Re: Chi squared statistic and Error measurements

Posted by bswalter on Jun 29, 2020; 1:18pm
URL: http://astroimagej.170.s1.nabble.com/Chi-squared-statistic-and-Error-measurements-tp1379p1381.html

Karen,
Thanks for the response. I expected to get nothing but noise from this run. From previous transit LCs with this telescope I judged the limit for td detection to be somewhere between 1 and 2 ppt with optimum sky conditions. Looking at the data I expected the model to give me a flat line fit. I was surprised to get a fit so close to the predicted transit values when I set the ingress and egress markers at the predicted values. I tried other marker locations, and the model "flat lined" if I moved them very much including to the ingress and egress locations of the model in the initial fit.  I left ingress and egress markers at the predicted locations simply because the data gave no clue where they should be.

Gain, dark current and read noise were set correctly the aperture settings page. I read the AIJ paper a a couple of years ago and forgot appendix B. I have found that measurement errors determined empirically from standard deviations of binned objects and standard errors of their means compared to the corresponding values determined from the CCD error equation are often two to three times as large. Aside from the scintillation effects Luca mentioned, our skies are not photometric. There are constantly changing inhomogeneities within the field of view. Finally, many stars that are considered non-variable have low level variability at the fractional ppt level and we can't separate variability at that level from noise. In retrospect I think that may be true of the C4 star I chose. It has more empirically determined noise than it should given its average source-sky counts compared to the target and other comps.

Thanks again for the helpful response

Brad Walter