Glass solar filters for white-light solar observing
A glass solar filter fits over the front aperture of a telescope and attenuates sunlight for observing the photosphere in white light. The Astrozap filters listed here are described as coated optical glass with a transmission of ND 5.0. Choose the cell for the actual outside diameter of your telescope tube, and check the filter’s clear aperture.
Safety: The filter must sit securely and fully in front of the optics. Before every session, inspect the glass, cell and fit. Finders and any additional optics must also be filtered or securely covered. For alternatives, see our solar filter film and Herschel prisms.
White-light solar observing with a glass solar filter
A glass solar filter is mounted on the objective side, in front of the telescope’s entrance aperture. It reduces incoming sunlight so that the visible photosphere can be observed in white light. Depending on aperture, magnification and atmospheric stability, observers can study sunspots and their structure as well as faculae near the solar limb. The Astrozap filters listed in this category are described as coated optical glass with transmission of 1/100,000 (ND 5.0); the product details and instructions for each individual model remain authoritative.
Choose the correct size
Measure the outside diameter of the telescope tube where the filter will attach. Select the cell by its stated diameter range, rather than by the telescope’s nominal aperture or model name alone. The products in this category cover several tube ranges from approximately 60 to 149 mm. Also check the clear aperture: the filter cell may reduce it compared with the filter or telescope diameter. This affects the effective light-gathering aperture and must be suitable for your instrument.
- Compare the measured outside diameter of your tube with the range stated for the product.
- Check that the cell fits evenly and can be secured without damaging the filter glass or tube.
- Take the stated clear aperture and intended application into account.
- For open-truss telescopes, cover the open tube section against stray sunlight as directed by the telescope manufacturer.
Safe fitting and inspection
A solar filter belongs in front of the optical entrance aperture, where it attenuates sunlight before it enters the telescope. A filter behind the objective or at the eyepiece is unsuitable and can be damaged by concentrated solar energy. Before each observing session, inspect the glass for cracks or other damage and check that the cell is firmly attached and cannot slip off. Add a secondary restraint if needed and permitted by the manufacturer’s instructions. Never aim a telescope at the Sun with an unprotected finder: cover or remove finders and protect every additional optical path. Do not leave the instrument pointed at the Sun unattended during the day.
Glass solar filters of this type are for white-light observing. Other white-light options include front-aperture solar filter film and Herschel prisms and wedges, each with its own compatibility and safety requirements. Observing the chromosphere and prominences requires a dedicated H-alpha solar telescope or a purpose-designed H-alpha filter system; a white-light filter does not show these features.
Check before purchase
Carefully compare the tube diameter, clear aperture, secure attachment method and product description with your instrument. Finders, guide scopes and camera lenses each need their own suitable protection. Use the solar filter only in its intended configuration and follow the manufacturer’s instructions. If you are unsure about fit or safe use, check with the filter manufacturer or a specialist retailer before observing the Sun.