The equipment used
Baader solar filter OD 3.8 was placed at the end of a dewshield for the Seestar S50 and held in place with glue and plastic adhesive tape.
A filter holder for the Seestar S50 with a 2” to 1.25” filter adapter.
The solar filter assembly and the adapter fitted with a Baader Ca K-line double stacked filter 394 nm peak transmission with an 8nm bandpass.
The whole assembly viewed from the bottom.
The Ca K-line filter can be seen in place.
The whole assembly seen from the top.
The Seestar S50 with the Double filter assembly in place
There were initially two unknowns in this experiment:
1) Whether a 1.25” filter would cause vignetting
2) Whether the built-in UV/IR cut filter would be so aggressive that it would prevent the Ca K-line light from being detected by the sensor.
Optical density 3.8 Baader photographic grade solar filter was used because it transmits 16 times the amount of light that is transmitted by an OD 5.0 visual/photographic solar filter. This will give the best chance for the sensor to detect the CaK light if the UV/IR filter will allow it to pass.
As seen below, in a short opportunity between the clouds, the system worked well and revealed more detail of the photosphere than normal white light filtering. There was no noticeable vignetting.
As this system works, it should also work with the Altair 2nm G-band Solar Contrast Filter 430.3 nm peak transmission.
Ca K-line image as seen on the Seestar S50 preview screeen
More work needs to be done to determine the best exposure setting when imaging with this setup. Because of clouds, very little time was available to optimise this. However, automatic exposure and gain seems to give a reasonable result.
The RAW AVI data were debayered and stacked in Autostakkert! with 1.5 drizzle (as the Seestar S50 is slightly under-sampled). The stacked image was wavelet processed in waveSharp, and post processed mainly in the Gimp 2.10.
Seestar S50 Ca K-line solar image August 9
The same situation occurred the next day on August 10 but with 252 debayered and stacked RAW AVI frames and post processing, another acceptable Ca K-line image was obtained
Seestar S50 Ca K-line solar image August 10
The results of this experiment are very encouraging. Whilst the assembly holding the OD 3.8 Baader solar filter and the double stacked Ca K-line filter worked fine, it may be possible to build a more compact assembly.
It is not known if this system would work with Ca K-line filters such as the Antlia 3nm CaK filter or whether it would yield even better results. It remains for others with different bandpass Cak filters to try.
