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The terms Reference star, Check star and Comparison star refer to different types of stars used for specific purposes during the Phoranso photometric analysis.
A reference star is a star with a known and stable brightness, which is used for calculating the magnitude of the Target star(s) present in the FITS image.
Consideration should be given to the criteria for selecting appropriate reference stars:
- Reference stars should be close to the target star and not near the edges of the image where it could be distorted. We strongly recommend to select reference stars within the center half of the image to minimize uncorrected extinction effects.
- Reference stars should be similar in color to the target star. This eliminates the need for color transformations. Phoranso assists you with the selection of appropriate reference stars, as explained here.
- Reference stars should be similar in magnitude to the target star. If you select very bright reference stars, you run the risk of saturation, invalidating the target star magnitude calculations.
- Reference stars should have no nearby companions that could contaminate the photometry apertures.
- Reference stars should be sufficiently bright, i.e. have a signal-to-noise ratio (SNR) of at least 100. In practice this may not always be possible, for instance if your target star is very faint, in which case you may have to select reference stars with low SNR values (e.g., around 20).
- Reference, check and comparison stars should all be well below the saturation level of your CCD/CMOS camera, typically in the range of 50-70% of the saturation level..
Phoranso allows to work with one or more reference stars (>=1). If more than one reference star is used, Phoranso will perform Ensemble photometry. Phoranso uses the convention to color an Aperture Ringset around a reference star in red.
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