Lunt Solar Systems utilizes the new Pressure Tune System, or Doppler True Tuning system in most of our Solar Telescopes.
The system works because the etalons used in the current Lunt designs are air spaced.
The center wavelength can be manipulated by several methods. Here are some Pro’s and Con’s:
Tilt Tuning: This changes the angle of the light at the interface of the high reflector/air layer, having the effect of moving the center wavelength toward the blue. Tilted Etalons are typically factory tuned slightly high of the H-alpha line so they can be tilted on band.
This process is fairly ideal for front mounted etalons where the F ratio of the Sun is within a pretty acceptable tilt/tune range of the Etalon. However, the etalon can only be tilted in one axis and significant tilt will lead to banding (a ripple effect) of the image. This effect is magnified when the Etalon is placed internally to the optical system.
Slight changes in barometric pressure and/or a change in altitude will effect the CWL due to the change in refractive index of the spacer layer caused by the change in air pressure. Air Spaced Etalon are Air Pressure Sensitive.
Heating: Heating has the effect of increasing and decreasing the distance between the Etalon plates by thermal expansion of the spacer material between the Etalon plates. While this is an effective way to tune and Etalon it does not allow for rapid Doppler shifting of activity.
Heating requires electricity.
Typically, heated etalons are solid etalons. In that the spacer layer is a solid layer of glass or Mica. This limits the size that these etalons can be made.
Because the Etalon is solid by design it requires an extended focal length. Typical solid Etalons need to be housed behind a f30 or greater system.
Because the F ratio of the system was expanded before the Etalon the image is generally highly magnified. The trade off is that the Etalon aperture would need to be large enough to accept the longer F ratio optical path so the f ratio could be reduced post Etalon for wide angle viewing. However, large solid etalon are VERY expensive to produce.
Doppler True Pressure Tuning: A method which solves most of the issues of tilt and heat systems is now described..
It should be noted that Lunt internal Pressure Tuned Etalons are matched to the Aperture and Focal Length of the Telescope. Our collimating system allows for the full aperture of the optical path through the Etalon at the optimized position. This allows us to re-focus the FULL optical path back down to the image plane, allowing for wide angle (full disk) viewing. Off course, various eyepieces can be used to zoom in on desired features. Our internal Etalons range in size from 15mm to 100mm.
The image at left shows the basic outline of this system. The internal etalon is at ambient pressure. The plunger of the pressure cylinder has just been removed and replaced. The factory tuning of the etalon is slightly low, putting the Center Wavelength (CWL) at the red wing of the Hydrogen line. This provides a view of less energetic features in the Chromosphere.
The diagram shown at left indicates that the air pressure inside the sealed chamber has been increased. At this point the CWL of the bandpass is at 656.28nm. At this position we are looking at the center of the Hydrogen-alpha line and the energy associated with that wavelength.
The sealing of the cavity is done utilizing the collimating and refocus lens so that the etalon itself is isolated from external pressure.
The piston applies from ambient to a pressure that is equivalent to taking an etalon from -500ft to 12,000ft above sea level.
This has the added benefit of making the etalon system altitude insensitive.
In addition the etalon can be used from -0 to 150 degrees Celsius due to the fact that the tuning can compensate for the very small changes that heat would have on the “feet” of the etalon.
However, it should be noted that the Blocking Filter has a narrower range of temperature range due to it being a dielectric filter.
Pressure Tuning removes the compromises associated with internal tilt systems. Only very small adjustments to the tilt of an internal etalon can be done otherwise the etalon system will begin to suffer from the off axis rays of the re-collimated beam causing observable banding on the CCD.
People have noted that in internal tilt systems the CWL is very sensitive to even small adjustments of the tilt wheel creating banding effects while imaging for example.
By removing the need for tilt we have placed the etalon in the most optimized position possible.
We install a very accurately tuned etalon. This etalon is tuned to the red side of the CWL. Given that it is already tuned to the red, the user has the ability to shift the tune of the CWL to the Hydrogen-alpha line and then Doppler tune to the blue or back thru to the red.
The diagram at left shows the system has been fully pressurized. This pressure is equivalent to about a very high altitude change.
The air inside the sealed chamber has been compressed due to the reduced volume. As a result the refractive index of the air has increased and caused the CWL of the etalon to move to the blue or high energy side of the Hydrogen wavelength.
Due to the fact that there is no tilt involved, the image field remains flat and very precise.