(Copyright François Rouvière - Christian Viladrich)
Test setup
From left to right : Ha discharge lamp, diffuser, etalon, Nikon Z6 camera with a 85 mm f/1.8 lens on a XY platform.
Test of Coronado SM III 60 mm (tiltable)
Characteristics : - 60 mm aperture,
- 12 mm obstruction (i.e. 20%),
- CWL set by tilting the etalon.
Example of test image :
SM III 60 mm - Nikon Z6 - Nikkor 85 mm f/1.8 S - ISO 100 - 0.5 s exposure - 14-bit RAW
Qualitative assessment: - The small dip in the central fringe pattern shows that the
etalon is not tuned at Ha at normal incidence (red shift). It means that the etalon
should be tilted to be tuned to Ha. This is meant
to reject the Sun ghost image outside main image when observing in a
double stack configuration.
- The red shift in CWL increases slightly from the central spacer to the
outer edge of the etalon (see images below). However this drift is perfectly symetrical
around the central spacer.
SM III 60 mm - Nikon Z6 - Nikkor 85 mm f/1.8 S - ISO 100 - 0.5 s exposure - 14-bit RAW
Fringe system seen from different positions over the etalon (from left to right): center of etalon, 20
mm away from center, 25 mm away from center. The measurement of the
radius of the central fringe allows to calculate the delta CWL at
each position: 0.56 A, 0.58 A and 0.60 A.
Measurements: - FSR = 8.5 A,
- delta CWL at
normal incidence : from 0.56 A (to the red) near etalon central
spacer to 0.60 A (to the red) at 25 mm from etalon center
(equivalent to
0.77° tilt). Average value over a 46 mm circular aperture.
- FWHM < 0.50 A when measured over a 46 mm aperture and < 0.55 A extrapolated to full aperture.
Test of Coronado SM III 60 mm RichView
Characteristics : - 60 mm aperture,
- 12 mm obstruction (i.e. 20%),
- CWL set by varying the air gap thickness. This is done by rotating
the outside ring of the mount, which is turn applies a pressure on the
center of the etalon the etalon (RichView mechanism)..
- The CWL can also be tuned by the usual tilt mechanism.
When used in a double stack combiguration:
- the tilt of the first etalon is tuned to Ha,
- then the second etalon is stacked and its tilt is ajusted to have the Sun ghost image away from main image,
- finally the CWL of the second etalon is tuned on Ha with the RichView mechanism.
Example of test image :
SM III 60 mm RichView- Nikon Z6 - Nikkor 85 mm f/1.8 S at f/1.8 - ISO 100 - 0.5 s exposure - 14-bit RAW
Qualitative assessment: - The dip in the central fringe is larger compared to the SM III 60 mm. This means that the CWL is farther away from Ha at normal incidence (red shift).
- The red shift in CWL increases slightly from the central spacer to the
outer edge of the etalon (see figure below). This drift is perfectly symetrical
around the central spacer in three quadrants only (out of four). Still, the umbalance with the fourth quadrant is minimal..
SM III 60 mm RichView- Nikon Z6 - Nikkor 85 mm f/1.8 S at f/1.8 - ISO 100 - 0.5 s exposure - 14-bit RAW
Fringe system seen from different positions over the etalon (from left to right): center of etalon, 10
mm away from center, 20 mm away from center.
The measurement of the
radius of the central fringe allows to calculate the delta CWL at
each position: 1.20 A, 1.22 A and 1.25 A.
Measurements: - FSR = 8.9 A,
- delta CWL at normal incidence
: from 1.20 A (to the red) near the etalon central spacer, to 1.25 A
(to the red) at 20 mm from etalon center (equivalent to 1.11° tilt). Average values over a 46 mm circular aperture.
- FWHM < 0.53 A when measured over a 46 mm aperture and < 0.58 A extrapolated to the full 60 mm aperture.
