With a clear filter we can usually get better S/Ns than with a V filter and it seems to be recognized that reporting CV measures (clear using a V comp) is useful. It seems to me that computing a CVI, say, transformation can certainly be done (my spreadsheet wouldn't know I was lying to it!) and, presumably, we could report CV measurements but claim they are transformed. BUT, would that be 'better?' Some of my recent targets are quite red (V-I ~ 3)(eg, V2492 Cyg) and the V images need much longer exposures than the I images to get reasonable S/Ns. Getting CV images at the same S/N would save considerable time (for more targets).
When I compute transformation coefficients I transform the instrumental values of the catalog stars to estimate how well it is working. For example, in my BVI values I find the average correction for 20 comp stars is 0.012(0.027) in B, 0.004(0.010) in V and 0.002(0.004) in Ic. From this I take it that my V and I filters are pretty close to "standard" and the B isn't too far off. Next chance I get (cloudy this time of year) I will re-run my M67 images using a clear filter in place of V and see what happens.
Jim Roe [ROE]