Filters...
Here I will again let an established site do some of the heavy lifting for me. There is no need for me to attempt to redo what has been well prepared by others as to what filters are best for what objects.
Guide to nebula and colour filters for astronomy
However, what I will do is offer some advice on how to select filters so that you make a better informed purchase.
There are four main nebula filters. Three of these, OIII, UHC, H Beta, are better suited to larger apertures. This is because they depend on a large aperture in order to make the transmitted light suitably strong enough to see. This is even more critical for the H Beta which is a very aggressive filter. These filters will perform BEST in apertures over 5", with the H Beta best over 10" it is so aggressive.
If your scope is 6" or below (no, I didn't make a mistake with the 6"), you may want to use a less aggressive filter, commonly known as a Broadband or Light Pollution Filter. These won't dim the overall image so much, but this also means that the contrast between nebulosity and surrounds won't be as stark. This is the tradeoff between the smaller aperture and wanting to more clearly see nebulosity.
There is a fifth type of filter. Narrowband filters can be expensive, and to purchase two or more even more so. One innovative company has produced a hybrid filter that combines the OIII and H Beta light bands in the one Hybrid filter. Omega Optical manufactures astronomical filters not just for themselves and its various subsidurary companies, but for some other high profile brands. Having all this sophisticated gear has also meant that they can fiddle around with innovative ideas, and produce very unique niche products. Their OIII + H Beta filter is one such product, and they provide it in both 1.25" and 2".
1.25":
1.25" OIII & Hb filter
2":
2" OIII & Hb filter
This filter is not as aggressive as dedicated OIII or UHC or Hbeta filters, but these really are a great compromise filter if $$$ need to be watched. Being less aggressive, it is probably a better option to a Light Pollution Filter. I have one of these, and I've managed to see the Horsehead Nebula with it too. But remember, the Horsehead Nebula is NOTORIOUSLY difficult to see, and it DOES require an aperture LARGER than 10" in order to have any chance of seeing it, AND
a very & transparent dark sky.
I'll talk about colour filters when I start a thread about viewing the Moon and Planets.
Alex.