Solar Filter Fitment Guide: Which Filter Fits Your Telescope?

Solar filters must fit snugly over the front of your telescope. A filter that is too small cannot be attached at all. A filter that is too large may fall off during use — a serious safety hazard. The filter must match the outer diameter of your telescope tube, not the aperture size printed on the box. These are often different numbers because the tube wall adds thickness and many telescopes have decorative shrouds that increase the outer diameter.

This guide lists the actual outer tube diameters for the most popular telescope models on the market, along with the recommended filter size. If your specific model is not listed, measure the outer diameter of your telescope tube yourself using a ruler or measuring tape, then match it to a filter that covers that size.

The August 12, 2026 total solar eclipse is 22 days away. If you plan to observe or photograph the partial phases through a telescope, you need a correctly fitted solar filter before eclipse day. Order now to allow for shipping time.

Safety First: The Only Safe Solar Filter Is a Front-Mounted Filter

Two types of solar filter exist, and only one is safe. Front-aperture filters mount over the front of the telescope tube, blocking 99.999% of sunlight before it enters the optics. These are safe. Eyepiece solar filters screw into the bottom of the eyepiece — they are dangerous and should never be used. The concentrated sunlight at the eyepiece can crack the filter glass instantly, exposing your eye to unfiltered solar radiation. No reputable astronomy organisation recommends eyepiece solar filters.

All filters referenced in this guide are front-aperture filters. They use either aluminised polyester film (Baader AstroSolar, Celestron EclipSmart material) or black polymer glass, both certified to ISO 12312-2 safety standard. When properly fitted and undamaged, these filters are completely safe for visual observing.

Telescope Model Solar Filter Fitment Database

The outer tube diameter (O.D.) is the measurement you need. It is always larger than the aperture because the tube wall, dew shield, or shroud adds width. Measure your telescope's outer tube diameter at the front end for the most accurate result; these numbers are typical values that may vary slightly by production batch.

Note: Outer tube diameters are approximate averages across multiple units and production batches. Always measure your own telescope before ordering if precision is critical. Universal adjustable filters can accommodate small variations.

How to Measure Your Telescope for a Solar Filter

If your telescope is not in the table above, measuring is straightforward:

1. Measure the outer diameter of the tube at the front end where the filter will mount. Use a ruler or flexible measuring tape across the widest point. For telescopes with a retractable dew shield, measure the outer diameter of the dew shield in its extended position.
2. For Schmidt-Cassegrain and Maksutov telescopes, measure the outer diameter of the front corrector plate housing — not the rear cell. The filter fits over the front corrector plate ring, which is typically 10–20mm larger than the aperture.
3. For reflector telescopes (Newtonians/Dobsonians), measure the outer diameter of the tube itself at the front opening. Truss-tube Dobsonians with open frames cannot use a standard solar filter — they require a full-aperture filter mounted on a custom cell that fits over the front ring.
4. For refractors, measure the outer diameter of the dew shield or objective lens cell. Many refractors have a dew shield that slides forward; measure the shield itself, as the filter will mount here.
5. Round up to the nearest common filter size. Universal filters with adjustable tension straps accommodate a range of diameters. If your measurement falls between two sizes, the adjustable universal filter is the safest choice.

Filter Types: Universal vs. Model-Specific vs. Custom

Universal Adjustable Filters

The Celestron EclipSmart Universal Solar Filter uses a three-arm adjustable mounting system that fits a range of tube diameters. The 56–102mm version covers most 60mm to 90mm refractors and smaller Maksutovs. The 102–152mm version covers 4-inch to 6-inch aperture scopes including most SCTs and mid-size reflectors. The 152–203mm version covers 6-inch to 8-inch SCTs and larger Newtonians. These are the most practical choice for most astronomers because they can be transferred between multiple telescopes and accommodate tube diameter variations between brands.

Model-Specific Rigid Filters

Celestron also makes rigid, model-specific solar filters for their own SCTs: a 4-inch (102mm aperture) filter, 5-inch (125mm), 6-inch (150mm), 8-inch (203mm), 9.25-inch (235mm), and 11-inch (279mm). These are precision-machined aluminium cells with Baader AstroSolar film and are the most secure option for Celestron SCT owners. They do not fit non-Celestron telescopes or non-SCT designs.

DIY with Baader AstroSolar Film

For large-aperture Dobsonians (10-inch and above) or unusual tube diameters, the most practical option is to buy Baader AstroSolar Safety Film and build a custom filter cell. The film is ISO 12312-2 certified and available in A4 sheets. You construct a cardboard or foam-core cell that fits over your tube and stretch the film across the opening. This takes about an hour of craftsmanship and is significantly cheaper than a commercial filter for large apertures. Detailed DIY instructions are available from astronomy forums and Baader's website.

Recommended Solar Filters for the August 12 Eclipse

With 22 days until the eclipse, these are the filters we recommend based on availability and fitment coverage:

BEST UNIVERSAL

Celestron EclipSmart Universal Solar Filter (56–102mm)

The most widely compatible white-light solar filter on the market. The three-arm adjustable mounting system fits telescope tubes from 56mm to 102mm outer diameter — covering most 60mm to 90mm refractors, small Maksutovs, and entry-level reflectors. The filter material is Solar Safe, a proprietary optical coating that blocks 99.999% of visible light and 100% of UV and IR radiation. ISO 12312-2 certified. The universal mounting system uses tension springs that hold the filter securely while allowing quick removal when clouds pass. For the August 12 eclipse, this is the filter most readers will need.

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FOR 70MM REFRACTORS

Celestron EclipSmart 70mm Refractor Solar Filter

A dedicated filter for 70mm refractors — the most common beginner telescope aperture. If you own a Celestron AstroMaster 70AZ, Travel Scope 70, StarSense Explorer LT 70AZ, Gskyer 70400, Sky-Watcher Startravel 70, or any 70mm refractor with an outer tube diameter of approximately 76–80mm, this is the correct filter. It is a rigid aluminium cell with the same Solar Safe material as the universal filter, providing a more secure fit than the adjustable version. The difference versus the universal filter is marginal in practice — choose this if you want a snug, model-specific fit for your 70mm refractor.

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White Light vs. H-Alpha: Which Filter for What?

The filters listed above are white-light solar filters. They show the Sun as a white or slightly orange disk, revealing sunspots, granulation, and limb darkening. They are the correct choice for eclipse observing because they show the Moon's silhouette against the solar photosphere.

H-alpha solar filters are a different category. They isolate a narrow wavelength band (656.28nm) to show solar prominences, filaments, and the chromosphere. H-alpha filters are significantly more expensive than white-light filters and require specialised equipment (a dedicated H-alpha telescope or a front-mounted etalon plus blocking filter). They are not necessary for eclipse viewing — the prominences visible during totality are spectacular but require an H-alpha system to see properly. For a comprehensive guide to white-light solar filters, see our best solar filter buying guide.

Frequently Asked Questions About Solar Filter Fitment

Get Your Filter Before Eclipse Day

The August 12, 2026 total solar eclipse is the biggest astronomy event of the year. If you plan to observe it through a telescope — and you should — a correctly fitted solar filter is the single most important piece of equipment you will need.

Use the fitment table above to identify the correct filter for your telescope, measure your tube diameter if your model is not listed, and order your filter now to ensure it arrives before eclipse day. A front-aperture white-light solar filter from a reputable brand, properly fitted, is completely safe and will give you stunning views of the partially eclipsed Sun with visible sunspots and the Moon's sharp silhouette crossing the solar disk.

For a broader overview of solar filter options — including dedicated solar telescopes and H-alpha systems — see our complete solar filter buying guide.