! have you recently moved there?

It's great, and even has two ice-cold lakes (half-frozen until May) I moved a few months ago; I live about 90 minutes away now, so I'll be going there again for sure (also for planetary work later this year).

[arnedanielsen]

Beautiful images and location! The comparison is very interesting!

Thanks for sharing,
Arne

[pedro]

Thanks for the interesting comparison. I have a similar Mak scope that I will use to image the SUN in WL HR

[GreatAttractor]

I have had a go with your images and find the continuum image gives the best granules but the 450nm-40nm image gives some lovely fine details around the spots and some interesting lines between the large spot groups to the single spot in the middle that's lost on the continuum image.

Thanks! As for those lines, I think they're just a transient phenomenon (it seems that way from the full AR animation).