The filter was measured using interferometry with an Ha lamp and diffused light.
| Filter | Test method | f-ratio | Surface measured | FSR | FWHM | |
| DayStar PE 0.6A#2 | Ha lamp | collimated | Full aperture |
26.4 A (asssuming a 1.575 index for mica) |
0.95 A |
Measurement of the etalon with a Ha lamp
Nikon Z6 with Nikkor 50 mm S f/1.8 lens at f/2 - center of the etalon - 20 s exposure - 800 ISO - 17 September 2023 - etalon at operation temperature.
Etalon gap index :
The main assumption to be made in the measurement with a Ha lamp is about the index of the mica gap. A value of 1.575 is assumed (same value found by spectrometer on other mica-spaced etalons).
The resulting FSR is 26.4.
On this basis, the average value of FWHM measured over the full diameter of the etalon is 0.95 A (collimated beam).
The main assumption to be made in the measurement with a Ha lamp is about the index of the mica gap. A value of 1.575 is assumed (same value found by spectrometer on other mica-spaced etalons).
The resulting FSR is 26.4.
On this basis, the average value of FWHM measured over the full diameter of the etalon is 0.95 A (collimated beam).