This Lumicon Light Red #23A 2" filter is commonly used for an even more noticeable increase in contrast than the #21 and is ideal for scenes where it is important to differentiate the green tonal values in both celestial observation and photography. It produces even greater blue suppression and leads to a further increase in the effects of darker red filters.
For planetary observing, color filters bring out the most detail because planets are naturally colorful. Different colors bring out different details of the planet you are viewing.
Always remember that you want to use the darkest filter your telescope will handle. Most small telescopes up to about 5" in aperture use the light colors. The larger the telescope, the darker your filter you can use. Color filters can sometimes be used in conjunction with a Neutral Density or Polarizing filter to aid in glare reduction. The more you use your filters the more you appreciate them and their affect.
The 1.25" filters employ industry-standard 1.125" x 42 tpi threads (except for Questar/Brandon, which use a thread pattern unique to Questar). The 48 mm filters employ industry-standard 48 x 0.75 mm threading, which thread into 2" eyepieces. All larger filters use the given size with 0.75 mm pitch (e.g. a 72 mm Minus Violet filter has 72 x 0.75 mm threads).