Just had a thought (there's a first time for everything) - what do you think?
Since stars with very small amplitudes are not - or shouldn't be - followed by visual observers, we deign to leave them to the CCD folks Since the variations of these stars are so small, then clearly the constancy of the check / comp stars is crucial. But we are constantly finding ever-increasing numbers of erstwhile 'constant' comps to be mini- or micro-variable, do these then become unsuitable for this purpose? Visual observers, who really don't mind if a comp star varies from 11.06 to 11.065, are now at an advantage as they have more 'constant' comps to choose from.
I would like to change your first sentence a little. In my opinion, stars with small amplitudes should not be followed by visual observers. Of course, "small amplitudes" needs to be defined. See below.
Unfortunately, many are, and they uselessly consume space in the database.
Perhaps the folk at HQ have looked at this problem, but if not I believe they should. My suggestion would be to look at stars with small amplitudes and many visual observations to find those for which (for example) analysis of the visual observations reveals no identifiable period in the case of periodic stars.
These stars should be labelled, somehow, "Visual observations not required" or some other phrase that captures the intent.
Of course the accuracy of visual observations varies among observers. In the past, I have looked at this for one particular low amplitude star, and found that the observations of most observers were very 'poor' for accuracy. One observer returned a constant magnitude for years! Another observer, on the other hand, had produced a nice light curve. It would be ideal if the 'accurate' observations could be retained and the others not.
With respect to the other issue (low-level variability in comparison stars), if the precision of your CCD/CMOS/DSLR observations will detect the variability, they should not be used as comps.
"Of course the accuracy of visual observations varies among observers. In the past, I have looked at this for one particular low amplitude star, and found that the observations of most observers were very 'poor' for accuracy."
Well, the same can be said of some CCD observers. LOL.
Yes, CCD results can vary among observers, but the comment is not in context. The point is that there are some low amplitude stars with many visual observations where the cycle is not evident wnen all visual observations are plotted.
Agreed. The previous both-siderist response does not address the issue at all.
Visual observations are needed when suitable comparison stars don't exist in the CCD field of view.
That aside: CCD observations have better precision; if done correctly (as most do) have better accuracy; they can be (and should be) transformed to (multiple) standard passbands which cannot be done visually; and unlike visual observations, CCD observation original data (images) can be reviewed at any later time. The supposed quality differences between CCD observers is much smaller than the wide difference between CCD and visual, and not in visual's favor. The most cursory glance at LCG is enough to see this.