PT observation of transit of HAT-3 on UT 2008 Apr 07 = MJD 54563

Michael Richmond
Douglas Tucker
May 03, 2008

On the night of UT Apr 07, 2008, the SDSS Photometric Telescope ("PT" for short) took a series of exposures of HAT-3. We saw a full transit 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.

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 -- good for the PT.

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 for the ingress and egress are: ingress at about 563.785 and egress at about 563.865.

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
2454563.77 2008  4  7  6 31   2454563.82 2008  4  7  7 46   2454563.88 2008  4  7  9  1

The ephemeris ingress of UT 2008 Apr 07 06:31:00 corresponds to JD 2,454,563.772, which is a bit earlier than I estimated. The ephemeris egress of UT 2008 Apr 07 09:01:00 corresponds to JD 2,454,563.876, which agrees with my eye estimate.

Note that PT observations on Mar 12 also suggested that the ephemeris ingress time was a little bit earlier than the observed ingress.

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, Apr 7, 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
Apr07.74310      4563.74310   4563.74636   0.727  0.003 
Apr07.74458      4563.74458   4563.74784   0.725  0.003 
Apr07.74962      4563.74962   4563.75288   0.724  0.003 

Last modified 05/03/2008 by MWR.