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.

A HR LS75F Un-obstructed filter with matchable BF at <0.6Angstroms (single stack).

During the process of R&D I have been working with new coating technologies that are very low stress and very high finesse. As you know, these coatings will be made available in the new LS152T.

One of our recent projects was the manufacture of some un-obstructed larger 75mm Hydrogen-alpha etalons. My intention was to build one and keep it for myself.

Due to batch costs, it was just as costly to make one as it was to make a few :)

As luck would have it, we have been able to produce a couple of these etalons and I have decided to make maybe 5 available. These filters are not part of our normal production and when they are gone, they are gone, we will not take any orders for more. They are a limited edition signature series filter. There is the possibility of matching 2 for a single DS system.

A little about the etalons:

Internal aperture obstructions are used to maintain the parallelism of the high reflector plates typical to an etalon.
Removing the internal obstructions, or feet, requires that the etalon plates be manufactured to the highest specification for flatness possible. Generally better than 100th wave.
In order to prevent stress to the plates (bending) after the coating process, the coating has to be done in a system that can put down a hard, accurate, dielectric at very low heat. This process is often refered to as ion assisted deposition.

In order to decrease the bandwidth, we had to improve the high reflector dielectric properties. However, in general the trade off has been that plates flat enough to take advantage of the improved coating, were too costly to produce. (higher R needed better flatness).

The coatings that we are currently introducing have higher finesse and lower bandpass. While also providing higher contrast (noise to signal and all that). We are making the blacker, black, and the brighter, bright. Sort of like HD for solar viewing..

I will discuss more about the coatings in a future blog..

The specifications are as follows:

Type: Single Etalon Front Mount Filter
Size: 75mm Un-obstructed
Bandpass: <0.6
FSR: 11 Angstroms
Blocking Filter: Price includes B1200
Adapter Plate: Not Included (POR)
Price: US$3,985.00
Availability: Limited from Stock (check your local dealer or call us)
 
These are subject to prior sale and we will NOT take orders after they are gone.

Each filter will be signed and dated.

We have a few other fun things coming up, so you will have to check back here to hear about them first. (How about an unobstructed 150????) ;)~

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.

During this week we finally finished our 6th Continuous Polisher.

All the polishers at Lunt are custom designed and fabricated on site. They are currently all 48″ diameter with a capacity for about a 12″ part. In our application we use plastic fixtures on the lap to hold multiple sizes. We also utilize a large conditioning wheel that runs continiuously on the lap to maintain the required flatness to the parts being polished.

The 48″ granite lap is supported on a thick steel plate which mounts via a very heavy duty machine bearing to our custom tables. Our tables are designed and manufactured to be very solid. The lap is driven by a 5HP motor with a large gear reducer, all controlled by a variable speed controller. Due to the fact that our laps generally run 24 hours a day, 7 days a week, we have ensured that all components are over sized for the job.

The granite forms the solid and thermally stable foundation for the polishing pitch. We use a Gugolz fine pitch. We heat the pitch up in a very large pot and once fully liquid, we add secondary components to ensure that the lap will remain firm and stable for long periods of time.
These additions make the lap very “hard”. The trade off is that it takes many hours just to push a lap 1/4 wave in each direction. The benefit is that the lap remains flat for extended periods of time, allowing us to finish an entire batch of glass in that timeframe. This also ensures that the laps cannot drift too far while being run overnight. The hardness of the lap and the fine polishing compound assure that our flats are very precise and have no roll off at the edge.

Our next project is the fabrication of a 36″ Continuous Grinder. David, our glass grinder, spends 8 hours a day grinding glass from rough cut to fine grind. A second machine will allow us to provide just a little more ground parts than the optics shop currently needs freeing David to hekp in other areas.

In addition to the 6 48″ CPs, we are in the middle of construction on our 72″ Continuous Polisher. The granite has arrived, and the steel base is finished. This machine will be used to manufacture precision flats up to 24″ in diameter. Recent large flats were finished on a large draper style machine. Although we eventually met the specifications of 1/10th wave over 20″, we have certainly concluded that a large CP is the way to go.

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 first week of November has flown by. A week ago today we were getting ready for Holloween night, it’ll be Thanksgiving before you know it!!.

Testing, new products, and website were the projects of the week.

The image at left is the New LS152T/PT. This is currently the largest dedicated telescope that we manufacture. This is a working prototype and was on display at PATS. It had been my intention to take it outside on the Sunday, but the mount that I was going to borrow was sold on Saturday. I did try the mount it was displayed on, but that was a drop waiting to happen.

The internal lens sets for the system are almost finished. The prototype lenses were “off the shelf” and weren’t quite optimized for the system proviving only about a 70% sweetspot in the center of field. The production lenses have been designed to the optical system. They are also slightly larger than the prototype given more aperture to the beam.

The LS152 contains an unobstructed 60mm HR (high resolution) etalon. This is a new etalon design and provides for high signal to noise ratio which greatly darkens that background allowing for stunning contrast on edge details.

I have already had one person ask about the unobstructed 60mm, and before the questions are raised and the assumptions made in other forums, I will state that we will not be introducing the LS60F until Summer of 2010. Due to the very high demand for the other products, we will not be tooling this particular size at the expense of current production. Our LS60F production is currently limited to the LS152T, and takes the place of the production schedule for the LS200T.

The estimated US$ retail for this instrument is $7,495.00 (this does NOT include the CaK module). For those that have the LS200T on order you will be receiving the LS152T at no additional cost. If you purchased the CaK option for the LS200T you will also be receiving that at no additional cost. As you know, the LS200T was introduced at 5k. It was the intention to increase this price once the system was shipping and had good reviews. As I will explain in another post, the LS200T could never meet both the performance requirements that I had set for it, while also meeting the strict safety requirements. The trade offs were leading to unacceptable compromizes on both sides. Safety will always be of the highest priority to ANY solar product.

For those that choose to accept the exchange, I promise that you will be receiving a solar system that exceeds my expectaions for what the LS200T was going to do. By defualt the delays have allowed for the incorporation of a 60mm etalon instead of a 50mm. The new HR (high resolution) coatings will be incorporated standard, and the value of the package itself will provide a good return on investment should you ever choose to sell.

For those that cannot accept the exchange, we will refund any deposit in full.