Telescope Reviews: I need a Primer on Filters

For a full article on filters, check out the following:

Filters for Deep-sky Use

In brief:

1. Light Pollution filters: there really is not a good general filter that will get rid of or significantly reduce a lot of the skyglow for all objects. The broad-band "Light Pollution Reduction" (LPR) filters can help a little with some mild skyglow, but the improvement they provide is slight and they quickly saturate under heavier light pollution. Examples of these are the Lumicon Deep-sky or Orion Skyglow. They are more useful for astro-imaging than visual observation, although I do find that they can help under slight skyglow at certain powers.

2. Nebulas: Nebula filters *can* often provide a significant boost in contrast, but they won't make the objects brighter. They come in two classifications: narrow-band, and line. Your Orion Ultrablock is a narrow-band nebula filter, and is a good choice for a "all-around" nebula filter. Like most narrow-band filters, it will make M42 appear more greenish in color, but at the same time, it will enhance the visibility of the fainter outlying structures of the nebula. Use it at low to moderate powers (3.5x per inch of aperture to 9.9x per inch of aperture) under a dark sky and it should make faint nebulae easier to see. There are also the even narrower Oxygen III (OIII) and Hydrogen-Beta (H-Beta) line filters which for some specific objects can provide even more contrast. However, you have to be fully dark adapted (20 to 30 minutes in total darkness with *no* nearby lighting), use averted vision, and use the right power ranges to get the most out of these filters. They won't work miracles, but they can help most emission nebulae. They also do not work well on objects like galaxies or star clusters.

3. Galaxies: No filter will provide a lot of improvement on galaxies. There are a few broad-band LPR filters (DGM Optics GCE filter, Lumicon Deep-sky, Orion Skyglow, etc.) that may provide a slight increase in contrast for some of the larger and more diffuse galaxies, but only under mild light pollution or under a dark sky. For galaxies, the best thing to do is observe them as far away from city lights as possible and with no moon in the sky.

4. Jupiter and Saturn. A pale blue filter (Wratten #82a) can help a little with making the belts stand out, but the improvement is often minimal at best. Some people also use a yellow filter sometimes, but again, in my observations, it doesn't help very much. There are a few "minus violet" filters that help a little with simple achromatic refractors, but beyond that, for the planets, I don't use any filtration at all. The only exceptions are for Mars during a good opposition, when I use a red filter to help bring out the dark markings or a pale blue filter to enhance the orographic clouds and polar caps of the planet.

As for 2" vs 1.25", for viewing larger emission nebulae, I do like having the 2" filters for use on my wide-field eyepieces that have 2" barrels. They may be a good investment for future use. Clear skies to you.