Solar disk imaging : comparison of 5 near-UV filters and 2 G-band filters

[christian viladrich]

Hi all,

The passage of good weather was short-lived here, but still allowed to compare 4 near-UV filters, a Ca K filter and two G-band filters on images of the solar disk.

I give the link to full-resolution images, so that everyone can make up their own mind. For this, it will be easier to load the images on your PC in order to make side by side comparisons.

The level of turbulence is about the same throughout the series.

Differences exist thought not very spectacular. Sticking to the visibility of the facular ranges (especially towards the center of the sun's disk), I would say that:

- the maximum contrast is obtained with the Edmund Optics of 394-10 nm and the Antlia 393-3 nm. The EO is also a bit of a surprise to me. Untill nouw I use it as a pre-filter for the Ca K filter, but it's a really good filter. With the EO, the difference in edge/center brightness is more marked than with the Antlia.

- then, we would have the 8 nm K-line and the 395-5 nm Andover, which are quite close,

- then, we would have the Andover 430-1.9 nm G-band filter, which has the advantage of being less sensitive to turbo (compared to previous filters),

- then, the Edmund Optics G band of 10 nm, which is less contrasted.

I also put the Ca K Alluxa of 0.37 nm for comparison.

It is possible that the result of the comparison will be different if we are interested in high resolution images (with a longer focal length). For example, on full disk images, the difference between a 0.37 nm Ca K filter and a 0.15 nm Ca K filter is rather small while it is obvious on high resolution images.

http://astrosurf.com/viladrich/astro/so ... 4-10nm.jpg

http://astrosurf.com/viladrich/astro/so ... -Kline.jpg

http://astrosurf.com/viladrich/astro/so ... 96-5nm.jpg

http://astrosurf.com/viladrich/astro/so ... 93-3nm.jpg

http://astrosurf.com/viladrich/astro/so ... -1-9nm.jpg

http://astrosurf.com/viladrich/astro/so ... 0-10nm.jpg

http://astrosurf.com/viladrich/astro/so ... -037nm.jpg

I think we have now everything we need in terms of near-UV filters.

We are more in need of a narrow-band G-band filter (which makes easier to handle turbulence) and of quality Ca K filter (say 2 A FWHM hard-coated).

Clear skies !

[marktownley]

Good comparison, thanks Christian!

[jdelburgo]

Thank you very much for this useful information.
I am considering now the Edmund Optics of 394-10 nm.
What is the maximun aperture you recommend to use this filter with no energy protection like triband ERF, KG3, IR-UVcut, hershell..?
The idea is to use this filter in a Synta 102/1000 reespaced achromat at 81mm aperture with Baader FFC working at 3X. As this is not a narrowband filter I would put FFC after the EO in the optical train, so the filter receives all energy flux.
I see that this filter is in 25mm aperture in thick and non-threated cell...any advise to set it in the optical train?
Thanks in advance
José C del Burgo

[christian viladrich]

Hi José,
My suggestion would be to put the EO filter after (and not before) the FFC. This way, the filter would not bother much about the thermal load.
Do you have any way to install a 50 mm sub-aperture blue filter in front of the FFC ?

I've found some 1"1/4 empty filter cells I don't remember where (e-bay, TS, Baader, ?). Then, I made some low tech adapters to arrange for the difference in filter size. More recently, I improved a bit making the diameter adapters with 3D printing.

[jdelburgo]

Hi Christian,
Thank you for your answer.
I can put a 2" nosepiece in front of FFC and use some m48 extensions to install a blue filter well ahead. I would not like to damage FFC which currently has a crazy price (730€).
The 3d printer adapter looks a good solution.
Best regards
José C del Burgo

[christian viladrich]

Here are some links for 1"25 filter cells :

https://www.teleskop-express.de/shop/pr ... thick.html

https://www.teleskop-express.de/shop/pr ... thick.html

The second one is better for 25 mm filters since you need some extra space to allow intermediate ring (3D printed).