Take the filter off ... and bin it. what type of filter is it anyway? if it's affecting daylight exposures by more than 1 stop there's something wrong unless it is an ND Being supplied with the lens I'd guess it would be a UV filter? A wide angle lens in daylight I'd expect to let in more light (than a longer lens) and so need less exposure - not more. Taking star shots It's not going to make any real difference to exposure - just capture more stars, making the starfield in shot look denser than with a zoomed in lens. Are you using a tracking mount? Found a great youtube vid here includes a diy tracker -
https://www.youtube.com/watch?v=vg3a0I0duMI