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The best way to determine the RA/DEC of an object?


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I am interested in knowing how to determine the RA/DEC of an object which I know nothing about, for example if I am observing a comet, which will not displayed on star maps - what would be the best way to get accurate coordinates for it in RA/DEC (bearing in mind I have an alt-azimuth Dobsonian mount)? Would it be best to try and find surrounding stars on a map and interpolate, or is there a program which will give you reasonably good co-ordinates?


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Most of the commercial planetarium programs have facilities to download ephemeris data for comets and hence you can use them to look up the Alt / Az coordinates for your time and location. It is possible that the free ones such as Cartes du Ciel and Stellarium do too.

Failing that, sites like Heavens Above provide finder charts for the brighter comets eg Comet 103P Hartley


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I am interested in knowing how to determine the RA/DEC of an object which I know nothing about

There are 3 ways of doing this.

1. Measure the position of the object with respect to one of known RA/Dec. This is is how astronomers such as Herschel determined the positions of new DSOs they found, describing the object as so much east-west, north-south of a catalogued star. With an alt-az scope (such as Herschel used) this is best done when the object is on the meridian, then E-W is horizontal and can be timed with a watch, while N-S is found from measuring the vertical movement of the scope.

2. Measure the altitude and azimuth of the object, note the local time and observing site longitude, then convert to RA/Dec co-ordinates. There is a standard mathematical formula for this, discussed in another thread: http://stargazerslounge.com/astro-lounge/118784-assigning-co-ordinates-objects.html. It can also be done using a universal astrolabe: The Saphea Arzachelis

3. Quickest but least accurate: plot the object on a star map and read off its co-ordinates.

Edit: just realised that you were the one who started the other thread!

Edited by acey
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