For those who have googled “what’s an etalon?” the returned results outlining complex looking optical equations, mathmatical relationships, and technical references probably didn’t do much to answer the question. Unless you are an Optical Physicist of course. (I love that stuff)
 
An etalon refers to an interference type filter typically used in Solar Telescopes because of the desire for an ultra narrow bandpass.

An etalon is probably one of the simplest designs for an optical filter utilizing some of the most precise optical specifications. Due to it both being simple and the need for precision, there are many compromises that can influence the quality of an etalon filter.

An etalon is comprised of 2 flat and parallel optical surface that have been optically coated with a high reflective dielectric layer with the high reflector layer peaking at the desired bandpass point for best results. These optical surfaces are seperated by a gap. This gap can be either air or a solid material. The light resonates in the gap by internal reflection off the highly reflective layers on the surfaces. Thru interference at this gap, only light meeting the correct angle of incidence to the surface and is not “interfered with” can pass, all other light is lost.

The main parameters that define an etalon are:

Bandpass: This is the width of the curve that defines the transmitance of the filter at 50% of the total transmision of the filter. An etalon’s transmission has a broad base and a sharp peak. Typically the peak transmission should be between 80-90%, so the bandpass of the etalon will be measured between the 40-45% points. For Solar applications it is generally accepted that the lower this number, the better. Typical Solar Etalons have a bandpass of <1.0.

Which is Better??

The defining difference between an External etalon vs an Internal etalon has typically been Price vs Performance… Well that gap is getting very narrow.

External etalons are used on the front of an existing scope, and the user typically asks that the etalon be full aperture. Internal etalons are situated internally to the scope, allowing for a full aperture objective to pass the light thru a smaller etalon due it being placed further down the optical path.

The basic rule of thumb for an etalon is the price is a function of the area. That adds up fast.

Another rule of thumb that I have heard is that the acceptance angle of an airspaced etalon is 0.5 degrees. What one should keep in mind is that, in any case, an etalon specified by surface reflection at the interface of the internal plates, thickness of gap, and gap medium only, will NOT perform to that optimal specification should there be anything other than collimated light passing thru it.

F30 does not apply specifically to airspaced etalons due to the higher index material utilized at the gap of the solid design. In most cases for solar this is solid material is Mica, and in some cases it is fused silica. All solid etalons that I have made for space flight have been fused silica.

The Sun is not a point source. In fact, the Sun is a very large extended source. It may look small at 93M miles away, but it is 865,000 miles in diameter. To put that in perspective, if the Sun were an objective and the front of your filter was the focal plane (I don’t think we need to look at filter size in this example ;) the system would have an F107 ratio. That’s still not entirely collimated..

Seeing the Sun at Sunset or Sunrise in relation to a distant mountain also puts it’s size into perspective.

Even so, the best position for an etalon filter is on the front of the scope. However, this etalon is still compromised from ideal by the Sun’s angle. These errors are small.
The use of tilt at the front etalon is simply a method of shifting the bandpass (the frequency of light passing thru the etalon). Due to the acceptance angle of the etalon, this tilt must remain very small. The more the tilt, the wider the bandpass becomes, and the non-collimated Sun angles begin to play a factor.

Tilting was introduced many years ago as a way of allowing etalons to be mass produced at slightly above the Ha line. They could then be tilted on line to accomodate the users changes in altitude and changes in barometric pressure at any given location. Without tilt, Solar Telescopes would be many times more expensive than they are today.

The move to internal etalons was done in order to bring the manufacturing costs of the Solar Telescopes down to less than 1k. By utilizing an etalon that was half the diameter of one required for the front, the cost of the etalon was cut by more than half. Even with the added cost of re-collimating and re-focus lenses, the system realized significant gains in affordability.

One of the huge benefits to internal etalons has been the fact that we only deal in one wavelength of light. This allows for the optimization of the internal lens system to remain fairly simple and straight forward. Ha! Right.. Simply aligning and collimating 4 optics now..

The evolution of internal etalons has taken several steps.
The original systems had internal fixed etalons that sat behind a re-collimating lens set (these lenses work to bring the light rays back to parallel). The light from the etalon was then re-focused back to the image plane.

These systems cannot compensate for changes in altitude and air pressure. Taking this system from sea level to 8,000ft would not work.

The next step is the introduction of some tilt. This works similar to the tilt on the front of a system. However, due to both off axis light rays that could not be re-collimated with a simple lens, and the angles created by the Sun’s size, the etalon can only be tilted thru a fraction of a front mounted system. These etalons have been designed to be only very slightly higher than the Ha line so minimal tilt is required. Front mounted etalon designs can be used for this process, but recent innovations to coatings on the etalon cannot be used due to the new design’s slightly narrower acceptance angle.

These systems maintain a very good flat field of narrow bandpass due to the optimization of the collimating set at Ha over about 80% of the entire field. Etalons with a theoretical bandpass of 0.65A are used, but are specified internally at 0.75A over the entire field due to the compromise of the ray angles. The center portion of these systems is typically better than 0.7A.

The current technology is the Doppler True tuning design.
The technology came about for the simple reason that tilted internal etalons are not optimized. The ideal design calls for an internal etalon (to reduce cost) in a perfectly collimated system, without the need for tilt, while also compensating for altitude and pressure changes (or being isolated from).
The new system may be simple, but it addresses 99% of all the relevant issues.

The incorporation of the air pressure system (see other blog entries for description of how this works ) has allowed us to improve the technology of the high reflector layers.
For a given optical system, we have been able to modify the design of the HR. We are still working on tweaks and changes, but have already seen some improvement to resolution, field flatness, and contrast. The new designs are moving in an area that provides for blacker backgrounds and sharper features. There’s still room for improvement..

So which is better? External or Internal??

The jury is still out on that one. It is still a function of Price vs Performance.

The new evolution of internal design has certainly gone a long way to improving the performance of internal systems. However, the added cost of the pressure system has now made the external system seem more affordable.

In my opinion the best option is to buy as much double stack system as you can afford ;)

All kidding aside. Going with an internal PT system is probably the most ideal start. Buying a system that you know you can DS later without much problem is good. The dedicated system is fully optimized and ready to go. We are familiar with these systems and can help with technical issues. We test every system that goes out. I cannot test a Lunt Filter on Your scope. I cannot answer questions about in focus etc. for Your scope.

The removal of the tilting from the internal system is providing stunning results. The ability to Doppler Shift instantly to and away from you adds a 3D feel to the image that cannot be matched. This technology has certainly improved the dedicated system.

Now… What about all that tilt on the front??

The see-saw of performance is about to swing again. Lunt will be introducing the front mounted filter system with the Pressure Tune capability in mid next year (or maybe sooner ;). These systems will be manual control and also remote operable. For those that don’t have 6 foot arms for your LS160F on that really long scope, this will certainly be a must.

Next blog: What is an Etalon and how do we make them?

About Lunt Solar Systems LLC…

Lunt Solar Systems is a manufacturing and sales facility located in Tucson, Arizona. Lunt Solar design, fabricate, assemble, and test solar telescopes and solar filters. Whether you are looking for a dedicated solar telescope or a solar filter for attachment to you own astronomy telescope, we can help. Solar telescopes models start at $499 for an LS35T (telescope) or the LS50F (filter) all the way thru the 152T and the 160F. Solar observing is both fun and educational. Don’t miss out on Solar Maximum. Feel free to contact us with question or comments and we’d be happy to help.

The New Lunt Solar Forum is live..

We are looking for a few enthusiasatic helpers who are familiar with this type of forum to do moderating and help set up the catagories etc..

Please let us know if you can help out by sending an e-mail to sales@luntsolarsystems.com

Working with us on the Lunt Forum will entitle you to discounts on Lunt Products. We can also arrange to send new products to you for test and review.

Thanks in advance…

We certainly appreciate being recognized by Sky and Telescope for a Hot Product of 2010 Award.

This award has been presented to Lunt Solar for the LS60T/Ha/PT system.

The Pressure Tuned System received an overwhelmingly positive response when shown at NEAF, RTMC, and PATS in 2009. I will be writing a full editorial about the Doppler True tuning system with insights into how to get the most from this system, and why this new technology is such an innovation to the Solar Telescope.

Thanks again to all the folks at Sky and Telescope for picking Lunt Solar and the LS60T/PT. I am very proud of this scope and enjoy testing them every chance I get.

About Lunt Solar Systems LLC…

Lunt Solar Systems is a manufacturing and sales facility located in Tucson, Arizona. Lunt Solar design, fabricate, assemble, and test solar telescopes and solar filters. Whether you are looking for a dedicated solar telescope or a solar filter for attachment to you own astronomy telescope, we can help. Solar telescopes models start at $499 for an LS35T (telescope) or the LS50F (filter) all the way thru the 152T and the 160F. Solar observing is both fun and educational. Don’t miss out on Solar Maximum. Feel free to contact us with question or comments and we’d be happy to help.

I’ll have to get Rikki to do a little write up about what it’s like living in Peru.

Here is an image of Rikki with Jesus, the guy who drives her too and from “work” every day.

Is that a NEAF shirt that Jesus is wearing??

As some people know, Rikki lives in Peru. She runs the Lunt office from a small office in the town near where she lives. It must be nice to be chauffeured to and from work.

When I get into the office in the morning, I see the mouse cursor on her virtual terminal moving around. She uses an internet based program to chat with Jen in the office all day for free!! You know she’s here all day, but you never see her…

I won’t say that it hasn’t been without it’s small glitches, but I have to say that the virtual commuting really works even over thousands of miles to a developing country..

What else does Rikki do in Peru and how did we get this worked out?? I’ll have to get her write up a little blog and send in some pics..

Stay tuned..

About Lunt Solar Systems LLC…

Lunt Solar Systems is a manufacturing and sales facility located in Tucson, Arizona. Lunt Solar design, fabricate, assemble, and test solar telescopes and solar filters. Whether you are looking for a dedicated solar telescope or a solar filter for attachment to you own astronomy telescope, we can help. Solar telescopes models start at $499 for an LS35T (telescope) or the LS50F (filter) all the way thru the 152T and the 160F. Solar observing is both fun and educational. Don’t miss out on Solar Maximum. Feel free to contact us with question or comments and we’d be happy to help.