Step 1: Getting Started
This guide will take you through the steps of building a Solar Telescope that fits your viewing expectations (whether they be Visual or Imaging or both) to your budget while answering the most common questions (along the way) about the various choices and the reasons behind those choices and how they effect performance and price.
Typically the first choice is desired Aperture of the System and/or the desired Budget Range. At this first step you can browse the various OTA’s (Optical Tube Assemblies) gather information on their specifications and view the range of price based on future decisions ranging from basic build through higher end upgrades.
Seeing conditions can play a big part in the choice of OTA aperture.
There is a misconception that “the bigger the aperture the better”. Solar viewing is done during the day and can be done from just about anywhere. High humidity, thermals, smog, and low elevation all take a toll on large aperture telescopes. It is often true that under typical seeing conditions a medium sized OTA will outperform a large OTA most of the time. The large OTA will suffer more than a medium scope due to less than good skies.
However, the large OTA will provided far more detail and magnification during great seeing conditions. If you intend to view in a area that has great seeing conditions then you are not restricted by Aperture, therefore, a larger Aperture OTA is probably the right choice.
If you intend to view in an area of less than ideal conditions you may want to take that into consideration. We find that the Lunt 80mm is best suited to average seeing conditions.
Single Stack vs Double Stack
There are several decisions to be made at this stage that will effect performance and price down the road.
If you look at the budget bar you will note a low end and a high end price point. The low end will assume you will use the system Visually and not for Imaging (explained in the next step). It will also assume that you will not be adding the Secondary Filter (DS).
You can always step back and re-configure.
If your decisions will be Budget driven (as most are) and you want the highest specification scope for the money it is best to pick an OTA that can be Double Stacked within that Budget.
The fact is that the larger the OTA the higher the cost of the Secondary Filter. However, any telescope can be Double Stacked (additional filter) at a later date without the need to return to the factory.
The lower the Bandpass the higher the resolution and detail. In general we find that people prefer the view through a DS system and find that a DS system is very beneficial to those that want to image.
Rule of thumb is to buy the largest OTA that will include a DS Filter within your budget.
How DS works will be explained in the “Secondary Filter” section.
Step 2: Visual Only or Visual and Imaging
The next step in the build process is to determine how the telescope will be used: Visual or Visual and Imaging.
We state “Visual and Imaging” because any Solar Telescope that is configured for Imaging can be used in Visual mode.
This decision will effect what recommendations and choices will be made at the next steps. If you choose Visual only we can offer a smaller Blocking Filter reducing the build cost. If you choose Visual and Imaging you will not be offered the smaller Blocking Filter because it will not work in Imaging mode.
Blocking Filters are a requirement of a Hydrogen-alpha system.
How Blocking Filters work will be explained in an upcoming step.
The decision on Visual and Imaging at this step will effect which Blocking Filter will be matched to your choice later. Thus effecting price.
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 50m 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.
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.
The advantage of getting a larger Blocking Filter for imaging use now is that, a) it removes the need to upgrade later should you decide to image later, and b) it provides more wiggle room around the Blocking Filter in Visual mode. (There is more dark space around the Sun when viewing). This also has the advantage of not having to adjust the mount as often when tracking manually.
It is also beneficial when there are large CMEs that require a larger Field of View (FOV).
Looking through the eyepiece of a Hydrogen-alpha Telescope can be a thrilling experience. The Sun is constantly changing and can become violent at any time. The Sun’s weather can effect life on Earth and being a witness to that cause and effect in real time never gets old.
If you are choosing to use your Telescope in Visual Only mode you will be offered a smaller Blocking Filter during that step. This keeps costs down.
Visual use will allow you to use an inexpensive mount with manual controls if you choose. You are looking through the eyepiece and can move around the Sun’s disk as needed.
Finding the Sun can be tricky at first. We definitely suggest the inexpensive Sol Searcher.
You will need eyepieces. Any simple eyepiece will work (you may already have some). We generally suggest 21mm to start and 12mm for higher magnifications. A Lunt zoom is a great way to scan around and zoom in on interesting features.
A special note for beginners:
Don’t expect to see fine details and/or surface details the first time you look.
Looking into a Solar Telescope is much like walking into a dark room.
You have probably been standing in the Sunlight and your eyes are contracted. A Solar Telescope not only transmits a small fraction of the light around you it is also transmitting a very narrow wavelength of that light. Generally 0.5-0.7 Angstroms at 656.28nm. It takes both time for your eyes to adjust and time for your eyes to “learn” what they are looking at. When you first walk into a dark room you see very little and would assume there is nothing to look at. However, 10 minutes later you navigate the room and start to pick out objects. The more you keep trying, the more you begin to see.
To put this in perspective. I have been viewing H-alpha for about 24 years. I use my right eye. I can pick up the very finest of details. I can Doppler shift from the Red to the Blue wing of the H-alpha line and can split spicules. That’s my right eye..
If I use my left eye I begin by seeing a red/orange ball with a few prominences around the edge. After a few minutes I pick up some surface details. After 20 minutes I can pick out filaments and begin to see details at the edge. I imagine this is exactly what looking through a scope for the first time must be like. But like anything else, spending some time behind the eyepiece and learning how to view is well worth the effort.
Visual and Imaging
A Solar Telescope that has been built for Imaging is also an ideal scope for Visual use.
By choosing Visual and Imaging we will only provide future decisions that will support both options. Lunt will help you design a Solar Telescope that will provide and excellent Visual experience while also taking into account the need for Imaging.
Lunt products were used to by NASA to image the 2017 USA Eclipse from Carbondale. 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 which 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.
A special note for imagers:
H-alpha is a very narrow emission line centered at 656.28nm. 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 and Color webcams have significant drawbacks when it comes to imaging monochromatic light.
The chip of the color 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 cutoff filter typically starts slightly below the 656nm line and significantly reduces the performance of the imaging system.
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 color balancing of all 4 sensors. The net result is that the red sensor will be “balanced” against the other sensors that received no light. Further reducing 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 resultant 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 color pallet that includes yellow.
Lunt uses a Monochrome Camera System in all our live feeds (including NASA Eclipse). We do not stack. We simply colorize and re-size in real time prior to broadcast.
Step 3: Picking the Right Focuser
Generally speaking there are going to be only 2 choices; the basic focuser that comes standard to the build or an upgrade to a Feather Touch focuser.
The choice can be based on personal preference, price, or a necessity for advanced imagers or people that want to have remote focus capability. (Feather Touch do offer a wide variety of add-ons for remote systems).
The non-rotating helical focuser standard to the 50mm Telescope is a good focuser for casual visual use. However, the focuser is not designed for larger eyepieces or heavy CCD cameras, or higher magnification viewing. The helical focuser does have a small amount of play/backlash, a normal characteristic of its design and function. If you have chosen the 50mm telescope with the 4mm Blocking Filter I would recommend that you upgrade the Blocking Filter before you upgrade the focuser.
The 1.25″ Feather Touch focuser is ideal for advanced observers and for general imaging due to its ability to provide a far more precise focus control and a more precise alignment while focusing. It is able to support more load than the helical focuser. Upgrading to the 1.25″ Feather Touch is an obvious choice for those that are using the 6mm Blocking Filter and have chosen to use their 50mm Telescope for some refular visual use and beginning to intermediate imaging applications.
The basic 2″ Crayford Focuser is an excellent choice for standard visual use and beginning to intermediate imagers on Lunt Solar Telescopes of 60mm or larger.
The 2″ Crayford focuser has 10:1 dual (course/fine) speed control, Drag and Lock adjustments, and a brass compression ring just inside the 2″ barrel to prevent marring of your blocking filter. The 2″ Focuser is a standard product in the Astronomy arena and is known for its excellent beginner to mid level performance.
The 2″ Feather Touch focuser is in a class of it’s own. Well known in the industry for its smooth feel and precise control. Feather Touch provide a full line of additional accessories which can be used to motorize or remote operate your CCD equipment. (sold by Feather Touch separately). The 2″ Feather Touch is a must have for the avid observer, or imager at all levels. The focuser is capable of securing loads up to 5.5lbs with essentially zero backlash and precise focal adjustment…. It’s simply a joy to use..
It should be noted that a special adapter is required to upgrade your system from a Crayford to a Feather Touch. Should you choose to upgrade to a Feather Touch at a later date please contact Lunt for this adapter.
Special Note about the 152mm Telescope:
The 152mm Solar Telescope is an advanced Solar Telescope for high magnification viewing and Imaging. For this reason the 152mm comes standard with the Feather Touch Focuser.
Step 4: Picking the Right Blocking Filter
In this step we are going to walk through the various Blocking Filter (required) options and choose the Blocking Filter that will best match the intended long term use.
This step generally has the largest effect on final system cost due to the high cost of the specialized cut-off filter that is used in the Blocking Filter.
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.
This cut-off filter is a mil spec’d product.
There are 2 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 34mm Blocking Filter is ONLY available in straight thru mode. However, a standard star diagonal can be used for visual. We do not recommend a star diagonal for anything less than a 34mm 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.
What does this Blocking Filter do?
Blocking Filters are essentially cut-off filters. They are 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 656.28nm (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 ND filter in this position we chose a filter that also absorbs 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 (more on that in a minute).
Element 4: A red filter. This filter is a 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 656.28nm 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 656.28nm 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
True Doppler Pressure Tuning allows for a shift into and away from the user. Adding a 3D component to the viewing experience.
While it has minimal effect on proms due to their being at the edge of the disk, it does have an effect on filaments and active regions.
While looking at a filament at the center of the Sun the user has the ability to Doppler shift from the base of the filament to the tip, following the filament thru it’s structure toward you and away from you. Allowing for enhanced visual and imaging capability for the observer as well as a research tool for the avid hobbyist.
The Lunt Pressure Tuning system provides an order of magnitude more precision to the tuning of the desired features then mechanical compression or tilt.
Because the Etalon is suspended in a sealed cavity it is 100% altitude insensitive. The Pressure Tuner changes the refractive index of the air in the ENTIRE sealed cavity and applies no differential pressure to the Etalon itself.
The Air Pressure system allows for immediate change to the CWL (Center Wavelength) without the use of electricity or heat. Heated systems require a waiting period while the CWL moves to the new position.
The Lunt Etalon is NOT compressed by mechanical methods. Mechanical compression relies heavily on the ability to produce optically precise components that will not change over time. Mechanical pressure systems physically squeeze the etalons plates together and are susceptible to differential heat expansion, “setting” over time, differential compression of the etalon, and bending of the Etalon plates causing a widening of the Bandpass of the total Etalon area.
The Lunt Pressure Tuned Etalon sits in an optimized position in the optical path, and because no tilt is required, produces flat, uniform images at the CCD.
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 tot he 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.
Step 6: To Double Stack or Not to Double Stack Explained
It is often stated that once you look through a Double Stacked Solar Telescope you don’t ever want to go back to Single Stack.
But what is Double Stack and what are benefits of having a Double Stacked system?
Double Stacking: The addition of a secondary narrowband Etalon into the telescope in order to reduce the bandpass of the system.
Bandpass: The specification of the etalon as taken at the FWHM of the measured transmission.
FWHM: Full Width Half Maximum (height) of the measured transmission curve.
Typically a secondary filter will reduce the bandpass from 0.7 Angstroms to <0.5 Angstroms as measured at the FWHM.
While the difference itself to the “specification” might seem small, it is what the secondary filter does to the base of the transmission curve that really matters. It is this reduction in transmission of light outside the desired wavelength that really matters.
All Etalons are defined by the same set of specifications. All Etalons exhibit the same transmission characteristics.
What is generally missing from the published specifications is the % of total transmission of the Etalon at the desired wavelength.
Lunt Etalons have high transmission at the peak wavelength as designed. Generally in excess of 85%.
Given the 85%T (Transmission), the Width (bandpass) of our Etalons is measured at the 42.5%T point.
Given the shape of the Etalon curve the T% widens at the base. The 2%T point is several Angstroms wide.
All single Etalon systems have a small amount of T at the 2% points that obviously lay outside the FWHM bandpass.
Even an Etalon specified at <0.4A has significant residual transmission at the base. How much residual transmission is dependent on the accuracy of the Etalon plates and it’s spacer.
The addition of a secondary Etalon significantly reduces this residual T, narrows the bandpass, AND cleans up the image allowing for better contrast.
Because Etalons are interference filters they can act together to reduce the T by a multiplication of the T%.
Lunt Etalons have a peak T of 85%. A DS (Double Stacked) system will have a peak T of 85% x 85% = ~72%T. A slight dimming of the image is noted but this is more than offset by the increase in contrast.
At the FWHM, or the 42.5%T point: The bandpass is measured in a single system at 0.7A. In DS system the bandpass is the multiplication of the 2 interference filters, 0.7A x 0.7A = 0.49A.
For the Single Stack the 2% residual Transmission points lays outside the desired bandpass. However, in the DS system the net effect is the reduction of the 2%T points to 2% x 2% = 0.04%T. In fact, the new 2%T points now lay well within the desired bandpass.
The DS transmission curve has become notably narrower at the FWHM, but more importantly, it has become significantly narrower at the base. This has a much larger impact on the contrast and details than what may be implied by the 0.7A to 0.5A specification.
The h-alpha emission line is approx. 1 Angstrom wide. A Single Stack system is narrow enough to resolve the features contained within this line and will display Prominences, Spicules, Filaments, Fibrils, and Flares. Edge details are particularly well resolved at 0.7A due to the higher transmission (when compared to DS) and the ability to contrast against the dark background.
I like to think of this as “looking at the details”.
The DS system provides a narrower slice of the details. The narrowing of the bandpass increased the contrast and “pops” the details. With the added ability to Doppler shift (explained in Tuning) from one wing of the h-alpha line to the other (red to blue) you can dissect the fine details.
I like to think of this as “looking into the details”. The larger the scope, the more “into” the details you can get via higher magnification.
Back when Lunt first began the only way to DS a system was to add an expensive Etalon filter to the front of the Telescope (Large Etalons are difficult to make and priced accordingly). In some cases the front filter was as much as the dedicated Solar Scope. However, the results were very impressive.
Technology now allows for the DS to be placed internally to the Solar Telescope. By placing the DS system in a smaller part of the optical path we can use a smaller Etalon. This reduction in the size of the Etalon significantly decreases the cost of the secondary DS system even when you take into account the added pressure tuning, mechanics and optics.
The addition of the internal Etalon has all the advantages of the front mounted version as far as narrowing of the bandpass is concerned.
The drawback to an internal DS system is the slight “glow” that the back reflections of the 2 Etalons have. Generally speaking this glow can be seen when viewing full disk images. However, it is generally not noticeable at higher magnifications especially when observing the surface details. This glow can be reduced via the use of an additional filter in the system (optional accessory) should full disk imaging be an issue.
It is generally agreed that the increase in resolution and significant increase in fine detail more than make up for the slight glow at low magnification.
It should be noted that the DS system is easily removed and re-installed into the Solar Telescope.
When choosing a Solar Telescope system I often advise people to get a Double Stack. If the choice came down to a 100mm Single Stack system vs a 80mm Double Stack system I would advise the 80mm DS. They cost about the same, but the cost of adding the DS to the 100mm later is significant.
However, I would take a DS 100m over a DS 80mm any day…
The DS can be added to the Solar Telescope at a later date. The Telescope does not have to be returned to the factory. Double Stack systems are easy to install and remove allowing the user to use the scope at both bandwidths.
Step 7: Make sure you View the Accessories Specific to Your Telescope to ensure you have Everything you Need
Thank you for considering a Lunt Solar Product. Our Family of employees take great price in hand building and personally testing every product we make at our facility in Tucson, Arizona.
We hope you enjoy your Lunt Solar System and please feel free to contact us at anytime with questions or comments. We are always happy to hear from you and keen to help.
The Lunt Solar Team.