[Aavso-photometry] photometry of 27 virginis

[arne]

trupti ranka wrote:
> hello everybody,
>   A month ago I started with photometry of a variable star named 27 virginis. i was using a 14" celestron telescope and SSP3-A photometer. Sky map pro software said that the star magnitude varies from 6.19 to 6.33 which we thought was good enough to be observed by 14" telescope. but after initial data reduction we found that star magnitude varies only by 0.05 magnitude. we checked some IBVS papers which confirmed are data. The papers (1975,1977,1980)reported a magnitude differences of about 0.02 to 0.03.
>   Now the problem is that since we were using a 14" telescope for such a small amplitide variation the data accuracy is poor. also the star is multi periodic delta scuti star. Its period varies fro 0.5 hrs to 3 Hrs (according to IBVS papers). I would like to know if there is a better and more relaible way to calculate error other that 1/SNR or some special data reductoin tricks to improve accuracy. also i would like to know if there is a specific software for calcualting periods of short duration multi periodic stars(i heard there is a FORTRAN based software but can't find it) . this has turned into a tricky star for data reduction but i would like have some help as i have spend lots of nights observing it and don't want the data to go waste.
>    
27 Vir == GG Vir, a low amplitude delta Scuti star.  As Trupti mentions,
IBVS papers from Sterken and Bartolini indicate amplitudes in the 0.03mag
range, so I'm not sure where the GCVS got their amplitudes from (Sky Map Pro
and the AAVSO's VSX program, http://www.aavso.org/vsx get their
variable-star information mainly from the GCVS).

As Jeff mentions, the usual practice with PEP observations is to
take multiple readings bracketed by the comparison and check stars
(see also Astronomical Photometry for details).  1/SNR is a first-order
approximation to the true error, and basically gives you the absolute
lowest error estimate (the true error will always be larger than this).
If you did not take your measures in this manner, it will be difficult
to obtain a better error estimate.

As for period analysis software, Peranso does a nice job:
http://users.skynet.be/fa079980/peranso/index.htm
Look at the nice powerpoint presentation by Matt Templeton
for other references:
http://www.aavso.org/aavso/meetings/spring03present/tsa.ppt

You picked a tough star to observe.  Not only are such delta Scuti
variables quite low amplitude, but the multiple periods make
analysis difficult.  You might try something easier next time,
like the new eclipsing cepheid!
Arne