Instrument: SPEX 1.26M Monochrometer
Here is an image of one of our Etalons taken by an independent lab.
Specifications: Lunt Solar 50mm Etalon
Center Wavelength: 656.28nm
Peak Transmission: 79.89%
Bandwidth: 0.7 Angstroms (FWHM)
FSR: 12.5 Angstroms
Our etalons were tested in a blind test along with several etalons from other manufacturers.
A) One of the other manufacturers had an etalon with a peak T of approx. 60%, a Bandwidth of 0.7 Angstroms, and an FSR of 10.5 Angstroms.
B) One of the other manufacturers had an etalon with a peak T of approx. 0.63%, a Bandwidth of 0.68 Angstroms, and an FSR of 15.5 Angstroms.
It should be noted that all etalons were tested by a 3rd party ISO certified test facility who had no knowledge of the manufacturer of any particular sample they received. The samples were taken at random from products that were either on the shelf at Lunt or purchased from a dealer. We have not published the actual scans of the competing etalons and can only talk to the results we received.
The next step will be to have additional samples tested for uniformity across the field of view while being tuned.
We would like to prove that there is NO differential change to the BW and uniformity of a Lunt etalon across the aperture while being tuned.
We invite competing businesses to publish their etalon scans and resulting data. Lunt would be more than happy to publish their data in this blog as a side by side comparison.
Lunt etalons are tuned by changing the refractive index of the air in the etalon chamber. We apply no heat or mechanical/physical compression to our etalons. Lunt barometric pressure change (PT tuning) is 100% uniform and will NOT change the uniformity of the etalon aperture when Doppler Tuning.
Physical compression tuning would need to be applied to precision optical tolerances. And etalons with a center foot would need to have the edge and the center compressed to 100% accuracy in order to prevent changes to the BW across the aperture during tuning. It would also require that the “feet” of the etalon be EXACTLY the same size. Physical compression tuning relies on the Young’s Modulus of the glass used to separate the etalon plates. A foot that is twice the size of another foot would only compress ~1/3 of the distance. So even small changes in foot size have a big effect on uniformity of tuning. Any change to the uniformity of an etalon across its aperture is a widening of the bandpass and loss of detail.
It should also be noted that some competing etalons have center obstructions. These obstructions limit the magnification that these systems can be taken to before the center obstruction becomes evident. I have spoken to this in another blog.
Solid etalons would require an optically precise spacer layer. Generally speaking solid etalons are manufactured with a spacer layer that is then polished to high optical tolerances. However, some solid etalons are manufactured with a Mica type spacer layer that is adhered to the etalon plates via a bonding process. (non optical contacting)
Solid etalons need to be either heat tuned or tilt tuned. Heating a solid etalon takes time. The larger the etalon, the more time it takes to reach equilibrium. The heat is generally done from the edge, so the spacer layer expands at the edge first. As the heat moves through the etalon, the spacer layer reaches equilibrium and the etalon comes on desired CWL. At any point the etalon is NOT at equilibrium, the etalon has a wider bandpass that results in loss of detail. The faster you try to heat the etalon to reach the desired CWL, the higher the differential BW will be.
Solid etalons are generally small. They need to be used at long focal lengths. Large aperture systems have a limited field of view when using a less than ideal etalon size. (Lunt manufacture etalons up to 140mm).
Doppler shifting a heated system is not instant. Moving through the wings of h-alpha can take many minutes to 10s of minutes (depending on etalon size). Fast moving Sun activity cannot be captured across the wings of H-alpha in seconds. As imaging systems gets faster, so must your imaging equipment.
Due to the acceptance angle (the amount the CWL changes as a result of angle) solid etalons are generally not tuned by tilt. Titling will result in rapid “banding” of the image. This is especially noticeable when imaging.
Lunt etalons have NO center foot. We have developed advanced methods of producing precision flat etalons plates to the same exacting standards that are needed by professional facilities.
Tuning a Lunt etalon requires NO heat and the tuning is instant.
Tuning a Lunt etalon requires no differential compression tuning by mechanical process.
Tuning a Lunt etalon is repeatable due to it being housed in a sealed chamber. Compression tuned etalons are effected by weather, barometric, and altitude changes. “Tuning” changes with the environment.
As part of my blog the opinion stated are simply my opinions. The data I received from the test facility was not surprising given what I have seen through various eyepieces. I invite anyone to simply take the information I have provided and ask questions of other manufacturers.
As always, we are here to answer any questions and listen to any comments.
If you have a question for the Lunt blog that you would like us to address, please send an e-mail to Lunt Solar.