[Aavso-photometry] Ensemble photometry question

[Brad Walter]

 Sorry, I answered in a hurry and stated my technique for ensemble error
incorrectly. I am looking for exoplanet transits, and therefore, I care
about changes in magnitude not the value of the magnitude. Precision is
important, accuracy is secondary. Since I am taking many measurements of
the same object in one night, on each measurement I use the std error of
the comp star least squares fit values for a single measurement from the
average values of the comp star values for the whole session. In
descriptive terms for a Maxim DL output, I take the average of all of
the values in each comp star column of the CSV table (or excel
spreadsheet). Then for a single row (one measurement) I take the std
error of the comp star mags for that particular measurement (row) from
their averages for the session.
SQRT(((C1-C1BAR)^2+....+(Cn-CnBAR)^2)/n)/SQRT(n). That gives me the
stochastic error for that measurement. 

-----Original Message-----
From: aavso-photometry-bounces at mira.aavso.org
[mailto:aavso-photometry-bounces at mira.aavso.org] On Behalf Of Brad
Walter
Sent: Wednesday, January 11, 2006 8:29 AM
To: aavso-photometry at mira.aavso.org
Subject: RE: [Aavso-photometry] Ensemble photometry question

 I have been wondering the same thing. To date I have been generating an
error for each measurement using the std deviation of the comp values
that Maxim DL generates using a least squares method from their listed
values. This results in a different error for each measurement that
should indicate which should indicate the relative precision of each
measurement. 

A different but related topic
I have been using a 106 mm aperture 530 mm F/L refractor to gather V
band light curve data on exoplanets. My 250 mm aperture 3000 mm F/L
scope doesn't have sufficient FOV to obtain suitable comp stars for
these bright HD catalog objects. I don't seem to be able to get better
than about .006 mag precision even though I have very good (several
thousand) SNRs. I am defocusing to around 4-5 pixel FWHM. I average
between 5 and 10 images depending on exposure duration and take readings
from the resulting combined photos. I am using 25-30 each flats darks
and flat darks. Exposure durations are constant during a session but
range over 5 to 20 seconds depending on the brightness of the target and
comp stars for the session. I am not auto guiding because I don't want
the re-centering delay. Therefore there is slight but steady drift of
the stars over a session. I suspect that the culprit is scintillation
due to the seemingly random variation, but it may also be differences in
pixel from movement of the stars over the CCD. Total movement over 4
hours is around 20 pixels. Does anyone have any suggestions for
improving the precision?

-----Original Message-----
From: aavso-photometry-bounces at mira.aavso.org
[mailto:aavso-photometry-bounces at mira.aavso.org] On Behalf Of Robert J.
Modic
Sent: Wednesday, January 11, 2006 4:20 AM
To: aavso-photometry at mira.aavso.org
Subject: [Aavso-photometry] Ensemble photometry question

I wondering what the best method was to calculate the total error of an
ensemble of comp stars.  I know that the error decreases with the square
root of the number of comp stars.  Since each comp star has its own
error that is usually different from other stars in the ensemble, what
formula should I use?

Bob Modic (MRV)


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