In my short experience?
Short answer is none.
Long answer is that most people end up with a pile of filters, used somewhere between sometimes and never!
My 2 cents worth:-
I guess you have a "Moon filter" already, which is some form of neutral density filter. Try this also on Jupiter and Venus when they are particularly bright.
A better option for this application is to buy two polarising filters and fit both as a set of crossed polars - just rotate the bottom one relative to the upper one until you have the brightness that is comfortable for you. A couple of 1.25" polarising filters are cheap (I got mine from
www.aoe.com.au).
I've never had great success with coloured filters, but I'm still learning. Here is a good site describing their application:-
http://sciastro.net/portia/advice/filters.htm
I do put a yellow filter on when I'm showing the sun through a white light solar filter (Thousand Oaks, over the end of the scope) - so people see the sun "nice and yellow", the way they drew it in kinder! Remember, the only place for a solar filter is at the field end of the scope, not the eyepiece end!
That web reference also describes other specialist filters that attempt to remove light pollution or enhance nebula emissions relative to the sky or other light sources.
I particularly like the DGM Optics NPB filter:-
http://www.iceinspace.com.au/forum/s...ad.php?t=22373
http://www.iceinspace.com.au/forum/s...ad.php?t=23307
Lots of good threads on filters in the forum, just search, eg:-
http://www.iceinspace.com.au/forum/s...ad.php?t=21056
Size:-
Your problem is deciding whether you need a filter in 2" since or 1.25" size?? If you can afford the 2" size, buy it. Then it can go on the end of a 2" eyepiece, or on the end of the 2"--> 1.25" adapter. Then you can change 1.25" eyepieces and not have to worry about swapping the filter to the new eyepiece. BE CAREFUL - some 1.25" eyepieces (eg. GSO plossls) are long and will hit the filter when you drop them into the adapter. Either tighten them up not fully in (and you'll probably forget) or put some form of ring on the eyepiece to prevent it going fully in (Your local Clark Rubber store has a stock of rubber O-rings. I have some which fit fairly neatly over the 1.25" barrel and prevent the eyepiece going all the way in. Not a precise solution, but it works.) However, if you are going to drop in a 2x or 3x 1.25" barlow, it will definitely be too long and will hit the filter.
Tip: If everything looks wrong, just check carefully that you haven't left a filter on somewhere and forgotten about it. Gee the sky is dark tonight! Woops, Moon filter still in place!
Have fun!
Eric
