thank you, i have now "the complete picture" (a local term). i think the best for me is buying 2 filters; one for the lunar and planetary observations, and another for the nebulae.
the most things i care for in planets is Mars` maria & polar caps, and Jupiter`s belts & red spot. so, i started to search for the apposite type of filters for me in the very useful link that you has gave to me. intially, i have found two good types of filters; #15 deep yellow and #29 deep red. but here i have a new question, can the polarizing filter makes the same work of the color filter in clearing these details? anyway, i will not buy a filter for the planets before i see how they seems in my telescope after i buy it (which would happen soon
). and for nebula, i will see what is the apposite for me.