LuntLS40FHa

Overall results

	Test method	f-ratio	Surface sampled	FSR	FWHM	Delta CWL	Note
Lunt 40 mm (#2)	Ha lamp	collimated	full aperture 15 mm diameter	10.7 A 10.7 A	0.61 A 0.56 A	+ 0.32 A + 0.28 A
	Spectro (fi=fc=200 mm, 2400 l/mm, 7 micron slit)	collimated	7 microns x 6 mm	10.7 A	0.56 A	+ 0.38 A

The transmission curve of the etalon is a textbook Lorentzian.
The measurements with the Ha lamp and spectro are fully consistent.

Test of the etalon with the Ha lamp in diffuse light

Optical setup: Ha lamp => diffuser => etalon => Nikon Z6 with 85 mmS f/1.8 lens

Interference fringe pattern

Lunt LS40FHa - Interference fringe pattern - Nikon Z6 - 85 mm S f/1.8 Nikkor lens at f/2.2 (i.e. 37.4 mm aperture) - 12-bit RAW file - 0.5 s - 100 ISO - 30 November 2024

Comments:

- The interference patern is quite uniform with a nice Lorentzian profile for the rings.
- The mesurements from the different rings are very consistent.
- There is a very slight improvement in the FWHM when the aperture is stopped down to 15 mm.

Spectro measurement in a collimated beam

Optical setup: Sun => etalon => Solex with 7 microns x 6 mm slit, fc = f i =200 mm, 2400 l/mm grating, ASI 290, 0.0471 A/pixel dispersion

Top image : solar spectrum.
Bottom image : spectrum transmitted by etalon.

The transmission profile (= transmitted spectrum/solar spectrum) consists in textbook Lorentzian curves (curve fitting with Fityk).

FSR increases with the order of interference (as expected). Fringe #5 is on Ha. The small bump on Ha is probably an artefact of the measurement. Base on the comparison with neighboring fringes, a conservative estimation of the FHWM is 0.56 A.

The values for Ha (fringe #5) are :
FWHM = 0.58 A and FSR = 10.7 A.