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UHC filter
As with any nebula filter, the bandwidth allowed through is narrow, i.e.the brightness of the "overall" image will be reduced 99%. That's less of a problem for a large scope than it is for a small scope.
Additionally, many small scope users tend to use them at higher powers, where the images are already darkened by magnification.
Keep the usage of such filters to 10X/inch or less (i.e.40X for a 4" scope) and the image will likely not be too dark.
These filters also have a different transmission at oblique angles than they do on axis. That means the nebula may be dimmed significantly if looked at off axis. If the nebula, just about the only thing still visible through these filters, disappears off axis, this is unlike the behavior of normal eyepieces where one can have one's head move around a bit and still see the image. Nebula filters require the head to be held much stiller. In practice, people don't have much of this problem with this wide a bandwidth (usually it's the line-band filters like a pure H-Beta), but to a newbie it can be disconcerting. When I explain to visitors about looking through my scope when I have a nebula filter in place, I explain this characteristic and no one has a problem with it. But people who have trouble holding their heads steady experience this more than others.
To avoid the problem:
--sit when viewing
--use low powers (larger exit pupils)
--adjust the eyecup up if it is adjustable.
But the nebulae themselves are dimmed only an inconsequential amount (0.1 magnitude or less), so there is no effective minimum size scope in which they can be used. A UHC filter will improve even the view of the Orion Nebula through a 50mm refractor.
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