Re: Aperture size to optimise SNR
Posted by karenacollins on May 08, 2023; 7:42am
URL: http://astroimagej.170.s1.nabble.com/Aperture-size-to-optimise-SNR-tp2228p2233.html
Finding the best aperture requires experimentation with different apertures types and sizes, and depends on the distance of other stars from the target star (and comparison stars if they are limited).
If the target star has a nearby relatively bright neighbor, you'd likely get a best result from a hand-selected fixed aperture radius that is half (or slightly less than half) the distance to the nearby star. Use the first AIJ option and the settings above it to define the aperture size.
For well-isolated target stars, I recommend using the "Auto Fixed Apertures from multi-image T1 radial profiles" option in the latest version of AIJ (with the default cutoff threshold of 0.01). This is the third option down. This mode will cycle through all images, calculate a radial profile from each image (and extract a FWHM), and then suggest a very good guess at an optimal aperture for most situations.
The 4th option down, "Auto Variable Apertures from each image T1 radial profile" may give a better result if there are large variations in seeing throughout the image sequence", and the target and comp stars are very well isolated from other stars. This option is the most sensitive to nearby stars and will produce larger variations in the target star lightcurve as the apertures grow and shrink in size as the FWHM in the each image changes.
It is on my to-do list to write a more detailed guide for how and when to make use of the different AIJ aperture options. Keep an eye on the forum for a post in the next couple of weeks.
Karen
URL: http://astroimagej.170.s1.nabble.com/Aperture-size-to-optimise-SNR-tp2228p2233.html
Finding the best aperture requires experimentation with different apertures types and sizes, and depends on the distance of other stars from the target star (and comparison stars if they are limited).
If the target star has a nearby relatively bright neighbor, you'd likely get a best result from a hand-selected fixed aperture radius that is half (or slightly less than half) the distance to the nearby star. Use the first AIJ option and the settings above it to define the aperture size.
For well-isolated target stars, I recommend using the "Auto Fixed Apertures from multi-image T1 radial profiles" option in the latest version of AIJ (with the default cutoff threshold of 0.01). This is the third option down. This mode will cycle through all images, calculate a radial profile from each image (and extract a FWHM), and then suggest a very good guess at an optimal aperture for most situations.
The 4th option down, "Auto Variable Apertures from each image T1 radial profile" may give a better result if there are large variations in seeing throughout the image sequence", and the target and comp stars are very well isolated from other stars. This option is the most sensitive to nearby stars and will produce larger variations in the target star lightcurve as the apertures grow and shrink in size as the FWHM in the each image changes.
It is on my to-do list to write a more detailed guide for how and when to make use of the different AIJ aperture options. Keep an eye on the forum for a post in the next couple of weeks.
Karen
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