Coronado SM 40 mm: qualitative evaluation
Optical setup (from left to right)
: Ha lamp, diffuser, Coronado SM 40 mm etalon, ASI 290 camera with 4 mm FL
f/1.2 lens (12-bit acquisition, 90 s cumulated exposure).
The
diameter of the area sampled by each position of the camera is given by
the aperture of the lens. Here : 4 mm /1.2 = 3.3 mm.
Three positions are sampled along a radius : one close to the edge
of central spacer, one half way between the edge of the central spacer
and the edge of the etalon aperture, the last close to the edge of the
etalon aperture.
For each position, the profile of the central fringe is drawn in order
to have a better assessement of the CWL offset compared to the Ha line.
Scanning the etalon along a radius from left edge of the aperture to the edge of the central spacer:
The peak of the central interference fringe is a bit
flatten on the left edge of the etalon and at half distance from center
spacer.
A small dip appears close to the central spacer betraying a slight
offset of the CWL.
Scanning the etalon along a radius from the central spacer to the right edge:
Again, there is a very slight CWL shift close to central spacer. However, the
CWL is right on Ha line from the right edge of the aperture to half distance from the central
spacer.
Scanning the etalon along a radius from central spacer to right edge:
There again, there is a slight CWL shift close to
the central spacer.The CWL is right on the Ha line from the bottom edge of the aperture to half
distance to central spacer.
Conclusion
: This etalon has quite an uniform CWL accross its aperture. There
is a minimal and negligeable CWL offset in a very small ring
around
the central spacer.
Coronado SM 40 mm: qualitative evaluation (FSR and FWM)
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 - 1/4s - 200 ISO
Results:
FSR = 12.1 A and FWHM = 0.74 A (average value over the full aperture of the etalon).