Re: [APML] Guiding far from the object

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

Hi Andy -

>Does not the position of the two stars in reference to the Meridian enter
>into the results?

Yes, it does. In fact this whole business of differential refraction is a
complicated function of the track the pair of stars will follow which of
course is a function of your local latitude, and the hour angle and dec of
the stars. The degree of refraction even depends on elevation and wavelength
(due to dispersion).

 >If two stars have the same altitude and are within a few
>degrees of the Meridian there would be very little change in the refractive
>index as they crossed the Meridian.  Near the Meridian there would be
>relatively little change in altitude and therefore virtually no change in
>refraction.

I believe that is exactly right. As you say, the degree of refraction is a
function of altitude, so in the special case of any pair of stars each near
the meridian, their relative separation would remain fairly constant for as
long as they were close to the meridian.

Of course none of this matters if you image from the Poles, where the stars
are all circumpolar and none of them change altitude with time...

BTW, It is possible to guarantee elongated stars even if you guide in the
center of the FOV, polar alignment is perfect etc, if the field is wide
enough, the altitude low enough and the resolution high enough.

Differential refraction is a problem even for the mighty Keck:
http://astron.berkeley.edu/~hroe/diff_refract.html

-P


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