Ca K filter selectivity

The solar spectrum transmitted by various combinations of Ca K filters was calculed on the basis of :
- high resolution solar spectrum, source : Meudon/Bass2000,
- transmission curves of Alluxa Ca K filters : source : Alluxa,
- 0.4 A and 0.6 A Ca K mica-spaced etalon : Lorentzian curves based on FWHM = 0.4 or 0.6 A, FSR = 26.8, n = 1.56, associated with square transmission BF 10 A,
- SHG : spectral resolution of 0.17 A, as measured with Sol'Ex SHG fc=fi=200 mm, 2400 l/mm, ASI290, Lorentzian transmission curve.

Transmission curves and transmitted spectra:
NB : both filter transmission curves are recentered on Ca K (just as if the filter was thermo-controlled).



The selectivity of a filter combination is defined as the ratio:
selectivity = transmitted solar spectrum on-band / total transmitted solar spectrum

Selectivity is equal to 100% if the all the energy transmitted by the filter combination is on-band.

For the visibility of filaments and fibrils in Ca K, we consider that the "on-band" window is the +/-0.5 A window centered on Ca K. Sensitivity to 0.8 A and 0.6 A windows is also examined.



As a comparison (source : Physical properties of bright Ca II K fibrils in the solar chromosphere. S. Kianfar. A&A 637 (2020)):
- the average distance between Ca K2 peaks is about 0.33 A,
- for Ca K observation CHROMIS/SST summed wavelength maps from -0.31 A to + 0.31 A.


Selectivity of various Ca K combinations:



The ranking of filter combinations does not change whether a window from 1 A to 0.5 A used for the claculation of selectivity, except for the SHG.
NB : the assumption made on the transmission profile of the SHG (Lorentzian curve) is very conservative. Actual profile is steeper.

The selectivity of the mica-spaced 0.6 A Ca K etalon is lower than the selectivity of the double-stack 0.13 nm because of the large wings of transmission of the mica-spaced etalon:





Tolerances on the CWL for double-stack combinations based on Alluxa Ca K 0.1 A filters

The question is "If the CWL filter is not exactly on Ca K, what is the impact on the selectivity of the combination ?"

In the following table, the 1rst column  shows different assumptions regarding the difference between the CWL of the filter and the center of Ca K, while the second shows the resulting selectivity.




We can conclude that we can double-stack two filters with a difference of CWL of 0.3 A. The impact on the selectivity (compared to a combination with the same CWL) is negligeable.




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