[Aavso-photometry] CCD 'fainter-than' question

[arne]

Bob Crumrine wrote:
> For CCD observing, what constitutes a 'fainter-than' observation?  If I 
> can see the variable on the image but only 10:1 S/N (50 minimum needed 
> for observation, below), is that a 'fainter-than'?
> 
I've been gone for 10 days, attending the SAS conference and
taking care of a few things in Flagstaff.  I've read the responses
to Bob's question.

The easiest answer is that, if you can see the target on your CCD
frame, you have a positive detection and should submit the magnitude
estimate, along with an estimate of the error.  At signal/noise of 10,
the error is on the order of 0.10 mag, which is acceptable.

If you cannot see the target on your image, then the simple approach
is to find the faintest sequence/comparison star that you can
see on your image and quote "fainter than" that magnitude.
You do not need to find S/N=3 stars on your image.  The sequence
is supposed to cover the entire range of any variable, so there
should always be a sequence star fainter than the faintest that
the variable can go (doesn't always work that way, but that is the goal).
In other words, something very similar to what you did as a
visual observer.

The other answers refer to what you do if you are in an unknown field,
or where you want to be as precise as possible.  That is a more complex
problem, as indicated by the discussion.  Not all software does a good
job of measuring things close to the limiting magnitude, so go down
that path with care.
Arne