Baseline Photometry of Exoplanet HD 74156
on 2004, Dec 02/03
and the AAVSO recently announced a photometric campaign to check for planetary transits of HD 74156 and HD 37605. The most probable time of a transit of HD 74156 occurs on Jan 1, 2005. 

CCD image showing the HD 74156 field. The host star is labeled "Obj1". Comparison star is GSC 219:1841 (Mag 8.98), labeled "Ref1". Finally, the check star was GSC 219:725 (mag 9.63).

In order to prepare for the transit observations, I started a baseline photometry session on HD 74156 in the night of Dec 2/3, 2004. Observations lasted for about 5 hours (1h22m UT till 6h22m UT), resulting in more than 1200 CCD images. I used a 0.35-m f/6.3 telescope, with a V-filter and SBIG ST-7XME ccd camera. Due to the brightness of HD 74156 (V = 7.61), I had to keep exposure times extremely short (3 to 4 seconds), to avoid pixel saturation. Clearly, this had a negative impact on the final precision of my photometric results (see further).

The photometric analysis was done using MaxIm DL/CCD v4, using both single and multiple comparison stars. After having performed bias-dark-and-flatfielding, images were stacked together per 5 and measured using aperture photometry. Each data point in the light curve below is the average of 4 photometric results (each result representing 5 stacked CCD images). Also shown is the standard deviation per bin.

The resulting light curve depicts - as expected - a nearly flat pattern. The level of noise is higher than what I'd like to see, and likely is caused by : (1) scintillation because of too short exposure times, (2) a nearby moon, (3) poor photometric sky conditions. I reached an overall accuracy of about 0.005 mag, which will need to be increased for the next campaign.

To further improve precision, I will probably have to use a technique, described recently by Ron Bissinger on the Transitsearch mailling list (instead of stopping down the aperture of my telescope - the latter apparently would have a negative effect on scintillation too). Bissinger's technique consists of using Kodak Wratten neutral density gel filter material, which is cut it into a semicircle that only covers the imaging chip of his CCD camera, and leaves the guide chip uncovered. When using 1.0 factor material, only 10% of the light is transmitted. Ron mounted the material in a scissor-cut cardboard mount and placed it between the CCD camera window and the focal reducer and/or nosepiece. 

Baseline photometric light curve of HD 74156, obtained at CBA Belgium Observatory.





Copyright © 2004 - Tonny Vanmunster.