I know most filters are pretty much useless, but ones like ND, and other light pollution filters I've heard can be useful. I found a 4 pack of different ND filters, at 4, 8, 16, and 1000, but I'm not really sure if I need all the different blocking abilities. Do I just get an ND16 for moon observing, and other bright objects, or something else.
The point of ND filters is to reduce image brightness; so, you'll only use them when the image is so bright as to be uncomfortable. For an 8" scope you might find an ND 1.8 (same meaning as ND64) useful for Moon and Venus. You shouldn't need anything more.
If you want some flexibility you might consider getting to ND 1.8 by stacking two ND 0.9: one ND 0.9 would be useful for a crescent moon, while the stacked pair for would be more useful for first quarter to full Moon.
For comparison I have a 22" scope and I stack and ND 0.9 and an ND 1.8 - same as stacking an ND8 and an ND64 - for lunar viewing. I find this plenty bright, but comfortable. The effect of ND 2.7 is to reduce the brightness by a factor 512. Your 8" scope has about 1/8 the light collecting area; so, for the equivalent brightness you need reduce your image brightness by a factor of 1/64, which is ND1.8 (or ND64).
Other filters - either color filters for planetary viewing, narrow-band light pollution filters, or other narrow-band filters (UHC or equivalent, OIII, H-beta) - are used to increase the contrast between the night sky and the object you are looking at. For example, much of the light from planetary nebulae comes from two emission lines associated with OIII ionization. A filter that passes just OIII will not dim these much; however, it will dim any continuum emission (such as other stars and incandescent light pollution), or light pollution from, e.g., sodium vapor street lamps.
The same is true for colored filters applied to planets. If there are features on a planet (e.g., Jupiter's Great Red Spot) that you want to enhance, you might choose a filter that is complementary (in this example, blue): this will increase the contrast of the feature against the planet background.
Like @stoest and @ascii I've found that narrowband filters - a UHC (Lumicon) or NPB (DGB) - make a huge difference in the ability to see bright nebulae, and an OIII makes a huge difference in the ability to identify and/or resolve planetary nebulae. In my previous location the Veil was simply impossible until I saw it through a UHC filter.
Edited by lsfinn, 26 February 2020 - 02:52 PM.