[Aavso-photometry] 70 Virginis observations

[Richard Miles]

As part of the current exoplanet campaign, I spent some time last night 
imaging the field around 70 Vir using a 6-cm aperture Takahashi FS60C 
refractor + V filter + SXV-H9 CCD camera operated unbinned.  This 
combination has a field size of about 1.0x1.5 deg so in addition to 
recording the closest comparison star (HIP 65749, V=8.58), I was able to 
also register three other 8th magnitude stars (HIP 65644, HIP 65589, and the 
furthest HIP 65534 some 48 arcmin from 70 Vir).

I defocussed the telescope so that the FWHM was increased from about 1 pixel 
to about 4 pixels and used a 5-pixel radius photometric aperture for the 
analysis.  Looking at a sequence of 100 x 5-second exposures made when the 
field was at an altitude of 46 deg, the following was found.

Standard deviation rel. to nearest comparison = 0.021 mag
Same rel. to nearest two 8th mag stars = 0.017 mag
Same rel. to furthest 8th mag stars = 0.016 mag
Same rel. to ensemble of all four 8th mag stars = 0.014 mag

Standard deviation of 4-star ensemble, 10-frame averages = 0.005 mag
Cycle time for 10 x 5-sec frames = 90 sec

One of the aims of the campaign is to identify possible flares of the main 
star, which may then translate (after a delay) in possible auroral activity 
from any orbiting planet, detectable in the radio spectral signature of the 
system.  Flares, if these occur will be significantly more intense in the B 
filter passband than in V.  However from 30 minutes of photometry to the 
above level of precision, no flare was detected in V.

Richard Miles
Golden Hill Obs, Dorset, UK