Common Imaging Issues
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This is an excellent resource written by /u/spastrophoto
about common problems seen in astrophotography. The original thread can be found here.
Reference Image
Original image of M42 for reference - This image was taken by the HST. The image shows no imaging, calibration, or processing problems.
Trailing
Star trailing example - Tracking or guiding problems during integration causes "streaked" stars in your raw images. This is inherent to the rig, so the only fix is to improve your tracking/guiding.
Hot Spot / Vignetting
Vignetting example - This is a symptom of an image that has not been "FLAT CALIBRATED". To fix this, take flat frames during your imaging session. You can read more about these in the stacking section in the wiki index.
Flat Over-Correction
Flat over-correction example - A flat calibrated frame that is over-corrected will typically show a ring-like vignetting effect. This is often caused by flats that are improperly shot or stacked. Fix: Make certain that your flats aren't over-exposed. When making a master flat, use an average combine; not sigma or other clipping.
Noise
Excessively noisy image - Noise is a symptom of poor signal to noise ratio (S/N). Fix: Expose longer and/or stack more frames. Ensuring you stack with Darks, Bias and a suitable number of lights can all help to reduce noise in your final image.
Noise Reduction
Excessive noise reduction example - In a quest to obliterate any trace of noise, it's tempting to run a noise reduction algorithm. It's quite easy to over do it and end up damaging the image. Over-reducing noise results in a weirdly smoothed "painting" of the object. Care must be taken to only apply noise reduction as sparingly as possible and only to the faintest/noisiest areas using masks.
Star Reduction
Image with flawed/excessive star reduction - Do not apply star reduction routines to non-star areas like nebulas or the background. The symptom of star-reduction applied to non-point sources is the strange filament-like or lacy look of the nebula. Also, over-reduction can eliminate all the faint stars and, if they are in front of nebulosity, it will look very bad. Fix: If you feel you absolutely must use star reduction, go very easy on it and use a mask which only lets the stars be affected.
Over Sharpen
Over-sharpened example - Unsharp masking and deconvolution can easily be taken too far. The best image looks sharp but does not look sharpened. The goal is to increase the visibility of real structures without introducing artifacts or clipping the histogram.
Clipped Histogram
Clipped histogram example - Setting the black point too high and the white point too low results in an image with excessive contrast and loss of data.
Diffraction Spikes
Artificial diffraction spikes - Imaging with a Newtonian will cause diffraction spikes on bright stars. This is normal and will look natural in images. Do not add fake diffraction spikes to your images. Just don't. They don't look natural.
