PT photometry of transit of TRES-2 on UT 2008 May 28 = MJD 54614

Michael Richmond
June 23, 2008

On the night of UT May 28, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of TrES-2. We observed a full transit under good conditions, showing ingress and egress times in good agreement with the prediction.

Notes from the night

This is a chart of the field. TrES-2 is the bright star indicated by the crosshairs. The labelled stars will appear in later analysis.

The host star of TrES-2 has a magnitude V=11.41 according to TrES-2: The First Transiting Planet in the Kepler Field.

Following the procedures outlined by Kent Honeycutt's article on inhomogeneous ensemble photometry, I used all stars available in each image to define a reference frame, and measured each star against this frame. You can find the software package used to do the ensemble photometry online; it's free!

The graph below shows the amount by which instrumental magnitudes from each image needed to be shifted to match the ensemble reference. On a clear night, this graph would show a straight horizontal line.

Below is a graph of the scatter in differential magnitude versus magnitude in the ensemble solution.

TrES-2 is the star at differential mag 0.87; it ought to show a small excess of scatter over neighboring stars of the same brightness, and does. The "noise floor" in these measurements is about 0.003 mag -- good for APO transit work.

Below are the light curves for the target (green symbols) and four comparison stars in the field.

In this closeup, I have shifted the data for two comparison stars to move them closer in magnitude to the target.

My eyeball sees the ingress at 614.78, and the egress at 614.86.

Justin's notes indicate that the expected time of ingress is 614.783, which agrees with my eyeball estimate, and expected time of egress is 614.860, which also agrees with my estimate.

You can grab the measurements for your own analysis. Below is a table with three flavors of time, plus the differential magnitude of the target and an estimate of the uncertainty in each measurement. I show the first few lines of the file to give you an idea of its format.

# Measurements of TrES-2 made with APO PT, May 28, 2008 UT. 
# Each exposure 50 seconds long in SDSS i-band; 
# Tabulated times are midexposure (FITS header time - half exposure length) 
#     and accurate only to +/- 1 second (??). 
# 'mag' is a differential magnitude based on ensemble photometry 
#    using a circular aperture of radius 5.25 arcseconds. 
# UT day      JD-2,450,000  HJD-2,450,000   mag    uncert
May28.73440      4614.73440   4614.73543   0.873  0.003 
May28.73596      4614.73596   4614.73699   0.872  0.003 
May28.73753      4614.73753   4614.73856   0.869  0.003 

Last modified 06/23/2008 by MWR.