Medium filters, represented by Lumicon's UHC and Orion's Ultrablock, are the bread and butter of deep sky observers. They are characterized by narrower passbands than the mild filters. More bad light is blocked, and they have a more noticeable effect on contrast. The improvement they bring to nebulas is truly dramatic. They also tend to work well on a wide variety of objects, from diffuse nebulas to planetary nebulas to supernova remnants. What are the trade-offs? The stars are dimmer in these filters than they are in the broadbands, making some fields a little less attractive than they would otherwise be. M42's nebulosity, for example, stands out beautifully, but the wondrous bright stars embedded within it are dimmed. A UHC, Ultrablock, or equivalent is the best filter for most objects and is probably the one to get first.
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