Most solar telescopes show the Sun in white light: the photosphere, sunspots, and granulation. Useful, but limited. A hydrogen-alpha telescope reveals what white-light filters hide — the chromosphere above the photosphere, the glowing loops and arcs of the magnetic field, and the sudden violence of flares and filaments that can only be seen in the hydrogen emission line at 656.3 nanometers. The Sky-Watcher Heliostar 100 is a dedicated H-alpha observer's tool. It shows solar phenomena that white-light observers can only read about in specialist journals. And it delivers sub-0.55Å contrast without requiring an external double-stack upgrade. The native bandpass of the Heliostar is under 0.55 angstroms — that is, it isolates the hydrogen-alpha line so precisely that you see contrast typically associated with double-stacked systems. It is the official solar telescope of the Charlie Bates Solar Astronomy Project, an organization that trains amateur astronomers in chromospheric observation.
What You See in H-Alpha
The Sun's visible surface, the photosphere, is a roiling layer of plasma at 5,500 Kelvin. Above it, attached to the magnetic field lines, is the chromosphere — a gas layer that is hotter and thinner, extending tens of thousands of kilometers into space. In visible light, the chromosphere is invisible; the bright photosphere drowns it out. In hydrogen-alpha light, the chromosphere shines. You see it as a narrow band of bright gas surrounding the solar disk, broken by sudden eruptions: prominences (cool plasma suspended in the magnetic field, bright against the hot corona), filaments (same prominences viewed from above, dark against the disk), and flares (sudden releases of magnetic energy that can outshine the entire solar disk for minutes). A white-light filter shows sunspots and photospheric granulation. An H-alpha scope shows the engine that drives solar activity.
The Bandpass Problem
Hydrogen-alpha is a narrow emission line — all the light is concentrated in a bandwidth of 0.0006 nanometers. To isolate it, an H-alpha scope uses an etalon filter: a pair of highly reflective surfaces separated by a thin spacer. When white sunlight hits the etalon, only light at the H-alpha wavelength reflects through; everything else bounces away. The narrower the bandpass — the smaller the range of wavelengths that get through — the higher the contrast between the chromosphere and the dark sky. A bandpass of 1.0 angstrom shows the chromosphere. A bandpass below 0.55 angstroms shows detail that a 1.0-angstrom scope simply cannot resolve.
There is a problem: etalons are temperature-sensitive. The spacing between the reflective surfaces changes as they heat or cool, shifting the wavelength the filter passes. Outside the lab, this means you must tune the etalon constantly as the scope reaches thermal equilibrium. Early H-alpha scopes used fixed etalons and had limited contrast. Better etalons were made tunable — you adjust them mechanically as the temperature of the air and glass changes. The Heliostar is tunable. Good. But traditional single-stage etalons still cannot deliver sub-0.55-angstrom bandpass. To get that level of contrast, you needed a second etalon filter stacked behind the first — hence "double-stack." Double-stack etalon filters are expensive add-ons, running several hundred dollars. The Heliostar is a double-stack design built into the scope itself, not an aftermarket filter. You get the high contrast without the extra cost.
The Heliostar Optical System
The Heliostar is a 100mm f/7.6 achromatic doublet. Wait — achromatic in an H-alpha scope? Yes. Achromatic lenses correct for the wavelength dependence of refraction in visible light, but they are far simpler (and cheaper) than apochromatic lenses. In H-alpha observing, you are looking at a narrow band of wavelengths around 656 nanometers, not the full visible spectrum. The chromatic aberration that would make an achromatic refractor uncomfortable for deep-sky work is largely irrelevant in this narrow wavelength range. The Heliostar prioritizes aperture and optical efficiency in the H-alpha band, not color correction across the whole visible range. The result is a 100mm scope with excellent light grasp and native sub-0.55-angstrom contrast.
The optics are fully multicoated to maximize transmission in the H-alpha band. The lens elements are a matched pair — factory-tested and paired for optical performance. The focuser is a 2.8-inch dual-speed rack-and-pinion.
What's Included
-
100mm f/7.6 H-alpha optical tube. Aluminum tube, 30 inches long, 13.2 pounds. Fully multicoated optics with matched lens pair and factory-calibrated etalon stack.
-
Integrated Heliostar solar finder. A low-power reticle scope that lets you center the solar disk without looking directly at the Sun. Critical for safety and centering accuracy.
-
Clip-on Sun shade. Extends over the front of the tube to protect the optics during setup and cool-down, reducing dew formation.
-
20mm 1.25-inch H-alpha blocking diagonal. Prevents H-alpha light from escaping back through the focuser and into your eye. Includes the H-alpha-blocking filter.
-
22mm 70° wide-field eyepiece. A 1.25-inch eyepiece included in the box — 35x magnification on the Heliostar, giving a 1.4-degree field of view. Shows the full solar disk and prominences.
-
Dual-speed 2.8-inch Crayford focuser. Fine focus knob with 11:1 reduction. Move the secondary mirror with micron precision.
-
V-style dovetail. Sits on a standard Vixen-style saddle. Mounts on any equatorial or alt-azimuth mount with a D/V-hybrid saddle.
-
Hard case. Padded aluminum case for transport and storage.
-
2-inch adapter. Allows use of 2-inch eyepieces and filters if desired (at reduced field of view).
Visual Observing with the Heliostar
Point the Heliostar at the Sun using the integrated solar finder. Focus the dual-speed focuser until the solar disk snaps into sharp detail. Adjust the etalon tuner — the integrated Trifid tuner — until contrast peaks. This usually takes 30 seconds on a stable day. You are now looking at the chromosphere. The white disk of the photosphere fills the center of the eyepiece. At the limb, you see the bright band of the chromosphere, studded with prominences. If there is a large sunspot group crossing the disk, you see filaments — dark lines of cool chromospheric gas suspended above the photosphere. During active days, you may witness a flare: a sudden brightening that persists for minutes or hours, sometimes bright enough to change the contrast ratio of the entire view.
The 22mm eyepiece in the box gives 35x, showing the full disk. For detail work, use 1.25-inch eyepieces: a 9mm eyepiece gives 120x, a 5mm gives 215x. Magnification is limited by the Dawes limit (1.16 arcseconds on a 100mm scope) but H-alpha is not like deep-sky observing. Chromospheric detail is big and bright. Even 80x shows remarkable structure. The real limit is often the atmosphere, not the scope.
Imaging with the Heliostar
The Heliostar accepts a camera adapter (sold separately) for planetary and video imaging. Mount a ZWO or other planetary camera in place of the eyepiece. Use a live-view application on your computer or tablet to view the solar disk in real time and capture video. Post-process the video with Registax or equivalent software to extract the sharpest frames, then stack them to yield a high-resolution image of the solar surface. Planetary camera imaging bypasses eyepiece optics and sees the raw optical performance of the Heliostar. Many experienced solar imagers prefer this approach to traditional film or CCD camera bodies.
Practical Considerations
First and most important: safety. Never aim a solar scope at the Sun without proper solar filters in place. The Heliostar has built-in solar filtration (the etalon stack), but you must also protect the finder. The integrated Heliostar solar finder is a safe low-power reticle scope, not a magnifying finder. Never look through the main tube finder-scope-style at the Sun. Second, thermal equilibrium. The etalon is temperature-dependent. After you set up the Heliostar in the field and point it at the Sun, allow 15–20 minutes for the scope to reach the ambient temperature. During this time, you may need to retune the etalon every few minutes as the glass cools. Once equilibrium is reached, tuning adjustments become minimal. Third, the atmosphere. Solar observation is best in the early morning or late afternoon when the atmospheric seeing is stable. Midday turbulence can wash out chromospheric detail. Fourth, mount considerations. The Heliostar weighs 13.2 pounds on its own. With a mount and camera, a complete system easily weighs 40 pounds. You will want an equatorial or alt-azimuth GoTo mount for automated solar tracking. A manual alt-azimuth mount works, but keeping the Sun centered while observing or imaging requires constant hand adjustments.
Frequently Asked Questions
What is the difference between a single-stack and double-stack etalon?
Bandwidth. A single-stage etalon has a bandpass of roughly 0.7–1.0 angstroms. It shows the chromosphere well. A double-stack etalon has a bandpass below 0.55 angstroms. The narrower the bandpass, the higher the contrast and the finer the chromospheric detail you see. The Heliostar is double-stack by design.
Can I use regular eyepieces or just the included one?
Any 1.25-inch eyepiece will work. The 22mm included in the box is ideal for full-disk views. For detail and prominences, shorter focal lengths (9mm, 5mm) give higher magnification. 2-inch eyepieces work with the 2-inch adapter, but the field of view shrinks — use them if you want extreme magnification (180x+) and don't mind a narrow view.
What mount do I need?
The Heliostar's V-style dovetail fits any Vixen-compatible saddle. For manual observing, a simple alt-azimuth mount works. For imaging or tracking over long observing sessions, a GoTo mount (such as the Wave 100i, sold separately) is ideal. The 13.2-pound OTA sits comfortably on mounts rated for 22+ pounds of payload.
Is it better for visual or imaging?
Both. Visual observing through the 22mm eyepiece is spectacular and requires minimal setup. Imaging with a planetary camera and post-processing yields higher-resolution final images. Most experienced solar observers do both, depending on the day and their goals.
Accessories
-
Camera adapter. Attaches a ZWO ASI planetary camera or similar to the Heliostar for video and imaging work. Sold separately.
-
High-magnification eyepieces. 5mm, 9mm, and 12mm 1.25-inch eyepieces for detailed chromospheric study. Magnification ranges from 80x to 215x.
-
Solar mount (GoTo or manual). An equatorial or alt-azimuth mount rated for 22+ pounds. The Sky-Watcher Wave 100i (sold separately) is an ideal match.
-
12V field battery. If using a GoTo mount, a lithium power bank with 12V DC output is necessary for portable operation.
Final Thoughts
Hydrogen-alpha observing is a discipline. There is a learning curve: understanding etalon tuning, learning how to center the Sun safely, recognizing the signs of thermal equilibrium. But the view is unlike anything visible in white light. Prominences that seem impossible scale, filaments that twist in real time, the sudden flash of a solar flare — these are phenomena that have captivated solar observers for over a century. The Heliostar 100 brings them within reach without the expense of double-stack etalon filters. It is the official instrument of the Charlie Bates Solar Astronomy Project, an organization dedicated to teaching amateur astronomers the discipline of solar observation. If you want to go deeper than sunspots and granulation, this is the scope that makes it possible.
