Solar Filter

Specifications

Observing parameters

Overview

A full-aperture solar filter reduces the Sun's output by a factor of roughly 100,000 before any light enters the telescope. White-light filters show the photosphere: sunspots, granulation and limb darkening. Hydrogen-alpha etalon filters are a different class of instrument — far more expensive — and reveal prominences, filaments and flares in the chromosphere.

Two rules govern everything else. First, the filter goes over the front aperture, never at the eyepiece: an eyepiece filter sits after the telescope has already concentrated the light and will crack from the heat. Second, inspect the filter before every single use against a bright light, and discard it if you see any pinhole, scratch or coating damage.

Also cap or remove the finderscope, and never leave an unfiltered telescope pointed at the Sun where someone else can use it. Solar observing is safe and rewarding when these rules are followed, and permanently damaging when they are not.

Where it fits

Practical notes

• Inspect the filter against a bright lamp before every single session. This takes ten seconds and is not optional.
• Strap or screw the filter on so it cannot be knocked off by wind or a bump.
• Cover or remove the finderscope before you start — a small unfiltered finder is a serious hazard.
• Let the telescope cool for a few minutes after solar viewing before you store it, and never point it at the Sun while packing up.
• If you have any doubt about a filter's condition, do not observe. A replacement filter costs less than the alternative.

Pairs with

• Herschel Wedge
• CMOS Camera
• Red Flashlight
• Lens Cleaning Kit