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Landolt Standards: Imaging before Astronomical Twilight?

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Landolt Standards: Imaging before Astronomical Twilight?

Hello! Before each night's run, I typically image a Landolt standard field at high and low altitudes in the unlikely event that such would be needed for future reference and for my own education.

    I was wondering, with ensemble photometry, for Landolt fields in particular (or imaging runs in general) is it OK to start imaging as soon as it is dark enough to obtain an image of the field or should I wait till astronomical twilight for all images? Would photometric and standard magnitude and color values be skewed when computed with images taken before astronomical twilight? Thank you and best regards.



HQA's picture
Twilight imaging

Hi Mike,

I usually start my imaging at Nautical Twilight (sun at -12deg), and on occasion will image in brighter skies, especially opposite the solar point.  To first order, it is a signal/noise issue.  If you are working with bright stars, then a brightish background doesn't add much noise beyond that found in the Possion noise of the star itself.  In many ways, this is similar to imaging on full moon nights.  Twilight sky is slightly different (for example, the illumination of the sodium layer at sun elevations around -10, and the obvious gradient away from the solar point).  If you can do your high airmass observation in the opposite direction of the Sun, that would be best.  I could usually extend my night by 15-30mins by doing bright stars in bright skies.


Thank you! Mike

Thank you!


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