The Wratten 25A is a red filter. The rationale behind using red and infrared light transmitting filters is that scattering and refraction are less at longer wavelengths. In fact scattering is inversely proportional to the fourth power of the wavelength. This means that red light from the Moon is less affected by adverse seeing. The image produced in red light is frequently sharper than that obtained using unfiltered light.
However when the seeing is good, using a blue filter can be slightly more advantageous. This is because the resolution of an optical system is linearly inversely dependent on wavelength; the shorter the wavelength, the smaller the Airy disk and thus the higher the resolution.
But seeing is rarely good enough to warrant the use of a blue filter and the benefits of this slightly higher resolution is offset by the fact that adverse effects from scattering and increased refraction in blue light will predominate.
Stick to the red and infrared transmitting filters. Also, most wecams, like the ToUcam, have essentially no blue sensitivity, but very high red and infrared sensitivity. The noise in the blue channel will be high making use of blue filters difficult with such cameras.
Incidentally, imaging the Moon in red can sometimes change the relative constrasts of the Mare and highlands since some Mare are red and some are blue, although few people appreciate this.
Zac Pujic
Brisbane, Australia
http://astroimg.org
