Re:Limiting Magnitude

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Posted by Martin Mobberley at 13:13 on 2013 Dec 05

Eliot,As no-one else has answered your question, I will have a go…..I think you have sort of answered your own query as you are detecting stars of 18th to 19th mag near the limit and so any 20th mag asteroids will be just beyond the limit. I’ve imaged quite a few faint objects over the years with my Celestron 14 and other telescopes and while I have reached as faint as magnitude 22 under exceptional skies, with long exposures, it becomes much harder to extract objects from the noise beyond mag 19, when the signal is only a few percent above the noisy sky background…. Even with an unfiltered exposure and a 14 inch aperture, getting fainter than mag 19 or so becomes quite a problem unless the sky is really dark and crystal clear and the tracking and focusing are spot on. It may be worth you having a look at Peter Birtwhistle’s web site: is the undisputed master at tracking down faint asteroids and his website shows what can be done with a large aperture from the UK. Of course, any kind of filter will hammer the limiting mag…a V filter can knock a magnitude and a half off the limit quite easily. Also, while an ephemeris might say an object is mag 20, there is often considerable error in the values given by planetarium packages….the phase of the asteroid plays quite a role too. A lot of the keenest imagers of faint objects use Guide 9.0 to tell them where asteroids and comets are and how bright they are too….it is not a flashy graphics package, but it is accurate and very modestly priced too.So, yes, I would say you are close to the limit of your equipment on an average UK night, but mag 20 is certainly possible if the night is crystal clear and focusing is perfect and tracking excellent, with good seeing. An image scale of around 2 arcsecs per pixel is often quoted as the optimum sampling value for getting faint stars recorded without the sky background swamping them.You ask how can images be aligned on something you cannot see? If you have Astrometrica there is a Tutorial in the help section which explains how to track and stack images allowing for the object’s predicted motion….It is Tutorial number III in my version of Astrometrica…..Martin