PT observation of transit of HAT-3 on UT 2008 May 12 = MJD 54598

Michael Richmond
Douglas Tucker
June 22, 2008

On the night of UT May 12, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of HAT-3. We detected the egress only, under mediocre conditions, at a time 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 0.005 mag -- not very 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.

The ingress occurs before the start of our measurements. I can see the egress at about 598.67, though it occurs in a noisy section of the data.

Justin's notes indicate that the ephemeris prediction is for ingress at 598.580 -- before our measurements -- and egress at 598.666 -- close to 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 HAT-3 made with APO PT, May 12, 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
May12.61731      4598.61731   4598.61937   0.740  0.003 
May12.61881      4598.61881   4598.62087   0.734  0.003 
May12.62028      4598.62028   4598.62234   0.739  0.003 

Last modified 06/22/2008 by MWR.