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Re: Everett and Howell Paper



I hope someone can figure out how to put this paper up on our web site.  I 
have written the authors, and will ask permission when I can get through to 
them.  At present the site seems to be down.  Possibly Michael knows the 
rules for doing this.

A partial answer to Andrews question is below:

>6. As mentioned above, we have little indication of whether a variable 
>star has been included in the ensemble stars until we have completed one 
>attempt at the differential photometry. To identify and remove any 
>variable stars from the ensemble, we examine a few basic statistics 
>calculated for each light curve. Those stars in the ensemble whose light 
>curves exhibit unusually large standard deviations for their predicted 
>errors and those with an unusually large value of 2 for the fit of a 
>constant magnitude to the light curve are removed from the ensemble. 
>Typically 10%20% of the stars are removed, leaving 1030 stars in the 
>ensemble. Those rejected include real variables, crowded stars, stars near 
>bad pixels, and stars with a cosmic ray falling in the aperture in one or 
>more exposures. The entire differential photometry process is repeated 
>iteratively, until a satisfactory ensemble selection is reached (e.g., 
>when no variables remain in the ensemble). Typically, after a full 
>reduction, the ensemble star light curves have standard deviations from 
>their means of 24 mmag, depending on their brightnesses. The fact that the 
>differential photometry correction is based on a large number of bright 
>stars ensures us that when it is applied to the ensemble stars themselves, 
>they will not have their natural noise or variations significantly 
>suppressed by "self-correction."


Tom Droege

At 01:35 AM 11/4/01 +0000, you wrote:
>On Sat, 03 Nov 2001 11:21:48 -0600, Tom Droege
><tdroege@veriomail.com> wrote:
>
> >The Everett and Howell paper is nice
>
> >Their (b) curve data is where we should be able to excel.  They state that
> >"This phenomenon is relatively common in our data (occurring in a few
> >percent of the stars).
>
>To save me from actually having to read the paper, I
>presume that "this phenomenon" is variability at the
>levels they can detect? (0.002 mag, single exposure)
>
>Can one translate this to what we could expect at the
>precision we are already getting with the Mk IV?
>(not much better than 0.01 mag)
>
>... does this mean that those high Welsh-Stetson
>statistics I got are real? I put most of them down to
>unknown systematic error. Wow! I must go back and
>count them.
>
>Andrew Bennett, Avondale Vineyard