PT observation of transit of TrES-4 on UT 2008 Apr 08 = MJD 54564

Michael Richmond
Douglas Tucker
May 03, 2008

On the night of UT Apr 08, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of TrES-4. We saw only the ingress, under good conditions, at a time roughly predicted by the ephemeris. This is our first measurement of this system (I think).

Notes from the night

This is a chart of the field. TrES-4 is the bright star indicated by the crosshairs.

The host star of TrES-4 has a magnitude V=11.592, according to TrES-4: A Transiting Hot Jupiter of Very Low Density.

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 clear, though the logs suggest dusty. 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-4 is the star near differential mag 0.38; it shows a small excess of scatter over neighboring stars of the same brightness. The "noise floor" in these measurements is about 0.003 mag -- good for the PT.

Below are the light curves for the target (green symbols) and some comparison stars in the field. For some reason, the measurements of the target stop a little before the end of the run; but since the egress lies well after the end of the run, I didn't bother to track down the reason.

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: ingress at about 564.90.

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
2454564.89 2008  4  8  9 26   2454564.98 2008  4  8 11 25   2454565.06 2008  4  8 13 24

The ephemeris ingress of UT 2008 Apr 08 09:26:00 corresponds to JD 2,454,564.893, which agrees reasonably with my eyeball estimate. The ephemeris egress of UT 2008 Apr 08 13:24:00 corresponds to JD 2,454,565.058, long after the end of our observations.

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-4 made with APO PT, Apr 8, 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
Apr08.86054      4564.86054   4564.86158   0.371  0.003 
Apr08.86212      4564.86212   4564.86316   0.370  0.003 
Apr08.86367      4564.86367   4564.86471   0.374  0.003 

Last modified 05/03/2008 by MWR.