Reference comparison images for the estimation of the FWHM of Ha etalons

[christian viladrich]

Hi all,

Without any measuring equipement, or at least any possibility of comparison with other filters, it is extremely difficult to get an idea of ​​the quality of an Halpha solar filter. This is specially true for people observing away from any associations or other solar observers.

This summer, I had the opportunity to have three Ha etalons at the same time. I had a morning with very good seeing conditions, which allowed me to take images with these three filters, in almost identical seeing conditions.
The equipment used was a Takahashi TOA 150 with a telecentric Baader TZ-4 and a Basler 1920-155 camera.
All the etalons were tuned on Ha line with the spectrograph.All the etalons were tuned on Ha line with the spectrograph.

I measured the FWHM of these filters with different approaches, and their FWHM turned out to be 1.0 A, 0.6 A and 0.3 A (in collimated beam). These are the actual measured values, and not the values indicated by the manufacturers.

This images are interesting because they show how the Sun looks like with different filters whose FWHM was actually measured.

The processing is a minimal and identical for all images : unsharp mask 1.4 8, linear visualization, gamma = 1 and that's it ;-)

1) First, a group of sunspot :
http://astrosurf.com/viladrich/astro/in ... ic-f31.jpg

As the FWHM decreases, the parasitic contribution from the photosphere decreases, and a higher layer of the chromosphere is sampled.
With a 1.0 A filter:
- photospheric granulation is still visible through the chromosphere. This explains the granular aspect of the image.
- the filaments are very pale.
With a 0.3 A filer:
- the photospheric granulation is no longer visible,
- the smallest spots (photospheric level) have disappeared, covered by fibrils (chromospheric level),
- the ombra of the largest spots are partly covered by fibrils.
- the spicules and filaments show more contrast,
- the general contrast of the Ha structures increases.

With a 0.6 A filter:
- we are half way between the two previous filters. The photosphere granulation has nearly disappeared.

2) Now, an image of the solar limb :
http://astrosurf.com/viladrich/astro/in ... ic-f31.jpg

In addition to the effects already described, we observe a reduction of the double-limb effect when the bandwidth reduces. But the double limb is still there at 0.3 A FWHM.

You have to go to double-stack to see the double limb effect disappear.

Off-course, the differences between filters also depends on the aperture and resolution of the instrument.
For example, the double-limb effect will be smaller in a smaller aperture instrument because of the lower resolution. In the same way, photospheric granulation won't be visible on a 70 mm refractor equiped with a 1 A Ha filter.

For those interested, here is a link to the raw files:
http://astrosurf.com/viladrich/astro/in ... ic-f31.zip

For reference, the effective FWHM of these 0.3 A, 0.6 A and 1.0 A etalons become 0.46 A, 0.70 A and 1.05 A in a f/31 light beam.

Hope this helps !

Clear skies,

Christian

[pupak]

Superb.

[marktownley]

Incredibly informative thanks Christian! A golden thread!

[TareqPhoto]

This is too much for me to follow or understand, it is truly impressive, excellent work Christian!

[robert]

Very nicely researched and clearly presented. Amazing work
Robert