Originally posted by Dr. Zee
For the more experienced star photographers here, what is the purpose of taking multiple shots and stacking them? Also, does anyone have techniques for getting sharp focus on a star? They seem to confuse the AF, and the difference between soft and sharp seems to be less than a mm on the throw.
The main purpose of stacking is to increase the Signal to Noise Ratio (SNR). The series of shots is shifted and rotated to get the stars (=signal) aligned and on top of each other and the "flattened" down to one single picture. In this process the signal persists or is even amplified while the noise is reduced significantly (by effects of math -> median, averaging, etc). Stacking enables you to use high ISO values and shorter exposure times especially for dark objects (e.g. nebula). A single shot of 10 s at ISO 800 might not catch enough photons to show the object. A single shot of 10 s at ISO 3200 would be too noisy to be used. But the stacked result of 100 such shots would show only little noise and sharpened stars, you see objects that never could be noticed in a sigle frame. take so called "darks" with the same camera settings and with lens cap on at the end of the series. Darks contain the sensor noise and are used by the stacking software to further reduce noise. A good and easy-to-use freeware is DeepSkyStacker. Shoot in RAW, pass the shots as 16 bit TIFF-files to the stacking software. Instructions on how-to-stack are a lot in the web.
Stars are tricky in matters of sharp focus. Good focus on stars is not possible with AF. Better use manual focus and live view. Being in the live view mode use the 10x loupe to get into good focus. Close the aperture one stop. Do test shots, use zoom to control the results and iterate until stars are pin-point-sharp. Use remote control and mirror lockup to avoid vibration blur. It is very important to turn of the Image Stabilisation (if it is not automatically turned of by camera)!!