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引述自Lunt Solar System：
http://luntsolarsystems.com/blog/intern ... erformance
Below is a graphical representation of the performance of an internal Etalon.
In response to a few inquiries that we have received I have decided to blog a little about the pro’s and con’s of internal etalons. I have written a few papers in the past that have been published in numerous online websites, but they are currently either archived or difficult to locate.
There is no doubt that in order to achieve the BEST performance possible for a PERFECT etalon, that etalon needs to be placed in “as-collimated” a light path as possible. Typically this would be the front of a telescope. However, while most of us understand that the light from distant stars is essentially collimated, the light from our Sun has an F ratio of ~109. So a perfect etalon is compromised even on the front.
The size of an Etalon has a very significant impact on the price. The cost to produce any Etalon basically goes up by the area of the surface being polished. This is mainly due to the fact that the polished surface of the Etalon IS the deciding factor for the Etalon’s performance. Maintaining 1/50th wave over 1″ is relatively easy when you consider the peak to vally error over the smaller distance. However, maintaining that same specification over 6″ is very much mroe difficult. I like to ask how many 1″ parts fit into the same area as a 6″ part? Yes, it is that much more difficult..
Given that the performance of the Etalon is a function of the surface accuracy you can probably understand that larger etalons, while collecting more light, may have higher ultimate bandwidth over the entire surface. Lunt have a proven track record of producing some fo the largest, most precise flats in the world.
In order to counter act some of the above issues and provide large aperture high performance Solar systems at reasonable cost, Lunt have gone the route of installing smaller Etalons internal to the optical system.
Given that the Etalon itself is the driving factor to the performance and cost, we feel that it is an overall advantage to have an Un-obstructed, highly accurate Etalon internally installed.
Original designs called for the internal Etalon to be placed within a re-collimating lens, and a re-focus lens set. These lenses are designed specific to a telescope design and take into account placement, F ratios, and wavelength.
Because the Etalon is AIR spaced, it is effected by changes in AIR pressure such as changes in altitude and barometric pressure due to changing weather conditions. In order to offset these issues, the Etalon was designed to be slightly high, such that the end user could effect the Etalon (tune) by a small amount of tilt to the Etalon as it related to the collimated light going thru it.
The idea is was taken from the fact that tilt has been the primary tool used to “tune” Etalons placed on the front of a telescope.
Lunt have learnt that internal Etalons are highly sensitive to tilt. I would guess that internal Etalons are about 5-6 times more sensitive to tilt than external Etalons. The reason comes down to the inability (physics) to PERFECTLY re-collimate the internal rays of the light path after the objective. Even though we are monochromatic, the compromises of light cone, image rays etc. cannot be overcome. Only optimized.
The result of overtilting of an internal Etalon is the tightening of the bandpass in a “band” perpendicular to the axis of tilt. This is due to the off axis light rays being very quickly exagerated at the point of tilt and the hinge, while remaining “normal” (not changing) along the perpendicular center axis. Most people ask why the entire Etalon cannot perform at the level under the band? The issue is that the Etalon has been over attenuated, and my feeling is that while the band did increase by a little, it becomes increasingly more obvious as the area outside it deteriorates quickly..
So, Lunt are producing internal tilt Etalons that are highly specified for internal systems. While a given system will work for a +/-2k feet change in altitude, people living at higher elevation need systems that are specific to their location.
The introduction of the Pressure Tuned Systems was a dramstic improvement to the performance of internal systems. I feel that these systems are currently at a level that allows for the least amount of compromise, and provide a level of performance that has never been available before. This technology has allowed us to make these improvements without a significant increase in cost to the system.
The Etalon is now placed in a sealed chamber. I will not discuss this in detail here because I have done so in other blogs.
The Etalon is Un-Obstructed. This allows for maximum throughput of the collected light without the reduction in contrast typical to obstructed systems. It also means that the Etalon is NOT contact in any way mechanically. This removes mechanical tolerances and their resultant errors from the precise optical system. Systems that we experimented with that had mechanisms that effected the Etalon directly often created “astigmatism” to the tuning of the Etalon, and “pinching” which often widened the bandpass.
The Etalon is precisely aligned to the optical axis. By fixing the Etalon perpendicular to the axis of light we are centering the Sweet Spot. However, in order to eliminate ghost images from other surfaces or other optics we are forced to tilt other filter systems. ie: the ERF in front of the Etalon housing that has a highly reflective surface due to the nature of it’s job. This tilt WILL have a slight offsetting effect to the sweetspot.
The Etalon is in a sealed cavity. This elimates the effects of altitude and barometric pressure changes. Allowing our systems to be used from sea level to 10k+ feet. We currently have a LS152T on the top of Hawaii. Being used at 200X magnification (lucky guys).
Titled systems need to be tilted more or less to allow for these changes. This also effect the persons ability to double stack later.
Doublestacking.. Let’s not overlook the importance that pressure tuned systems have to doublestacking.
It is impossible to determine the position of a tilt system (how much tilt) for a customer’s Etalon. Tilt is important to doublestacking becuase when we DS, we need to narrow the bandpass, but ALSO eliminate ghosting that is created when the 2 very highly reflective Etalons are put in close proximity to each other. Imagine for a moment 2 highly reflective Etalons placed perpendicular to the optical axis, their surfaces being parallel to each other.. You would see infinate ghosts as the light bounced back and forth.
Etalons placed in a PT systems are perfect to the optical axis and we therefore know that we can use minimal tilt to the secondary Etalon to remove the ghosts. By using minimal tilt we also reduce potential banding issues, and provide maximum performance.
Tilting an Etalon brings the CWL on band, but does result in a slight widening of the bandpass.
I often see when testing that the ultimate performance is found when there is no tilt. However, that secondary ghost sure is annoying.
Despite everything that I have said above.. The internal system is not without it’s own compromise. While we have done everything we currently can to perfect the system, we are still dealing with the physics of light.
An imaging system cannot be perfectly re-collimated after passing thru an objective.
Compromises need to be made to size of sweetspot, bandpass, and contrast as a function of bandpass specification/performance.
It is my opinion that the smaller systems require a larger sweetspot at the slight compromise of bandpass. Given that the smaller aperture results in a minimal effect by seeing conditions and air turbulance, I like to see a fairly bright image that provides an nice overall Full solar image with a balance of proms and surface (typcial to <0.7Angstroms).
However, as the aperture increases we are faced with a few more decisions.
Increased aperture, in my opinion, allows the user the advantage of light due to image scale. It allows the use of higher magnifications under ideal conditions to see within the details of the Sun without significant loss of brightness.
Increase in aperture has the disadvantage of being effected by poor seeing conditions.. However, I would prefer to know that while my system may not have ultimate performance under less than average conditions, it will provide the best resolution and contrast under ideal conditions, AT HIGH MAGNIFICATION. If it is not your intent to use the systems at high magnification, you do not need a large aperture scope.
ALL Etalons are created equal.. What???
All Etalons at Lunt are created using the modified refelectivity, spacer gap, materials etc.. The important difference is the maintaining of the higher quality surface (in order to maintain the theoretical bandpass) required for the higher end systems.
If Lunt did produce custom Etalons per systems, we would NOT be able to mass produce and the cost of these systems would be many times higher than what it currently is. By maintaining high polishing standards accross the board and only making changes to the R (R effects bandwidth), we can provide very economical high performance systems.
Under low magnification the user will note a sweetspot in the eyepiece. If the Sun is placed in the center and an eyepiece is chosen that allows the Sun to be about 40% of the field of view, you should see uniform detail around the entire disk. In some cases the sweetspot maybe slightly off center. This is an unavoidable result of tilted optics and mechanical tolerance holding those optics.
With the Sun at about 40% of view, you will have an dark area around the Sun equal to about a Sun radius..
Moving the Sun around in the field of view you will note that the proms will drop off quickly as the image moves to the edge of the field of view. This is due to the off axis rays becoming greater than the acceptance angle of the Etalon. This is NOT the result of a poorly tuned Etalon (however, you can re-tune to get this detail back at the loss of the center (basically de-tuning the Etalon to match that angle)), or the result of mis-aligned optics.
Note that when you magnify the image you are using a much smaller area of the field of view and will note that the sweetspot quickly disappears as the heart of the system is utilized.
Note: the narrower the bandpass of the Etalon (higher R for narrower bandpass) the faster, or more pronounced the sweetspot.
The image at top (had to get there eventually
represents the ideal internal system. We try to provide what we refer to as a slight “bird” shape. The Sun would sit within this area provided that the magnifiaction was set such that the Sun was 40-50% the field of view. The center comes down very slighly in order “push” out the sweetspot.The narrower the bandpass of the Etalon, the narrower it’s acceptance angle. Therefore, lower bandpass systems will deteriorate faster at the edges.
The above deterioration can be ofset by placing a wider bandpass Etalon in the system. ie: if we placed a <.08A Etalon in the 152T, we could increase the sweetspot to 70-80%. Hmmmm, I designed the 152T for high resolution, high magnification viewing. Research of the active areas, and NOT for a quick overview of the entire Sun. That can be done with a 60/PT or a 100/PT. I believe that I would be doing a dis-service to the customer if I provided a systems that worked okay at low mag by simply showing a uniform, if less contrasty Sun, but showed very little detail close up. There’s a reason the University of Hawaii moved the 152T to the top of the volcano and typically use it at 100-200X.
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