Re: [APML] Guiding far from the object

[Paul Howell <paul@howell-ltd.com>]

Tony - 

The problem is indeed due to atmospheric refraction, which is a bigger
effect than most people realize. This is also the reason that no one
sidereal tracking rate works, unless it is trained to account for refraction
(like I think, the new ProTrack will from Bisque).

All stars, except those at zenith are of course refracted towards the
zenith. That is, they appear higher than they actually are. The effect is
stronger near the horizon so as a given star rises the effect decreases.
Thus rising stars are continuously slowing down until they cross the
meridian, and then they speed up.

Consider two rising stars that will eventually pass overhead, one higher
than the other. Near the horizon, the refraction will be stronger for the
lower star than for the higher one. This makes them appear closer than they
really are. At zenith, the refraction is negligible and they appear
separated by the correct (larger) amount. In other words, the trailing star
'falls behind' the leading star. The reverse happens as the pair sets. The
consequences of guiding on one of the stars while imaging the other are
obvious. 

Here's a link to some graphs I made that quantify the effect:

http://www.howell-ltd.com/Astronomy/html/Differential%2060.html

Basically if you are within 30 degrees or so of zenith and your guide star
is within a half degree or so of anything being imaged you have nothing to
worry about.

- P


Tony Hallas wrote:

> Matt,
> 
> Jim Riffle once told me thousands of years ago that guiding on a star that was
> not in the central area of the imaging FOV would result in trailed stars no
> matter how well the scope was polar aligned. I didn't believe him... it didn't
> make sense... but he's right. I have taken images with a scope that was dead
> nuts polar aligned, but with a guide star at the edge of the FOV and sure
> enough... trailed stars.
> Does anyone have an explanation for this? I guess it might have to do with
> angular velocities... in a given time a set part of the sky will move a
> greater distance than another part? Like to hear what the more mathematically
> gifted than me can make of this.
> 
> Tony
> 

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