Quick ensemble solution of Mark IV data around HD 155229

Michael Richmond
Aug 8, 2002

John Greaves wrote about offsets between nights: 

> Granted the
> mean mag on the two nights is different enough to make one feel that
> different parts of a varying light curve are being sampled.  Trouble is,
> I'm getting different signals on this.

Andrew Bennett weighed in with his experience: 

> The basic problem seems to be that two frames on different
> nights don't cover the same patch of sky, don't share the 
> same Tycho stars and do manage to have unexpectedly large
> zero-point offsets. What is worse is that these offsets
> seem to manage to afflict individual stars without
> affecting all nearby stars in the same way ...

I can't do a real analysis of this problem right now, because it would take forever :-( However, I tried something quick last night as a check. John was writing about the star HD 155229. I decided to pull out the magnitudes for this star and all others within roughly 1 degree from Tom's July, 2002, Mark IV dataset, and subject them all to an ensemble solution.

It turned out there were just two nights with measurements -- not enough, really, but I'd already gone this far, so ... I ran the ensemble solution, and found the following:

  1. HD 155229 dominates the ensemble solution; if one doesn't mark it as variable and exclude it from the calculations, it imprints its own variation on all the other stars

  2. the stdev from mean value has a floor of about 1% at the bright end (around V=8), rising to 10% at about 4.7 mag below the brightest (around V=12.7)

  3. there is a slight zero-point shift between the two nights, but only about 0.02 mag; it's just a bit larger than the frame-to-frame scatter within a single night

  4. the outliers in the sigma-vs-mag plot (and so, I think, in the Welch-Stetson value) are a pair of stars which have one anomalously bright measurement per night: see blue squares and black stars below:

  5. finally, on HD 155229 itself, I think John is right: it's not going to be easy to measure the variation of this star with Mark IV data. Here's a closeup of HD 155229 (red squares) and another bright star in the ensemble.

Eventually, after data is placed into a database, I need to figure out a way to run ensemble solutions on it in a systematic way. This is an area which needs lots of investigation.

But I haven't yet seen a strong case for large night-to-night zero-point variations...