Barr Ca K filter



Filter Test method f-ratio Surface  measured FWHM BW at 10% peak transmission CWL
Barr Ca K 0.37 nm
 Sol'Ex spectro
(fc=fi=125 mm)		 f-115
 10 micron × 4.5 mm 0.26 nm 0.48 nm
2-cavity filter		 - 0.7 A
Sol'Ex spectro
(fc=fi=200 mm)		 f-115 10 micron × 4.5 mm 0.26 nm 0.44 nm
2-cavity filter		 - 0.6 A


Temperature coefficient : 0.01 A/°C

Measurement in collimated light

# 1 : Optical set-up:
Sun => Sol'Ex spectrograph with 10 micron slit : 10 micron × 4 mm. 125 mm f.l. collimating lens (re-focused on Ca K). Grating : 2400 l/mm. 125 mm f.l. imaging lens.  ASI290 camera. 12-bit acquisition. Dispersion: 0.0932 A/pixel.
Very transparent blue sky. Sun near meridian.


Estimated accuracy: +/- 0.1 A for CWL, FWHM and bandwidth at 10% transmisison.

The filter is exposed to nearly perfect collimated light beam: converging light beam with a 0.5° cone (solar disk), equivalent to f-115 ratio. The surface of the filter sampled is set by the slit size (10 micron x 4 mm)

The following two images show:
- the solar spectrum near Ca K and H,
- the spectrum transmitted by the Barr filter at normal incidence.

Transmission profile at normal incidence





The bandpass at 10% peak transmission being < 2 × FWHM, we can conclude this is a 2-cavity filter.
Measured peak transmission is in the range of 20%.
These measurements with the 125 mm f.l. collimator are in close agreement with the measurements made with the 80 mm collimator (0.1 A difference).


# 2 : Optical set-up:
Sun => Sol'Ex spectrograph with 10 micron slit : 10 micron × 4 mm. 200 mm f.l. collimating and imaging lenses (re-focused on Ca K). Grating : 2400 l/mm. ASI290 camera. 12-bit acquisition. Dispersion: 0.05822 A/pixel.
Blue sky. Sun near meridian.


Note:
The small dents in the transmission profile are artefacts due to non linearity of the camera and/or defocussing.



NB : peak transmission is underestimated due to the unsufficient resolution of the spectrometer.


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