Image Preprocessing Techniques for Astrometry in Adverse Weather

[Sharaf Mohanna]

If the weather was not clear when I took the photo, can astrometry still work on the image?
If the photo contains clouds, how can this be handled in order to estimate the right ascension (RA) and declination (Dec)?
Are there any algorithms that can preprocess the image to reduce blur, noise, or fog, so that astrometric calibration can still be performed?  

[Dustin Lang]

Hi,

For astrometry.net, you usually need 12-15 of the brightest stars to be detected.

Fundamentally, detecting stars is all about signal-to-noise.  In an idealize CCD image, you have "sky" background, which varies pixel-to-pixel, giving you noise, as well as readout noise.  And then the signal from the stars gets spread out into multiple pixels by the Point Spread Function.  Uniform clouds would just reduce the amount of signal.

In the web version, if you have an image with a wide PSF (blurry), you can use the "Downsample" option (default = 2).  With the "solve-field" command, you can use "--downsample"; there's also the "--nsigma" option that says how much signal-to-noise is needed to detect a star - good when there are clouds or noise making the stars faint.

cheers,

dustin

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