H-Alpha & CaK Filters

How a Solar Filter Module Works

Two components do the work. The etalon defines a very narrow passband around the hydrogen-alpha line; the blocking filter removes everything the etalon lets through elsewhere in the spectrum, including the infrared that would otherwise reach your eye.

• Front-mounted etalons: Fitted ahead of the objective, with a maximum aperture and minimum focal ratio specified by the manufacturer. Exceeding either degrades performance and can cause heat damage.
• Blocking filters: Fitted at the eyepiece end, sized by their clear aperture. The size determines usable field and whether full-disc imaging is possible.
• Tuning: Tilt or pressure adjustment shifts the passband to favour disc detail or limb prominences during a session.
• Focal ratio requirements: Etalons perform best within a specified range, commonly around f/7 and slower. Faster telescopes may need a telecentric amplifier to present the correct light cone.
• Telecentric amplifiers: Present parallel light to the etalon rather than a converging cone, which is what allows a fast refractor to be used correctly.
• Double-stacking: A second etalon narrows the band further for markedly better surface contrast, with a corresponding loss of brightness.

Why Choose David Astro for Solar Modules

Compatibility here is not a matter of whether the parts thread together. It is a safety and performance specification.

• Telescope suitability verified: We check your aperture and focal ratio against the module's stated limits before supplying anything, because exceeding them risks thermal damage.
• Refractors only, and we say so: Front-mounted etalons are for unobstructed refractors. We will not supply a module for a reflector or catadioptric where concentrated heat inside the tube is a hazard.
• Blocking filter sized to your intent: Full-disc imaging with a larger sensor needs a larger blocking filter, and we check that against your camera.
• Authorized dealer: Genuine equipment with full manufacturer warranty, which is a safety requirement in solar filtration.
• Ships from Canada: No duties or brokerage fees for Canadian buyers.

Solar Modules in Canada — Frequently Asked Questions

Can I fit a solar module to my telescope?

Only if it is an unobstructed refractor within the module's specified aperture and focal ratio range. Tell us the exact telescope and we will confirm before you order.

Why does focal ratio matter for an etalon?

An etalon's passband shifts with the angle of incoming light. A converging cone from a fast telescope smears the passband and reduces contrast. A telecentric amplifier restores a parallel cone.

What blocking filter size do I need?

Larger blocking filters pass a wider field. Visual use and small sensors are generally fine with a smaller one; full-disc imaging on a larger sensor needs more clear aperture.

Is a module cheaper than a dedicated solar telescope?

Often, if you already own a suitable refractor, and it gives you a telescope that does both jobs. A dedicated instrument is optimized for the wavelength and is simpler to set up.

Converting a refractor for solar?

Send us the exact telescope and we will confirm compatibility and specify the module. Canadian stock, worldwide shipping.