I'm looking to find a V filter which is 2" in diameter and will thread on to ther end of the 2" adapter on my ST8XME.
Are V filters made in this size?
I have a set of 2-inch round filters in ring mounts, but unfortunately they don't have screw threads. The ST8 has a physically small sensor, and doesn't need 2-inch filters. I'd recommend using a 1.25" nosepiece, and then a 1.25" to 2" adapter to fit your focuser. Otherwise, I know Chet Schuler provided some threaded filters in that size, so you might check with Don Goldman at Astodon to see if he has such rings and is willing to make you a filter.
Does everybody submit their results to the AAVSO with a V filter?
Can a Clear filter be used?
We highly recommend using a standard filter, as this makes it much easier to compare your magnitudes with others. This is especially important with red stars, where the red sensitivity of the CCD skews the results dramatically, often by a magnitude or more from those who are using filters (or even just a different kind of camera). There are certain projects where unfiltered photometry gives acceptible results, though even there your measurements will find a wider audience if they are done with a filter. An example is the cataclysmic variable monitoring done by Joe Patterson and his team, where the unfiltered photometry gives good temporal resolution of the short orbital or superhump periods. Joe can adjust the offsets between the various time series to get the science he needs, but other researchers who might want to use the same datasets for temperature studies or long-term variations might discard the unfiltered results in favor of the standardized ones.
Thanks Arne for the clarification.