Mon, 11/29/2021 - 22:36
Not sure if this is the right forum to post my question, but I'll give it a try.
When doing photometry of stars that vary wildly, some stars have ensembles that cover a lot of magnitudes.
Thus, it may be possible that for some magnitudes of the variables some stars of the ensemble are not reachable : too bright, or too weak.
So the question is :
How does one cope with that when reporting to webobs one used the "other stars of the ensemble as they were available" ?
Kind Regards,
pieter.
(tought I found an answer, but it's not correct)
Hi Pieter,
When you download a chart for a variable, you also get a sequence of comparison stars. The sequence team generally makes the sequence for visual observers, and so covers the entire range of the variable in roughly 0.3mag steps. For a high-amplitude variable like a Mira star, this can mean stars ranging over several magnitudes.
When using this sequence as an ensemble in something like VPHOT, all of the comp stars above a certain signal/noise threshold are included in the ensemble, with the fainter stars being discarded. This is fine, as the fainter stars basically contribute noise to the final measurement. Using the best comp stars to form the ensemble is fine, and you don't have to report that some comps were not used.
There are lots of reasons why some sequence stars are not used. Your camera field of view might be too small to include some of the stars. Some might be saturated, or blended with a neighbor at your imaging/seeing resolution. A satellite or a hot pixel might have compromised one star. Just do a good job of selecting the best stars for the ensemble, make your estimates and report them.
Arne
Thanks, wanted just…
Great !
Thanks, wanted just to be sure that it is not a problem if I do not use all compstars of a sequence.
Thank you !