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Re: Distance calculation for lots of stars [message #91424 is a reply to message #91423]

Mon, 13 July 2015 11:49

matthewportman93
Messages: 1
Registered: July 2015

Junior Member

On Monday, July 13, 2015 at 2:36:32 PM UTC-4, wlandsman wrote:
> A couple of comments:
>
> You are not using the correct formula for the distance on a sphere. (Near the pole, stars can have very different right ascensions but be separated by less than an arc second.)
>
> If stars are separated by more than 1 arc second in declination, then they must be more than 1 arc second apart. So a first step is to just search in declination, and you only need the full distance computation when the declination differs by less than 1 arc second.
>
> Jeremy Bailin's program matchall_sph.pro uses many such tricks to speed up the processing. It is available in his tar file jib-1.2.tgz available from
> http://www.simulated-galaxies.ua.edu/jbiu/ --Wayne
>

Thanks Wayne, I'm looking at matchall now.

I apologize, I didn't note that I have already taken into account the position on the sphere. The code I posted is a gross over generalization of the bigger picture!

I've run into some issues with very close stars but the reduction comes later anyway. I'll implement the declination change and check that against matchall.

Matthew

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[Message index]

		Distance calculation for lots of stars By: Matthew on Mon, 13 July 2015 10:45
		Re: Distance calculation for lots of stars By: wlandsman on Mon, 13 July 2015 11:36
		Re: Distance calculation for lots of stars By: matthewportman93 on Mon, 13 July 2015 11:49
		Re: Distance calculation for lots of stars By: Jeremy Bailin on Thu, 16 July 2015 09:08

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