The Sundancer etalon was measured using four different approaches :

(1) Sol'Ex spectrometer in collimated beam (f/115 beam):
        - The etalon is exposed at normal incidence to the Sun light (no optics between the Sun and the etalon),
        - Temperature regulation is ON (Sundancer BF and telecentric lens are removed).
        - Measurement is done over a 10 micron x 4.5 mm surface, accordingly CWL and FWHM non uniformities are not captured.

(2) Sol'Ex spectrometer with f/24 telecentric beam:
        - Sun is the light source,
        - The Sundancer with its 3x telecentric and BF (temperature regulation is ON) is placed at the focus of a 55 mm f/8 refractor.
        - Measurement is done over a 10 micron x 4.5 mm surface, accordingly CWL and FWHM non uniformities are not captured.

(3) Interferometry using an hydrogen lamp:
        - Temperature regulation is ON (Sundancer BF and telecentric lens are removed).
        - The full aperture of the etalon is sampled.

(4) Ha collimated beam:
        - The Sundancer (without its BF and telecentric lens) is exposed to Ha collimated beam.
        - The full aperture of the etalon is sampled.

Futhermore, the BF was tested with Sol'Ex spectrograph (5).

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	Test method	f-ratio	Surface sampled	FSR	FWHM	Delta CWL	Note
Baader Sundancer II	Interferometry with hydrogen lamp	collimated	Full aperture		1.1 to 1.2 A
	Sol'Ex spectro	f-115	10 micron x 4.5 mm	18.1 A	0.90 A
	Sol'Ex spectro	f-24	10 micron x 4.5 mm		1.1 A		consistent with FWHM = 0.9 A in collimated beam
Blocking Filter	Sol'Ex spectro	f-8 (non telecentric)	10 micron x 4.5 mm		10.9 A	- 2.0 A	3-cavity filter

Three successive images are presented:
- (1) solar spectrum arround the Ha line,
- (2) solar spectrum transmitted by the Sundancer etalon with temperature ON in a f/155 telecentric beam, the etalon is normal to the incident light.
- (3) X profile of the  spectrum transmitted by the Sundancer.

The profile of each fringe is fitted with a Lorenztian function (with Fityk) in order to measure its position and its FWHM. The measure is done directly on image (2), using a profile along the X axis (only one pixel width). There is no need to divide this by the solar spectrum because of the lack of strong solar lines near each fringe. The measured FWHM is similar to one fringe to the other ± 0.05A.

The CWL is not perfectly centered on Ha. No effort was taken to tune the etalon on the Ha line with the thermoelectrical unit, since there is no impact on the measurement of FWHM.

As with the spectrograph Sol'Ex, we note the presence of a faint secondary fringe system, possibly due to a lack of blockage of the polarizing filter (the mica being bi-refringent a polarizing filter is used in the stack to cut "extraordinary" rays):


Visual inspection shows that the CWL is not  uniform over the aperture of the etalon. Furthermore, the width of the fringes is not uniform.

The two inner finges are curve-fitted with a Lorentzian function in order to calculate the peak position of each fringe and the FWHM of the etalon.
For the calculation of the FWHM of the etalon, we need the value of the FSR. This value was taken equal to 18.1 A from the direct measurements with Sol'Ex spectrograph. This is consistent with an index of 1.56 for the mica spacer.

The measurements were made over the full aperture of the etalon, 13 mm aperture and 9 mm aperture, as well as the etalon heated or cold. Results remain unchanged to about ±0.1 A.


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