Coronado SM40#2 test

Test of the etalon with the Ha lamp in diffuse light

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

Interference fringe pattern

SM40 #2 - Interference fringe pattern - Nikon Z6 - 85 mm S f/1.8 Nikkor lens at f/2 (i.e. 42 mm aperture) - 12-bit RAW file - 1 s - 100 ISO - 10 January 2022

Some comments on this filter:

- This tested examplar is less uniform than SM 40#1, with a larger CWL drift accross the aperture (from central spacer to the outer rim).
- A fainter secondary fringe system can be observed when the lamp is seen with the etalon side of the filter toward the lamp. This is clearly an internal reflection within the filter stack. The reflection disappears when the etalon side is toward the eye, as it would be when observing the Sun.

SM40 #2 - Reflected fringe pattern seen when the etalon is in the "reverse" position (RG630 side to the camera lens) - Nikon Z6 - 85 mm S f/1.8 Nikkor lens at f/1.8 (i.e. 42 mm aperture) - 12-bit RAW file - 1/2 s - 100 ISO - 18 January 2022

Results:

The filter was measured with lens f-ratio of f/2 (42 mm aperture), f/4 (21 mm aperture) and f/8 (10.5 mm aperture):

Test of the etalon with Sol'Ex spectrometer

Optical setup: Sun => Coronado SM 40 => Vixen 55 mm f/8 fluorite refractor => Coronado SM40 (with.without BF5) => Sol'Ex => ASI 290 (0.0753 A/pixel dispersion)

Three succesive images :

- SM40 alone,

- SM40 + BF 5 diagonal,

- SM40 + BF 5 diagonal, but with different visualisation threshold in order to make visible de secondary transmission.

The fringes profiles are fitted with a Lorentian curve:

Taking the average of the two peaks surrounding the Ha transmission peak:

FWHM = 0.76 A
FSR = 11.8 A

NB : we get the same results when the etalon is very slightly tilted to the blue side of the Ha line.

Test of the etalon in collimated Ha light

Optical setup: Ha lamp => Fluorite 55 f/8 used as a collimator => Coronado SM 40 => Nikon Z6 with 85 f/1.8 lens at full aperture

1/30 s - 100 IS - 14 bit acquisition - RGB - Linear vizualisation - 17 Jan 2022

1/30 s - 100 ISO - 14 bit acquisition - Red channel - Linear vizualisation - 17 Jan 2022.

Interpretation of the image:

Non uniformities in Ha transmission appears as changes in brightness level. The bright areas correspond to areas where the transmission of the etalon in Ha is the highest.

If the etalon was completely uniform, the brightness level would be the same over all the surface of the etalon.

Small scale linear defaults (scratches, inclusions, etc.) can also be seen as well and small scale polishing default (circular pattern) and surface roughness. These defaults have no impact on the image quality.

Scanning the etalon CWL

In the following set of images, the etalon is tilted progressively from left to right.

- Tilting the etalon moves its Central Wavelength (CWL) to the blue.

- For each tilt position, the area whose CWL is on Ha is the brightest. So tilting the filter is a way to scan the CWL over the aperture of the etalon.

- The bright areas in the image with no tilt are the areas which are on Ha at normal incidence (no CWL offset).

- The areas which require more tilt to become bright are the area farther from Ha (their CWL is offset from Ha to the red side of the spectrum).

- The central image is the most balanced. This is the one closest to the optimal tilt (most of the etalon aperture CWL is on Ha). The tilt is 0.9°, equivalent to a delta CWL of 0.8 A, which is close to the more accurate value found in the test in diffuse light.

Etalon tilted with increasing angle from left to right. 14 bit acquisition - Red channel - Linear vizualisation - 23 Jan 2022

We note symetry of the images on each side of the optimal tilt: bright (or dark) areas in DSC_1980 turn dark (or bright) in DSC_1992.
Finally, the area at 12 o-clock position and arround the central spacer need a tilt of about 0.7° to be on Ha, while the outer rim of the etalon need a tilt of about 1.1°, meaning its CWL is farther to the red side of Ha.
Accordingly, we can guess than the thickness of the central spacer is smaller than the thickness of the spacers on the rim.

It is probably possible to built a map of the CWL drift accross the etalon aperture from this type of scan.

Technical note on the test in collimated Ha light beam :

The following two images were taken with the etalon tilted for the best overall transmission in Ha. On the left side, a diaphragm of 1.2 mm was used at the focus of the 55 mm f/8 refractor while a 5 mm diaphragm was used on the right image, resulting in a lower separation of the CWL variations. Still, the overall CWL variation map is about the same.

Test of Coronado BF 5 diagonal with Sol'Ex spectrometer

Test n°1 (12 Jan 2022)
Optical setup: Collimated Led source => Takahashi 106 FSQ => 1.8x Barlow lens => Coronado BF5 diagonal => Sol'Ex => ASI 290 (0.0753 A/pixel dispersion)

20.5 C temperature.

Results

FHWM = 6.9 A

Bandwith at 10% peak transmission = 11.8 A

2-cavity filter

Test n°2 (13 Jan 2022)

Optical setup: Sun => Fluorite 55 mm f/8 refractor => 1.8x Barlow lens => Astronomik 12 nm Ha filter => BF 5 Coronado BF5 diagonal => Sol'Ex => ASI 290 (0.0753 A/pixel dispersion)
Ambiant temperature (in the shadow) = 7°C

Transmission is calculated as the ratio between the spectrum transmitted by the BF5 and the solar spectrum maesured without the BF5.

NB:

- The dent at 6562.8 is an artefact.

- The incident light beam falls onto the BF5 at f/14.4. When measured at f/8, the FWHM enlarges slightly to 7.2 A.

Results

FHWM = 6.9 A

Bandwith at 10% peak transmission = 11.5 A

CWL offset : +1.5 A

2-cavity filter

	Test method	f-ratio	Surface sampled	FSR	FWHM	Delta CWL	Note
Coronado SM40 etalon #2	Ha lamp	collimated	Full aperture	11.9 A	0.95 A	0.9 A	variation of CWL from central spacer to outer rim of the etalon
	Sol'Ex spectro	f-155	10 micron x 4.5 mm	11.8 A	0.76 A
BF 5 diagonal	Sol'Ex spectro	f-14.4	10 micron x 4.5 mm	Not relevant	6.9 A	+1.5 A	2-cavity filter