Re:Limiting Magnitude

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Posted by Martin Mobberley at 16:02 on 2013 Dec 08

You’ve got most of it right Eliot, yes.Seeing is basically down to turbulence….which determines how bloated the stars will appear in long exposures…. Of course, if your drive is poor and the stars are drifting around by arcseconds during the exposure, this will have a similar effect in limiting the magnitude. Bad light pollution will mean the signal-to-noise ratio gets worse….so 20th mag objects are sitting on a wall of light pollution… For example, you may find that objects on the limit of detection are only 1% brighter than the background sky, so given that the background sky is noisy and noise is random, the signal gets swamped by the noise. Without filters the stars can spread out, yes, but this rarely becomes a factor with reflectors, although for planetary imaging filtering is usually essential due to the spreading of light due to dispersion and the desire for sub-arcsecond resolution. Some telephoto lenses produce bloated stars and do need filtering to cut out violet haloes. Filters are desirable for photometric work on bright objects, but dim the light so much below mag 15 or so that they make the image very noisy…..OK if you have a huge telescope and like long exposures, but they do hammer the limiting magnitude.Clearly you are already getting to grips with letting Astrometrica allow for the object’s predicted motion…….Martin