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Photometry Settings

Default Photometry Settings are used by the Photometry. It is designed to be as robust as possible, but you need at least to understand how those settings are defined. When you select "Photometry Settings", the window below will pop-up :



The two first lines are concerned with the signal you want to measure : star's center and radius (all are set in pixels, even if the scale is correctly set up before). Forcing the coordinate of the star might prevent you from random errors if you want to measure the flux of the same star in different images. However, be careful that your images need to be calibrated correctly so that the star is always at the same position in pixels. This is never the case in practice, the pointing is never the same from one exposure to another. Hence, in practice, this procedure computes the pixel flux barycentre and is quite robust. Unless, you have a very special configuration (e.g. you want to measure two very close stars), you do not need to force the star's centre. Forcing the star radius is more needed in practice, even though we do not entirely understand why. With ground-based observations, the FWHM (Full Width Half Maximum) of the PSF (Point Spread Function) varies as a function of time, hence from one exposure to another. In principle using the 'Auto' function should be the best option as the star's radius should be proportional to the FWHM. For exercice on transit based on spatial Spitzer data, we do not expect that the FWHM varies as a function of time as the observations are not affected by the atmosphere and its turbulence. However, the auto star radius varies as a function of time. Hence, it is necessary in this case to force the star radius so that the same applied to the whole image series (each image observed in the same observing conditions).

The two last lines are concerned with the noise you have to expect : it is computed using the same coordinate center, but within a larger radius. You may even force the sky value if you know it already, if you do not trust the way it is computed by the software, or if you are not interested in it.

Description of the automatic procedure

The position of the star is given by the position of the mouse as you click

The radius of the star.....

The radius of the sky...


How you shoud set up the star radius, accounting for the PSF of the instrument

The objectif is to integrate all the light coming from the star, with a minimum contribution from the other sources of light (sometimes the moon...) or instrumental noise.

The star is usually a point source. Yet, its light is spread over a given number of pixels - almost isotropically - due to the response of the instrument. This response is called the point-spread function. To have an idea about the extent of this response, you may proceed as follows :

     Draw a line along the star (the yellow line below). Then, plot the profile. You should end up with something similar to this (note, if your object is close to a strong source of light, as a galaxy in the SuperNovae exercice for exemple, you may see this feature on top of an additional slope. The choice of the star's radius is then a bit more difficult...)


You may then set the radius of the star to half the width of the signal (in pixels)



Once you have set the photometry, simply close the window...