I get excellent images from MPO and use AAVSO sequence. SNR of target and comps lays in the range 50-400, check star estimations are close to real values. All is OK, except abnormal high errors about 0.08-0.09. All attempts to use shorter sequence increase check star deviations. I do not understand the reason of so high errors. Maybe longer exposures are needed or something else?
Hi,
most of your comp stars are dimmer than the target star (FW Boo) that increases your errors on the photometry. Try to remove comps 152, 140, 144 and rerun your analysis. SNR of 100 gives approximately an error of 0.01.
Josch
Thank you, Franz! Of course, I tried to use shorter sequences. Indeed, according to you recommendations errors decreased to normal level, but regrettably check star deviations increased dramatically:
5 comps: 0.002B, -0.007V
2 comps: 0.092B, 0.075V
Which way is more correct for photometry - high error and low check star deviation, or reasonable error and higher deviation?
Mikhail:
You have been observing for a while. What is your opinion/judgement? Can you think of circumstances where one or the other may be appropriate? Which puts more scatter in the lightcurves of ‘multiple’ observers on the LCG?
Ken
Hi Ken, I’m glad to see that you keep your method of questioning 
Truly speaking, I suppose good coincidence of check mags is more important than error. One can get extremely low error with single comp star, but longer sequense is more preferable.
But in this case I’m not so confident… Maybe two comps will be enough, in spite of check star deviations?.. Errors are so huge!
Mikhail,
Remember what the purposes of the check star are. The first is to ensure that the comp star is not itself variable. If it does exhibits short term variability, the check star light curve will not approximate a horizontal line. And of course it may be that the check star is variable! If you make enough observations over years, you will experience both situations. Experimenting with different sequences solves the problem if it arises.
The second purpose of the check star is just as it name suggests - to provide a reality check on your measurements and calculations. The measured magnitude of the check star should be close to its catalogue magnitude.
Provided that the precision of the check star measurements is acceptable, it does not matter if its measured magnitude differs from the catalogue value by a little more than you would like to see, particularly if at the same time the error for your variable star measurement is very small. In my opinion, this is the preferred situation.
Concerning comp star ensembles, the current guideline supports their use, and there are good reasons for this. Also, and particularly if you use VPhot, iteratively selecting different ensembles allows you to see easily how to optimise your errors.
However, I have personally never considered it mandatory to use ensembles. It is interesting to compare the calculated magnitude of the variable using the individual members of a comp star ensemble over a number of nights. For example, in my experience the check star magnitude will stay pretty much constant with each particular (individual) comp star, and there will be consistent differences between the magnitudes of the check depending on what comp star is use for the calculation. You might expect this, because it may be that the catalogue magnitudes of the comps will vary in their accuracy.
One other point about ensembles of comps is that, if one of them is in fact slightly variable, it may be that this will not be revealed in the check star light curve because the variability is ‘swamped’ by the data from the other comps. You may argue that does not matter because one purpose of the ensemble is to average out (minimise) errors, but it has always bothered me that variability may be hidden in this way.
Considering all of the above, and because I obtain time series of non-transformed V magnitudes of short period eclipsing binaries using a camera with a 14 bit sensor, I prefer to have the comp B-V and V as close as possible to those of the variable, and therefore mostly do not use ensembles. This is a personal situation. It will not apply to many observers.
Roy
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Thank you, Roy, for detailed explanations!
Well, probably compromise search between check star precision and target error is correct way, but it leaves too much for human factor…
Your note about check star LC is very important! Is it possible to see in VStar plot both check and target light curves???