What 40 mm Lunt Telescopes Can Do

The all new Lunt 40 mm Solar Telescope provides the basic essentials perfect for a first-time introduction to solar observing.

A true grab-and-go dedicated hydrogen alpha telescope, the Lunt 40 mm F/10 system is sure to impress — allowing both amateurs and seasoned professionals to experience the awe-inspiring phenomena of our closest Star!

Easily portable and a joy to use, the Lunt 40 mm joins the Lunt Telescope lineup as our smallest aperture offering, while providing the power, amazing detail and quality you expect from a Lunt Telescope — all at an impressive ~4.0 mm Solar image size!

This scope includes a non-rotating helical focuser, integrated Vixen Style Dovetail and a 5 mm blocking filter for both visual and photography purposes.

Experience the thrill of viewing the Sun’s explosive activity with Lunt’s entry-level 40 mm Dedicated H-Alpha Solar Telescope. Surface detail, hot spots, prominences and more — all available in the stunning Hydrogen Alpha wavelength.

Enjoy Portable, Affordable Solar Viewing

Our Dedicated H-Alpha Solar Telescopes come standard with Lunt’s exceptional Hydrogen Alpha Modules, providing excellent viewing prowess at <.7 angstroms bandpass.

The excitement of viewing our Star through a dedicated solar telescope is sure to provide many hours of visual enjoyment and educational insight. As the Sun becomes increasingly more active, your love for this instrument is sure to grow!

The Lunt 40 mm Hydrogen Alpha Telescope includes a non-rotating helical focuser, an integrated Vixen Style dovetail plate, and a 5 mm Lunt blocking filter for both visual and photography purposes.

The Etalon – Heart of the Solar Telescope

The most dynamic, exciting way to view the Sun is by zeroing in on a specific, narrow bandwidth of light: the hydrogen-alpha (H-alpha) bandwidth.

The light arriving here from the Sun at the H-alpha frequency (656.28 nanometers) comes from a rarified layer of hydrogen gas slightly above the photosphere, the bright surface of the Sun. This hydrogen layer is called the solar chromosphere, and it’s invisible without using instruments to filter out brighter, competing bandwidths of light.

Due to its being dominated by magnetic energy, the chromosphere is where the most intense solar activity can be observed, including filaments, prominences, spicules and active regions.

The etalon, along with other components, provides us with this amazing view.

Lunt etalons

What is an Etalon?

An etalon is 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 in need of great precision, there are many compromises that can influence the quality of an etalon filter.

An etalon is comprised of 2 flat and parallel optical surfaces that have been optically coated with a highly reflective dielectric layer, with the high reflector layer peaking at the desired bandpass point for best results. These optical surfaces are separated 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. Through interference at this gap, only light meeting the correct angle of incidence to the surface is not “interfered with” and can pass; all other light is lost.

The main parameters that define an etalon are:


This is the width of the curve that defines the transmittance of the filter at 50% of the total transmission 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.

Bandpass is a function of the gap size between the high reflector plates. The larger the gap size, the narrower the bandpass. Bandpass is also a function of the reflectivity of the etalon high reflector plates. The higher the reflectivity, the narrower the bandpass.

Peak Transmission

In order to obtain good contrast it is important to maximize the peak transmission while minimizing the “out of band” transmission. To explain this statement, a surface that had zero reflectivity would have 100% peak transmission. However, because there is no reflectivity in the cavity, there is no interference, and thus, no filter (bandpass). A surface with 100% reflectivity would reflect all light before it entered the cavity, thus having zero transmission. The compromise is somewhere in between.

Peak transmission is a function of the reflectivity of the surfaces. The higher the reflectivity, the lower the peak transmission. Of course, there’s more to it than that.

Free Spectral Range (FSR)

The free spectral range is defined as the gap between the peaks of transmission plotted against wavelength. An etalon produces a “comb” of peak transmission across a broad range of the visible spectrum. This would be like a hair comb. One tooth of the comb would represent a peak transmission. This comb would then be missing about 12-14 teeth before the next peak transmission, or tooth.

In our case the FSR is more than 10 angstroms. This becomes important to our ability to block the unwanted peak transmissions using simpler filters. The narrower the FSR, the harder it is to block the transmissions you don’t want. Letting another peak transmission through will wash out the details.

FSR is a function of gap size. The narrower the gap, the wider the FSR.

Optical Flatness and Parallelism

Probably the most critical aspects of the etalon performance — to put it simply, the better the flatness and the parallelism, the better the etalon. The quality of an etalon is very much the function of precision polishing and gap maintenance.

When it comes to the specification of a solar telescope I often hear the bandpass stated out as a matter of fact. However, this bandpass is typically the theoretical value of the system based on the known parameters of reflectivity, gap size, and optical flatness. What one should realize is that bandpass is not the all defining specification of a quality system.

I could quite easily manufacture a 0.3 Å bandpass filter and everyone would be happy… Right? Not if its performance was any factor. An etalon with a peak transmission of 40% and a FSR of 4 Å is quite easy to manufacture. But I assure you, you wouldn’t want it in your scope.

When it comes to etalons there are typically a few things to look at: The bandpass is of very high concern as long as all other aspects of the optical system have been addressed. An FSR of greater than 10 Å is required in order to prevent out of band leaks.

Peak transmission is also important as long as it comes with a zero baseline. Signal to noise ratio is critical because it is what makes the brights bright, and the darks dark. No one wants to see a significant orange glow in the space around the Sun’s image, this simply washes out the edge details.

Double Stack Compatible

Lunt Solar System has the Lunt 40mm Double Stack in the works! Adding a second etalon enables a narrower bandpass down to less than 0.5 angstroms throughout the system! With a narrower bandpass, surface detail and features appear to explode off the Sun’s surface. Double stacking provides an extra dimension of solar viewing for both photographic and visual use!


Lunt Etalons are the Best

Lunt Etalons are precision crafted at our manufacturing facility in Tucson, Arizona.

We start with precision cut UV grade fused silica blanks that we purchase from a glass supplier in Massachusetts.

We use UV grade due to the higher specifications that UV grade material has for reduced bubbles and striae, both of which would negatively effect the performance of the finished etalon. UV grade costs more but we believe that high quality materials result in a superior product. 

Interchangeable Blocking Filters

All solar telescopes must have a blocking filter.

This cut-off filter meets military specifications.

We offer two choices of blocking filter configuration:

Straight Through and Diagonal.

Straight thru Blocking Filters are generally used when the system will be used in imaging mode only.

The 34 mm Blocking Filter is only available in straight through mode. However, a standard star diagonal can be used for visual. We do not recommend a star diagonal for anything less than a 34 mm Blocking Filter due to the issue of not being able to get the eyepiece to focus. Diagonal systems are generally used when the system will be used for both Imaging and Visual.

It should be noted that ALL Lunt blocking filters utilize the same specifications of cut-off filter in their design. A larger Blocking Filter does not have a “better” specification than a smaller one. All blocking filters are cut from the same material and have the same bandpass (6 Angstroms), the same out-of-band blocking and the same temperature range.

What a Blocking Filter Does

A Blocking Filters (BF) is essentially a cut-off filter. It’s a combination of several filters that are designed to provide additional safety to the viewer and remove all out of band transmission from the etalon, allowing only the transmission at 656.28 nm (H-alpha) to pass.

Element 1

The blue glass filter in the nose of the BF is designed to attenuate the brightness of the image. Given that we needed basically a neutral density filter in this position we chose a filter that also absorbs infrared light (IR).

Element 2

The diagonal mirror itself is actually not a mirror. This element acts as a Long Wave Pass Filter while also designed to reflect the H-alpha light at a 45 degree angle. The effect is that any residual IR passes through the filter into the backing plate and the H-alpha light is reflected up to the eyepiece.

Element 3

The cut-off filter. This filter is designed to cut out-of-band transmission from the Etalon.

Element 4

A red filter. This filter is an ultraviolet light (UV) blocking filter. It is also coated with a very good anti-reflection coating so you do not see any back reflections when looking in the eyepiece. Element 3 is highly reflective and would cause serious back reflections without the addition of Element 4.

The Etalon is a highly precise optical filter that puts out a very narrow bandpass of light (<0.7 Angstroms) at the 656.28 nm point. It also puts out that same bandpass of light every 10 Angstroms in most of the visible spectrum. Basically the transmission curve of an Etalon looks like the teeth of a comb.

The Blocking Filter is used to transmit only the 656.28 nm tooth in the comb and remove all other transmission lines. It also provides additional safety features to the user by blocking and absorbing and residual IR and UV radiation.

It should be noted that all harmful IR and UV radiation are also removed in the Solar Telescope main body.

  • Lunt Blocking Filters contain a 6 Angstrom FWHM Trimming Filter
  • Lunt Blocking Filters eliminate ALL harmful UV and IR radiation
  • Lunt Blocking Filters incorporate a standard T2 thread for adapting to imaging equipment

Viewing Directly vs. Capturing Images

Lunt telescopes are ideal for both direct viewing and imaging (capturing images with a camera). However, the telescope must be configured a bit differently to be used for both viewing and imaging than if it’s to be used only for viewing.

The solar telescope creates a cone of light that is focused onto an “image plane” at the back end of the telescope. The size and distance from the objective of this image plane is defined by the focal length of the system and the apparent size of the object (Sun).

When using the telescope in visual mode with an eyepiece the slide tube and/or focuser is generally racked out such that the eyepiece can be brought to focus onto the image plane. Various eyepieces have various back focus requirements but most remain within a fairly standard length. This generally places the Blocking Filter at the smallest portion of the light cone allowing the user to use a smaller aperture Blocking Filter.

However, if the Telescope is to be used in Imaging mode the slide tube and/or focuser will need to be racked in to bring the image plane onto the CCD surface. Generally speaking this can be as much as 50 mm for larger systems. This requirement for “in focus” effectively moves the aperture of the Blocking Filter up to a larger diameter of the light cone. Using a small (visual only) aperture Blocking Filter in imaging mode would effectively cut off the edges of the image. It also results in a vignetting of the image on the CCD.

A Lunt customer’s camera setup

The Sun is a fixed object. We can therefore pre-determine what size Blocking Filter is required after the decision of visual or imaging has been made as a function of the focal length of the telescope.

Lunt products were used to by NASA to image the 2017 USA Eclipse from Carbondale, IL. Lunt standard products have also been used in the past by NASA for the transit of Venus and Mercury, and by National Geographic for the Easter Island Eclipse. All of these were very successful live streaming events. Our products are ideally suited to imaging due to the ability get full disk images, rapid Doppler tuning, and ease of CCD adaptation to our systems.

Use a Monochromatic Camera

H-alpha is a very narrow emission line centred at 656.28 nm. This narrow wavelength is found in the red area of the visible spectrum.

The Solar Telescope transmits only this small portion of the spectrum. It transmits no Blue or Green. The most ideal imaging system is a monochrome camera or webcam.

DSLRs, mirrorless cameras and colour webcams have significant drawbacks when it comes to imaging monochromatic light.

The chip of the colour camera contains sensors that are designed to capture light in the green, blue and red. Due to these sensors being very sensitive to the red portion of the spectrum they only utilize 1 sensor for every 3 sensors in the blue and green. It should also be noted that these chips also have a red/IR blocking filter placed over the sensors to reduce the red sensitivity of the camera making it easier to control exposure settings. This red cut off filter typically starts slightly below the 656 nm line and significantly reduces the performance of the imaging system when used for H-alpha imaging.

Because only one 1 sensor in 4 are actually sensitive to the red, the camera is actually only using 1/4th of its CCD, significantly reducing resolution.

Software within the camera system also uses colour balancing of all 4 sensors. The net result is that the red sensor will be “balanced” against the other sensors that received no light. This further reduces the image quality. The simple answer is that the software has no idea how red the reds are because it has no other point of reference. This typically creates washed out and muddy red images that require significant retouch and post-processing.

Monochrome cameras are not only ideal, they are simpler to use and are relatively cheap. Monochrome cameras utilize all sensors as a photon dump and are excellent at defining contrast. The resulting image can be colorized by a simple method of turning the darker shades of grey to red and the mid shades to a lighter more orange hue. Some images even create a colour pallet that includes yellow.

Lunt uses a monochrome camera system in all our live feeds (including the NASA Eclipse). We do not stack. We simply colorize and re-size in real time prior to broadcast.

Industry-Leading Quality and Safety

High Safety Standards

At Lunt Solar safety is our top priority. When Lunt Solar started making solar telescopes and filters, the subject of eye safety was at the forefront of our design. Our designs were approved by a senior ophthalmologist professor at a leading university of ophthalmology in Canada. A safety criterion was determined for both UV and IR transmission. This criterion basically set the bar at less than 1×10-5 (T) for any hazardous radiation.

Several standalone filters in Lunt products meet this criterion as a single unit. However, Lunt sets double and sometime triple standards for this requirement so that in the unlikely event one filter fails, the user will still be fully protected.

Our Filters

Energy Rejection Filter

The filtering of a Lunt system starts with a “true” energy rejection filter at the front of the system. This filter is unique to Lunt, and blocks both dangerous UV and IR.

On smaller telescopes the ER filter is seen as a red-looking filter installed at a slight angle (to remove internal ghosting). This is either installed at the front of the scope or just inside the main objective.

On larger aperture telescopes, Lunt puts an additional IR blocking filter onto the front surface of the main objective. This will remove all heat load from the internal parts. Even on these large aperture systems we still provide the secondary red ERF just inside the objective.


The next “filter” in the system is the heart of the system, the etalon. While the etalon was not designed as a safety filter, it does have a very high reflective surface that rejects most UV (T). Significantly, this would reject the majority of all IR if no prior IR filters were present.

BG Filter

The third filter is the Schott-designed BG (Blue Glass) filter. This filter is also created to absorb any residual IR.

Long Wave Pass Filter

The next filter is commonly called the diagonal “mirror” However, it is not a mirror at all. Inside the diagonal is a Long Wave Pass filter. To begin with, it is designed to reflect a specific percentage of the 656 nm wavelength to attenuate the image to a manageable brightness. It sits at a 45º angle and passes through any IR into the backing plate.

Blocking Filter

The next filter is the blocking filter. Again, this is not a safety filter unto itself. As the name implies, it blocks out-of-band wavelengths and allows the H-alpha waves to pass.

Red Glass Filter

The final filter is another piece of the red glass (without the IR coating). This glass blocks 100% of all UV. It also acts to stop the back reflection of your eyeball from the very bright BF.

Redundant Filters

People ask why we incorporate so many IR and UV filters in the system. The multitude of safety features we employ insures that our customers will be protected. They are protected even if they use our products improperly. For instance, should a person accidentally place a standard night time diagonal in the rear of a solar telescope, the view would be bright, but safe.

Because of the addition of multiple filters and safety features, a person simply standing in the sunlight will receive more ambient UV and IR radiation to the eye than when they are looking through one of our solar telescopes.

Lens Coating:

Lunt purchases our raw etalon glass materials from an ISO-qualified company on the East Coast of the United States. We grind, edge, bevel, and polish all the glass needed for the etalon and filter systems in-house in Tucson, AZ. Some coatings are outsourced to a facility that maintains a coating specific to our requirements.

Our coating facility has the required ability to produce anti-reflective coating at less than 0.1% reflectivity (R) (typically in the 0.06%R range). They also hold the high reflector coatings to better than +/-1%. The ability to control the coating processes to such high accuracy has allowed us to make precision modifications to the coating formulas, which have proven to increase contrast through the reduction of background noise.

Quality Assurance:

Each coating batch is provided with full scans of the coating applied and is certified to meet any and all safety requirements. Some of our precision-coated filters are provided to us from a US military-qualified company who provides full military certifications with every filter.

All Lunt Solar products are 100% safe when used as directed and are shipped from the factory free of any damage or defects. If a Lunt instrument is ever dropped or damaged, it should be returned to the factory for testing and re-certification.

Due to different optical arrangements in each design, a Lunt solar product should never be mixed and matched with components made by other companies.

One of the most important questions to ask when looking at a Solar Telescope is whether or not is has taken your safety into the highest consideration.

Does the system have redundant safety features to protect you if something should fail?

Does the system come with a Blocking Filter that contains additional safety features?

Lunt 40 mm Telescope vs. Competition

40 mm Lunt Coronado DayStar
Objective Aperture 40 mm 40 mm **NA
Objective Type Custom Designed Doublet Optimized at 656 nm Commercial Grade Doublet **NA
Objective Coating Ion Assist Ultra-Narrowband AR *Not Specified **NA
Universal Capabilities H-alpha H-alpha **NA
H-alpha Option H-alpha Fixed Etalon H-alpha Fixed Etalon **NA
Etalon Clear Aperture 35 mm *Not Specified **NA
Etalon Bandpass <0.7 Å <1.0 Å **NA
Etalon Wavelength 656.28 nm 656.28 nm **NA
Etalon Material UV Grade Fused Silica Commercial Grade Fused Silica **NA
Etalon Tuning Tilt Mechanical Compression **NA
Etalon Doppler Shift Timeframe Instant Instant **NA
H-alpha Blocking Filter CA 5 mm, 6 mm, 12 mm *Not Specified Fixed **NA
H-alpha Blocking Filter Bandpass 6 Å *Not Specified **NA
H-alpha Blocking Filter Type Interchangeable Straight Through or Diagonal *Not Specified Fixed **NA
Double Stack Compatible No Yes **NA
Focuser Options Helical, Dual Speed Feather Touch *Not Specified **NA
Mount Type Vixen Style Foot 1/4-20 Mounting Holes **NA
Country of Manufacture USA Mexico **NA
*Not Specified – Information is not available on the company website.
**NA – This option is not available for this product.

Lunt 40 mm Telescope Specifications

  • Telescope Type: Dedicated H-alpha
  • Universal Capabilities: H-alpha
  • Aperture: 40 mm
  • Objective Type: Custom Doublet Optimized at 656 nm
  • Objective Coating: Ion Assist Ultra-Narrowband AR (656 nm)
  • Focal Ratio: F/10
  • Focal Length: 400 mm
  • Focuser Options: Helical or Dual Speed Feather Touch
  • Mounting: Mounting Foot
  • Storage: Box with Fitted Foam, Hard Case Optional
  • H-alpha Etalon Type: Internal Dedicated
  • Etalon Wavelength: 656.28 nm
  • Etalon Bandpass: <0.7 Å Single Stack, <0.5 Å Double Stack
  • Etalon Material: UV Grade Fused Silica
  • Etalon Tuning: Tilt
  • Doppler Shifting: Instant
  • Blocking Filter Size: 5 mm, 6 mm, 12 mm, Diagonal and Straight Through
  • Blocking Filter Bandpass: 6 Å
  • Sun Finder: Tele Vue Sol Searcher