Lunt LS230THa Double Stack Solar Telescope with B3400 Blocking Filter

LS230THa — 230mm Double Stack H-Alpha Solar Telescope The world's only 230mm solar telescope. Five units. When they are gone, they are gone.

New-generation DS etalon — each unit individually assembled, hand-matched, and verified on Lunt's precision metrology equipment. Every specification confirmed. Every unit unique.

There are instruments that push the boundary of what is commercially possible. And then there is the LS230THa.

At 230mm of fully unobstructed aperture, the LS230THa is the largest dedicated hydrogen-alpha solar telescope ever offered for sale to the public — and the only 230mm solar telescope available anywhere in the world today. 230mm refractors of this specification are not currently in production by any manufacturer for any purpose. These five instruments are not simply the finest solar telescopes Lunt has ever made. They are the finest that can be made at commercial scale, and they will not be made again.

Five units. One lead time of approximately six months. When the fifth is delivered, the LS230THa is retired.

The Optical System

The LS230THa is built around a 230mm f/7 FLP51 air-spaced doublet objective — a lens set optimised at the hydrogen-alpha wavelength of 656.28nm from design to coating. The front objective element carries a custom IR reflective coating that removes the thermal energy load from the system before it reaches the precision optics behind it. Additional IR filtering throughout the optical train ensures that only the narrowest, cleanest slice of the solar spectrum reaches the etalon system. The result is a thermally managed, optically pure optical path designed entirely around one target: the solar chromosphere.

This design makes the LS230THa unsuitable for night-sky use. It was not designed for night-sky use. It was designed to be the finest solar telescope that can be built — and that required choices that serve solar observation completely, without compromise.

The Double Stack Etalon System

The LS230THa is double-stacked as standard. There is no single-stack version. The dual pressure-tuned etalon system places two independently tuned etalons in series within the optical path — one at 0.6Å, one at the new-generation 0.35Å specification. The combined system bandpass is ~0.30Å.

This pairing was not straightforward. Matching etalons of two different generations — with different finesse characteristics and different pressure-to-wavelength response curves — requires extensive individual testing. Each of the five instruments has been assembled with a hand-matched etalon pair, tested to verify the combined FWHM, and verified per unit. No two units are identical in their internal configuration. Each is the result of a matching process that finds the optimal pair for that specific instrument. The result across all five units is consistent: ~0.30Å combined bandpass with excellent uniformity across the full 230mm aperture.

The two etalons work as a matched system. Both must be tuned in tandem through the hydrogen-alpha line — they are not independently optimised for different wavelength positions. The combined FWHM specification is achieved only when both etalons track together. The dual PC-USB controller system provides the precision and independent trim capability required to maintain that combined performance across the full tuning range.

The High Resolution Glass Filter

Integral to the LS230THa optical train is the High Resolution Glass (HRG) filter. In a double-stack solar telescope without HRG, off-band solar light — residual red and orange continuum transmission around the H-alpha bandpass — creates a background glow across the field of view that reduces contrast and limits the perceived depth of chromospheric features. The HRG filter eliminates this glow entirely, producing a genuinely dark background against which the chromosphere is rendered with maximum contrast. Combined with the Lunt-modified B3400 internal blocking filter, it delivers the cleanest, highest-contrast chromospheric image achievable in a portable solar instrument.

Dual PC-USB Pressure Tuner Controllers

The LS230THa includes two PC-USB Pressure Tuner Controllers — one per etalon. This is not a convenience feature. At 230mm aperture, physically touching the telescope to adjust a tuning knob introduces vibration that takes seconds to settle and disrupts the combined etalon alignment. Remote pressure control is operationally essential at this aperture class.

Each PC-USB controller is a microprocessor-controlled pressure management system accurate to 0.3 PSI. Two buttons: pressure up, pressure down. Digital feedback and indicator lights confirm the current state. The system auto-compensates for temperature-driven pressure fluctuations inside the sealed etalon cavity, maintaining the set wavelength position without continuous manual intervention.

The two etalons work as a matched pair — both must be tuned in tandem to maintain the ~0.30Å combined FWHM specification at any given position on the hydrogen-alpha line. The two PC-USB controllers allow independent fine trimming of each etalon's pressure contribution as you scan through the line, compensating for the different pressure-to-wavelength response characteristics of the two etalon generations. As you move from line centre toward the blue or red wing, both controllers track together — with the independent trim capability ensuring the combined bandpass remains at its optimal specification throughout the full Doppler tuning range.

Because the PC-USB provides closed-loop feedback of internal etalon pressure, users can record specific tandem pressure positions for any point on the hydrogen-alpha line and return to them precisely on any subsequent session — 0.3 PSI repeatability, session after session. This turns Doppler tuning from an art into a reproducible science. For imaging workflows that require consistent, calibrated Doppler positions across multiple sessions, this reproducibility is genuinely transformative.

Each controller runs for more than 12 hours on internal battery — a full day's observing without external power. External power input accepts 12–15V DC or 120/240V AC via the included adapter, which simultaneously powers the unit and charges the internal battery. A 1/8" airline connects the controller to the telescope at any length — the operator can be separated from the instrument by the full length of the airline without any loss of control or feedback. USB connectivity allows software integration for observatory automation and remote operation.

What You'll See

At 230mm aperture and ~0.30Å combined bandpass, the solar chromosphere reveals itself at a level of detail that previously required access to a dedicated solar observatory.

On a typical day of reasonable seeing, the LS230THa shows more than any other commercially available solar telescope. Filaments resolve into multi-strand systems with sharp dark cores and defined lateral boundaries. Plage regions display internal fibril organisation — the detailed structure of the magnetic network visible as texture within texture. Active regions show penumbral flows, light bridge evolution, and the minute-to-minute development of magnetic structures that drive solar activity. Prominences reveal internal threading and flow patterns from base to tip with a clarity that makes serial observation feel like watching geological forces in real time.

On a day of exceptional seeing — the kind that serious solar observers plan entire trips to access — the LS230THa reaches the finest scales that ground-based solar observation allows. Supergranulation cell boundaries become traceable across the quiet sun. Individual spicule structures at the solar limb resolve as distinct features rather than a diffuse fringe. The magnetic architecture of active regions — fibril networks, moat flow patterns, the small-scale reconnection events that precede flares — all of it becomes accessible to a single observer at an eyepiece rather than requiring a spectrograph and a research institution.

With dual Doppler True pressure tuning and closed-loop PC-USB control, you are not just observing the chromosphere — you are mapping it in velocity space. Scan both etalons in tandem from red wing through line centre to blue wing, maintaining the full ~0.30Å combined bandpass throughout. Fast-moving plasma, eruptive events, the early signatures of solar flares — all of it resolves at 230mm with a clarity and a depth of Doppler control that changes what solar observing means.

For Imaging

The B3400 blocking filter provides 34mm of clear aperture — sufficient for large format sensors. The Starlight Instruments Feather Touch focuser with 2" barrel and 10:1 fine focus reduction provides the mechanical precision that imaging at 1,610mm focal length demands. The 1,610mm f/7 optical path produces a solar image of almost 16mm at the focal plane — an image scale that fills a large sensor and reveals chromospheric detail at pixel scales that smaller aperture systems cannot approach.

The dual PC-USB closed-loop control makes the LS230THa the most capable solar imaging platform available for Doppler imaging work. Establish calibrated tandem pressure positions for specific points on the hydrogen-alpha line — record both values — return to them with 0.3 PSI repeatability on any subsequent session. Build a site-specific pressure-to-wavelength calibration for your instrument. Run systematic Doppler imaging sequences at known, reproducible bandpass positions with both etalons maintaining combined FWHM throughout. This level of operational precision and repeatability has previously required professional-grade instrumentation. It is now available in five units, from Tucson, Arizona.

Package Includes

• LS230THa OTA: 230mm aperture, 1,610mm FL, f/7. FLP51 air-spaced doublet — custom IR reflective coating on front element. Internal dual pressure-tuned etalon system. Dedicated solar telescope.
• Dual Pressure-Tuned Etalon System: 0.6Å + new-generation 0.35Å — hand-matched pair, individually verified per unit. Combined bandpass ~0.30Å. Both etalons tuned in tandem for combined FWHM performance.
• High Resolution Glass (HRG) Filter: Integral to optical train. Eliminates off-band background glow. Delivers dark-background, high-contrast chromospheric image.
• B3400 Blocking Filter: 34mm clear aperture. Internal mount. Lunt-modified for enhanced solar performance.
• Starlight Instruments Feather Touch Focuser: 2" precision with 10:1 fine focus reduction.
• PC-USB Pressure Tuner Controller — Etalon 1: Microprocessor-controlled remote pressure system. 2-button interface. Digital feedback and indicator lights. 12+ hour internal battery. External power: 12–15V DC or 120/240V AC adapter. Accurate to 0.3 PSI. Auto-compensates for temperature variation. 1/8" airline connection at any length. USB remote control compatible. Closed-loop pressure feedback for reproducible tandem Doppler positioning. Retail value $995.
• PC-USB Pressure Tuner Controller — Etalon 2: Identical specification. Works in tandem with Controller 1 to maintain combined ~0.30Å FWHM across full Doppler tuning range. Retail value $995.
• Tube Rings and Mounting Plate: Observatory-grade mechanical assembly.

Key Specifications

• Aperture: 230mm — fully unobstructed
• Focal Length: 1,610mm
• Focal Ratio: f/7
• Objective: FLP51 air-spaced doublet — custom IR reflective coating on front element
• Etalon System: Dual internal pressure-tuned — 0.6Å + 0.35Å (new generation) in series
• Etalon Operation: Tandem tuning — both etalons track together to maintain combined FWHM
• Combined Bandpass: ~0.30Å — hand-matched and individually verified per unit
• HRG Filter: High Resolution Glass — eliminates off-band background glow
• Blocking Filter: B3400 (34mm clear aperture) — internally mounted, Lunt-modified
• Tuning: Dual Doppler True Barometric pressure tuning — tandem operation
• Controllers: 2× PC-USB Pressure Tuner — one per etalon. 0.3 PSI accuracy. 12+ hr battery. USB remote. Auto-temperature compensation. Closed-loop tandem Doppler feedback. Retail $995 each — both included.
• Focuser: Starlight Instruments Feather Touch 2" — 10:1 fine focus reduction
• Image Size at Focal Plane: ~16mm solar disk
• Night Sky Use: No — dedicated solar telescope
• Production: Limited edition — 5 units only. Will not be remanufactured.
• Lead Time: ~6 months
• US Retail: $39,995

A note on resolution: At 230mm aperture, the etalon system is not the limiting factor. The atmosphere is. This instrument will regularly outresolve what the sky allows. Observatory sites with excellent solar seeing will reveal the full capability of the LS230THa. For observers at variable-seeing sites, the instrument will still dramatically outperform everything else available — they will simply encounter the atmospheric ceiling more clearly, because the telescope itself imposes none of its own.