[Kstars-devel] 1e8 stars in KStars

[Jason Harris]

Hi Akarsh,

On Sep 9, 2008, at 12:26 AM, Akarsh Simha wrote:
> What is the reason for such tiles? Are we seeing them in KStars too?
>
> Whenever 'V' is undefined, I use B-V = 1.6. There are many such cases,
> so you might see a large density of reddish stars. But I haven't
> noticed sharp edges.
>
I can see such an edge in the screenshot you attached.  I've attached  
a gimp-ed version to highlight the edge: left of the yellow line, the  
average star color is noticeably bluer.

> Are these tiles (in density / colour) a result of telescopes that
> create the catalog capturing the sky tile-by-tile?

Yes, exactly, they're catalog artifacts.  The catalog is constructed  
by measuring the brightnesses of stars in images; this is a  
complicated, imperfect process requiring many calibrations to get  
reasonable results.  The images were surely taken over many different  
nights (probably over many years), during which weather conditions and  
instrument properties varied in ways both subtle and dramatic.  What  
we are seeing are the edges of these individual images on the sky.   
Small mismatches in the limiting magnitude of two adjacent images lead  
naturally to a discontinuity in the stellar density at the boundary.

I'm actually surprised to see that the artifacts are so prominent in  
the screenshot Mederic posted.  Hopefully, we will only see the subtle  
effect I noticed in your screenshot.

regards,
Jason

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