Re:DSLR and the Horsehead

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Posted by Graham Relf at 09:49 on 2013 Dec 19

Grant, A good question. I did NO flats or darks. I rarely do.This is another thing which I think puts beginners off. Yes, you must do those things for photometry but otherwise there is no need (with today’s cameras – and mine is 5 years old now). I know this attitude will be controversial.My method takes into account the inevitable fact that the unguided mount is inaccurate. That makes the camera’s fixed pattern noise and defective pixels wander around among the sequence of star images. So when they are stacked those problems are watered down by a factor equal to the reciprocal of the number of exposures taken. I find that is sufficient – except when doing photometry.Of course the mount must not be so inaccurate that a star image is elongated by more than the radius caused by atmospheric turbulence. That determines the maximum length of each exposure. My HEQ5 fits the bill fine for exposures of half a minute and sometimes 1 minute. In suburbia you cannot do exposures as long as that at high sensitivity anyway without complete fogging. (But the Horsehead cannot be photographed from suburbia because it is below the background level. I have tried.)