PT observation of transit of HAT-3 on UT 2008 Jun 04 = MJD 54621

Michael Richmond
Douglas Tucker
June 24, 2008

On the night of UT Jun 04, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of HAT-3. We detected ingress and egress under good conditions, at times roughly predicted by the ephemeris.

Notes from the night

This is a chart of the field. HAT-3 is the bright star indicated by the crosshairs. The labelled stars will appear in later analysis.

The host star of HAT-3 has a magnitude V=11.59 according to HAT-P-3b: A Heavy-Element-rich Planet Transiting a K Dwarf Star.

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 very clear. 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. The spike due to clouds occurs at nearly the same time as the predicted egress, alas.

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

HAT-3 is the star near differential mag 0.7; 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 -- pretty good.

Below are the light curves for the target (green symbols) and some 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 estimates the ingress at 621.78. The egress is tougher -- there is one rise at about 621.86, a short plateau, and second small rise to 621.88. I guess the second one at 621.88 is the egress.

Justin's notes indicate that the ephemeris prediction is for ingress at 621.776 -- close to my eyeball estimate -- and egress at 621.863. That is close to the initial rise I noticed in the data.

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 HAT-3 made with APO PT, Jun 4, 2008 UT. 
# Each exposure 45 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
Jun04.73999      4621.73999   4621.74082   0.724  0.004 
Jun04.74150      4621.74150   4621.74233   0.726  0.004 
Jun04.74300      4621.74300   4621.74383   0.725  0.004 

Last modified 06/24/2008 by MWR.