What to expect in the eyepiece All celestial objects get a distinct orange hue. The #21 orange filter has an overall transmission of about 50% and dims the telescopic image noticeably, but not as much as the red filters #23A or #25A would do. 

For a small telescope it is the ideal filter on the red side of the spectrum. Moon: For observing small detail along the terminator, the #21 is a very good choice. Contrast is strongly improved. Mars: 

The #21 orange filter is very useful to enhance the dark albedo features on Mars. These have a bluish/greenish hue, with the #21 in place they stand out better against the bright orange background of the planet's disk. Great Mars Filter Simulated small telescope view of Mars unfiltered and #21 filtered Jupiter: On nights of very good seeing, the filter may be used to enhance the bluish features that can be found in Jupiter's atmosphere, like festoons or the polar regions.

All objects: Filter #21 orange is especially useful for telescopic twilight observations, for example of Mercury, Venus or the Moon. Since the filter cuts off blue colors, it considerably darkens the twilight sky and celestial objects stand out better.