Bruce L. Gary (GBL), Hereford Arizona Observatory (G95)

This web page is meant to present a photometric sequence for the exoplanet HD 74156 star field (from observations taken 2004.12.10 UT).
This exoplanet system is located at RA = 08:42:25.1, Dec = +04:34:40.2 (equinox 2000.0). The star is very bright, with Mv = 7.6.

Links Internal to this Web Page

    Overview of Star Field
    Photometric Sequence
    Nearby Star Observing Consideration
    Precision and Stability Test
    Related Links

Overview of Star Field

The following color image shows a large field of view (FOV) with the exoplanet HD 74156, hereafter referred to by a name that I can remember: E0842.

 RGB full frame

 Figure 1. Color image of bright stars surrounding E0842. FOV = 1.2 x 0.8 degrees.

 RGB crop

Figure 2. Crop of previous image, showing colors of fainter stars (brighter stars are saturated white). FOV = 29 'arc square. E0842 is the brightest star in the center of the image.

 Photometric Sequence

The following figure is a large FOV image showing the star field for E0842.

Star field 

Figure 1. Star field centered on the exoplanet star E0842 (square). FOV = 71.2 x 47.0 'arc, north up, east left. [Celestron 14-inch CGE-1400, prime focus, Starizona HyperStar, R-filter, 20 10-sec exposures, SBIG ST-8XE CCD; 2004.12.01, Hereford, AZ]


Figure 2. Crop and zoom of previous image. FOV = 21.8 x 17.5 'arc. (The limiting magnitude is 18.5, SNR = 3.)
 The following table lists all-sky photometric results for the 23 stars analyzed.

BVRI table 

 Figure 3. BVR magnitudes (left section) and star colors (right section), based on 2004.12.10 all-sky photometry observations (version 4C15b).

There was no measured brightness change for the stars in the table between 2004.12.01 and 2004.12.10, based on R-band images (at low air mass). The largest change was Star #15, which differed by 0.08 magnitude between the two images. My estimate for SE uncertainty (stochastic plus calibration) is 0.05 0.04 and 0.03 magnitude for B, V and R (for all stars with V-mag <12.5).

R-Band Chart

Here's a chart of R-band magnitudes that I plan to use for my R_band observations.

R-band mags

Figure 4. R-band magnitudes for stars suitable for use as reference (cicled) and possible check stars (ovals). FOV = 21.8 x 17.5 'arc. (The magntudes in this figure should be adjusted by +0.01 mag due to a last minute correction not yet aplied to the image.) 

 Nearby Star Observing Consideration

Observers should note the presence of a star close to E0842 for its potential for affecting measured E0842 brightness measurements. This potential problem is illustrated in the following figure.

 Photometry aperture issue

 Figure 5. This is one way to deal with the 5 magnitude fainter star 44 "arc north of E0842.

Since the fainter star north of E0842 has a brightness of only 1% that of E0842 if it is included in themiddle of the sky reference annulus, as hown in the image above, it will decrease the "intensity" of E0842 by a small and constant amount. For the aperture pattern depicted above the interfering star decreases the E0842 intensity by only 0.18 %, or 2 milli-magnitude. Provided the point-spread-function FWHM does not change dramatically this decrease of intensity will be constant. Since we're searching for a 7.5 milli-magnitude, this constant offset of 2 milli-magnitude is a small nuisance compared with alternative ways of removing its influence.

Precision & Stability Test

On December 26 I monitored E0842 (HD 74156) for the purpose of evaluating use of the R-filter and 8-second exposure time. I quickly concluded that E0842 was too bright, and would require use of an exposure time of 2 or 3 seconds to avoid saturation, so I then evaluated use of the I-filter. This was a better observing strategy as the following plot of brightness versus time shows.

4C26 LC

Figure 6. Brightness of E0842 versus time for a 2-hour test interval using an I-band filter and 7-second exposures. These measurements were made at 15-second intervals, and they exhibit a RMS about the average of 7.2 milli-magnitude. 9-minute averages of this data (red squares) exhibit an RMS about the average of 1.9 milli-magnitude.

Considering that the goal for this exoplanet is to measure a transit fade of 7.5 mmag (milli-magnitude) depth lasting 7 hours it is difficult for me to imagine that measurements such as these would be adequate. Whereas the RMS of the 9-miute averages is 1.9 mmag there seems to be  a systematic wander of a few mmag. The observing interval is insufficient to establish the temporal scale for these systematic wanders, but I shall take the position that observations like these are inadequate for the task of detecting the exected fade for this exoplanet. I won't be observing it during the predicted Jan 1 transit, therefore.

The check stars exhibited slightly worse RMS scatter, as the following graph shows.

RMS vs I-mag

Figure 7. RMS variation about the average for the 2-hour test observing interval versus I-magnitude. A model trace assumes a constant component (due, presumably, to scintillation) and a component that is related to SNR.

Related Links

    Details for this photometric solution
    Exoplanet HD 37605 (E0540) photometric sequence
    AAVSO web page for December Exoplanet Campaigns (HD 37605 & HD 74156))
    TransitSearch web page
    Tutorial for Observing Exoplanets
    TrES-1 Transit Observations
    HD 209458 Transit Observations
    Return to Bruce's AstroPhotos

    You may e-mail me at B L G A R Y @ u m i c h . e d u


This site opened:  December 9, 2004 Last Update:  March 30, 2006