PT photometry of transit of TrES-3 on UT 2008 May 08 = MJD 54594

Michael Richmond
Douglas Tucker
June 20, 2008

On the night of UT May 08, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of TrES-3. We detected egress well, but the run started just after the predicted ingress.

Notes from the night

This is a chart of the field. TrES-3 orbits the is the bright star indicated by the crosshairs. The star labelled "C" will appear in later graphs.

The host star of TrES-3 has a magnitude V=12.40 according to TrES-3: A Nearby, Massive, Transiting Hot Jupiter in a 31-Hour Orbit .

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 night was okay. 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-3 is the star at differential mag 0.74; it shows a small excess of scatter than neighboring stars of the same brightness. The "noise floor" in these measurements is about 0.004 mag -- not bad. Many of the outliers in the plot above are real variable stars (which were removed from the ensemble solution).

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 estimate for the ingress is 594.715, but it's hard to say; there isn't much data before ingress, and the measurements are somewhat noisy. But I can see the egress clearly at 594.765.

Justin's notes indicate that ingress should have occurred at 594.681, before the start of the measurements, and egress at 594.767, just at the time I see the egress.

The egress occuring at the predicted time is not consistent with the indications from earlier nights,

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-3 made with APO PT, May 8, 2008 UT. 
# Each exposure 30 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
May08.70102      4594.70102   4594.70322   0.732  0.004 
May08.70235      4594.70235   4594.70455   0.727  0.004 
May08.70369      4594.70369   4594.70589   0.730  0.004 

Last modified 06/20/2008 by MWR.