Howdy everyone.
Thanks. Actually is is sort of Ha and OIII as the UHCS filter favours Ha and OIII wavelenghts
I actually tried imaging M42 with my 7 Nm Ha filter and it was disapointing, not as much detail as the colour UHCS image.
Here is an article that I learn the masking technique from
http://www.astropix.com/PFA/SAMPLE4A/SAMPLE4A.HTM
HOWEVER, I dont know whay they say in point 4 to paste the LONG exposure into the mask, I get much better results if I pase the SHORT exposure into the mask. That was the inner detail of the short exposure appears, and the faint outer detail of the long exposure is maintained.