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.
