From: Jason Rowe <jasonfrowe@gmail.com>
Subject: Re: Some follow-up questions re: MOST observations of sigma Ori E
Date: January 23, 2012 8:56:26 AM GMT-02:00
To: Richard Townsend <townsend@astro.wisc.edu>

Hi Rich,

It's my understanding that column 2 (the instrumental magnitude) and column 13 (the star flux) represent essentially the same data, but with a couple of differences, viz:

(i) column 2 is in magnitudes, with a mean of zero; while column 13 is in flux units.
(ii) column 2 has had a variable stray light correction applied, while column 13 is 'raw'.

My questions are as follows:

(a) In the column 13 data, there is a significant jump in the flux (by about a factor 2) about 3/4 of a day after the start of the observations. How did you go about correcting for this jump? (To illustrate this jump, I attach a plot of the flux light curve for the first part eight days of the observing run). Is the jump perhaps a result of a change in gain, or in exposure time?

Column 13 is in ADU, so no corrections for exposure time are applied.
The instrumental magnitudes (column 2) are correceted for exposure
time.

(b) How precisely did you apply the correction for variable stray light effects (in particular, the correction for the modulations on the orbital period)? I'm guessing you didn't use the Reegen et al. (2006, MNRAS, 367, 1417) approach, since that is for Fabry rather than direct mode. Do you instead follow the procedure you discuss in section 2.2.6 of Rowe et al. (2006, ApJ, 646, 1241)? Or is some other method used?

Part of my delay is replying was finding my notes from the reductions.
My 2006 paper is a good reference, but here are the details:

-no background gradient was fit (this was not necessary)

-pure aperture photometry was used to estimate photometric flux.  An
aperture with a radius of 4 pixels was used.

-The 'sky' was estimate from all future than 8 pixels from the
photometric center.  Centroids where measured by fitting the PSF with
a 2-dimensional Gaussian.

-Photometry variability of sigma Ori E was observed to be much larger
(~0.1 mag) than instrumental variability related to the amount of
stray-light (~0.02 mag).

-An 8th order sinusoidal series was fit to the data centered on the
primary period of 1.1908 days and removed from the time-series.

-A polynomial was fit and subtracted from the residuals the
relationship between the sky-background and instrumental magnitude.
If the above step is not taken than the stray light correction can
produce aliases related to 14.1 c/d

-outliers where clipped at the 5-sigma level.

-The sinusoidal fit was added back to the data.

(c) As a parallel question, am I correct in interpreting column 13 as the result of applying the steps described in sections 2.2.4 and 2.2.5 of Rowe et al. (2006) -- that is, background gradient fitting, cosmic ray removal, sky subtraction, and PSF-fitting/aperture photometry? So, column 13 is the flux prior to the variable stray light correction? And then column 2 results from the variable stray light correction, converted to magnitudes with the zero subtracted?

column 13 will only include sky-subtraction.  I use it as a diagnostic
to search for saturation and non-linearities and sometimes for flux
calibration.

As a final question -- given that you've already invested time in these observations, and moreover given that I'm taking up more of your valuable time right now, I'd like to include you on the author list for the paper I'm writing. Are you OK with that? Of course, I'll be circulating a draft once I get it done.

If you want me to write up a section of data-reduction then let me know.

Cheers,
Jason



-- 
Jason Rowe - Kepler Mission Transit Scientist
NASA-Ames Research Park
MS-244-30
Building 244, Rm 108
Moffett Field, CA 94035-1000
ph# 650-605-3468 / fax# 650-604-2338
