Raw Data Quality

The best way to produce good images is to have excellent raw data. Capturing good data requires patience and practice. The following steps are mandatory:

  1. Ensure that the images have good focus, sampling, and guiding.

  2. Calibrate your images. As a minimum subtract a dark frame. Even better, use dark subtraction and flat-fielding, and combine multiple calibration frames to ensure the best signal-to-noise ratio. For very deep exposures, the accuracy of the flat-fielding can be the limiting factor for sensitivity, since the target will be much fainter than the sky background.

  3. Expose for a long period of time. Modern CCD and DSLR cameras can easily record images of 300 seconds without saturating out from dark current. Capture as many of these frames as possible and stack them. Stacking has the advantage that you can eliminate bad pixels (airplanes, cosmic rays, etc.) or entire bad frames (wind gusts, rough spot in the worm gear, etc.) from the stack.

  4. Shorter exposures (30-60 seconds) can be used if you are not autoguiding and have trouble with trailed images. Under suburban skies you may not even notice the extra contribution of read noise, since the sky background contributes photon shot noise. Unfortunately imaging with narrowband filters will generally require longer exposures, since the read noise may be large compared to the photon shot noise.

  5. When using the LRGB color technique, use more exposure in the luminance band than for the color bands. Increase the exposure in any color band that has poor sensitivity relative to the others (e.g. some cameras have poor blue sensitivity). Consider binning the color exposures to increase sensitivity, bearing in mind that this may actually make things worse in high sky brightness conditions if you grossly undersample.

  6. Most cameras have a number of "hot pixels" that have much more dark current, and dark current noise, than the rest of the array. These can be eliminated very easily; use the Dither capability during image capture. This moves the hot pixel slightly relative to the target between each exposure. Then when you stack the individual frames, use Sigma Clip or SD Mask to eliminate the outlier pixels. This very effectively suppresses the hot pixels, leaving a much cleaner and lower-noise final result.