The exposed lens surface of an eyepiece should be dusted periodically with a:

The best choice for dusting the exposed lens surface of an eyepiece is using a camel-hair brush. This type of brush is designed to effectively and gently remove dust and debris without scratching the delicate surface of the lens. Camel-hair brushes have soft bristles that are flexible and less likely to cause damage compared to harder materials.

Using a wet tissue, soapy cloth, or dry sponge can all pose risks. A wet tissue or soapy cloth could introduce moisture or chemicals that might harm the optics, and a dry sponge, depending on its texture, might not effectively remove dust and could potentially scratch the lens. The camel-hair brush, therefore, provides a safe and effective method for maintaining the cleanliness of eyepiece lenses.