MBCAA Observatory

Data processing

Michel Bonnardeau
31 Dec 2007
Revised 25 Dec 2012


A partial list of what I do to process data for photometry.


Here is a partial list of what I do to process my data, in the order I do it. Most of it is done with the AstroMB software package.

Position filter

This process is used only when the variable star is both faint and close to a bright star, e.g. BG CMi.

The celestial coordinates of the centroid of the PSF of the star being analyzed are recorded. They are compared with those of the star. If they are different, an alert is raised: the variable may have become very faint and is corrupted by its bright neigborhood.

Extra losses filter

This a procedure to reject low quality photometry measurements, from Gary (2007) (chapter 13).

The magnitude V of the variable star is computed from the magnitude C of the comparison star from:

V-C = -2.5*log(IV/IC)

where IV and IC are the intensities (number of ADU from the CCD) of the variable and of the comparison respectively. I call the quantity:

ZP = 2.5*log(IC) + C

the "zero point" of the photometry.

For each session, the zero points of the photometry are plotted against the air mass. The plot show a linear decrease of ZP with the air mass (the slope is minus the extinction) and, when the sky is poor, "extra losses". An example of such a plot:

Red circles: the measurements;
Blue line: the trend ZP(air mass). The measurements below this line are "extra losses" due to poor sky;
Green line: the images with the "extra losses" below this line are rejected.

Extra losses greater than a threshold (e.g. 0.1 mag) indicate poor air transparencency and the images may be rejected.


The statistical uncertainty of a star is computed from the signals in the aperture. The background noise is computed as the standard deviation of the outer ring of the aperture, the star noise as the square root of the star signal in the inner circle of the aperture.

The statistical uncertainty of a variable star and of the comparison star are quadratically added to obtain the total statistical uncertainty. This is valid only if the fluctuations of the variable star and of the comparison star are not correlated. However they may be: atmospheric variations or turbulence may induced correlated fluctuations for both stars. The quadratic sum may then lead to an overestimate of the statistical uncertainty.

The statistical uncertainties of the check star are computed the same way as for the variable, and the average uncertainty is compared with the standard deviation. The SD is expected to be larger because it takes into account errors other than the Poisson's errosr, but it should not be much larger.


Gary B. (2007) Exoplanet observing for amateurs Reductionist Publications.
(May be downloaded from http://brucegary.net/book_EOA/x.htm.)

Technical notes

Telescope and camera configuration.

Computer and software configuration.
The plot of the ZP versus the airmass is straightforward with the AstroMB software package as both these quantities are computed and recorded.

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