| Filter | Test method | f-ratio | Surface measured | FWHM | BW at 10% peak transmission |
| Alluxa Ca K 0.37 nm |
Sol'Ex spectro (fc=fi=125 mm) |
f-115 |
10 micron × 4.5 mm | 0.37 nm | 0.57 nm 3-cavity filter |
| Sol'Ex spectro (fc=fi=200 mm) |
f-115 | 10 micron × 4.5 mm | 0.39 nm | 0.59 nm 3-cavity filter |
Substrate : N-BK7
Temperature coefficient : 0.0018 nm/°C (source : Alluxa)
n eff = 1.645 (source : Alluxa)
Temperature coefficient : 0.0018 nm/°C (source : Alluxa)
n eff = 1.645 (source : Alluxa)
Measurement in collimated beam
#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 three images show:
- the solar spectrum near Ca K and H,
- the spectrum transmitted by the Alluxa filter at normal incidence,
- the spectrum transmitted by the Alluxa filter when tilted for Ca K.
- the solar spectrum near Ca K and H,
- the spectrum transmitted by the Alluxa filter at normal incidence,
- the spectrum transmitted by the Alluxa filter when tilted for Ca K.
Transmission profile at normal incidence
The bandpass at 10% being close to 1.5×FWHM, we can conclude this is a 3-cavity filter.
Measured peak transmission is in the range of 60-65%.
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).
Transmission profile when tilted for Ca K
Notes:
- the FWHM and the BW at 10% peak transmission increase slightly due the filter tilt.
- the dent at 3933.7 A is an artefact due to non linearity of the camera (the core of the Ca K line is very dark) and / or change of focus.
- measured peak transmission is again in the range of 60-65%.
- the FWHM and the BW at 10% peak transmission increase slightly due the filter tilt.
- the dent at 3933.7 A is an artefact due to non linearity of the camera (the core of the Ca K line is very dark) and / or change of focus.
- measured peak transmission is again in the range of 60-65%.
Transmission profile with increasing tilt (collimated light)
#2 - Optical set-up:
Sun => Sol'Ex spectrograph with 10 micron slit : 10 micron × 4 mm. 200 mm f.l. collimating and imaging lens (re-focused on Ca K). Grating : 2400 l/mm. ASI290 camera. 12-bit acquisition. Dispersion: 0.0584 A/pixel.
Blue sky. Sun near meridian.
