PT photometry of transit of TrES-3 on UT 2008 Mar 27 = MJD 54552

Michael Richmond
Douglas Tucker
Mar 28, 2008

On the night of UT Mar 27, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of TrES-3. We detected an entire transit. The transit is (as usual with this system) briefer than the ephemeris predicts.

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 not good. 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. This night's data is very close to horizontal -- it changes by only 0.025 mag over the course of the run.

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 very small excess of scatter than neighboring stars of the same brightness. The "noise floor" in these measurements is about 0.003 mag -- about as small as I've seen for the PT. 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. The lowest three light curves are eclipsing binary stars; one shows no features during this run.

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 start of the transit is 552.92, and my estimate for the end is 552.98.

An ephemeris grabbed from predicts for this night

 Begin Transit Window           PREDICTED CENTRAL TRANSIT     End Transit Window
                                     All Times UT

                               HJD        Year M  D  H  M
2454552.91 2008  3 27  9 50   2454552.95 2008  3 27 10 47   2454552.99 2008  3 27 11 44

The ephemeris ingress of UT 2008 Mar 27 09:50:00 corresponds to JD 2,454,552.910, which is about 14 minutes EARLIER than the observed ingress. The ephemeris egress of UT 2008 Mar 27 11:44:00 corresponds to JD 2,454,552.989, which is about 14 minutes LATER than the observed egress.

This pattern confirms 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, Mar 27, 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
Mar27.88504      4552.88504   4552.88553   0.730  0.003 
Mar27.88637      4552.88637   4552.88686   0.730  0.004 
Mar27.88770      4552.88770   4552.88819   0.728  0.003 

Last modified 03/28/2008 by MWR.