Re: [APML] The perfect astrophoto book (Was: North American Nebula (or dodging aurora))

["westergren" <westergren@netzero.net>]

Gene,

Field rotation gives stars away from the guide star an apparent arc with the
center on the guide star.  ADR also holds the guide star as a point by
definition, and refracts the star image up or down relative to the local
horizon.  The worst case for ADR is to use a guide star that is locally east
or west of the target (at the same local altitude as the target).  This case
gives elongated stars that look exactly like field rotation.

The way I control or eliminate ADR is to pick a guide star that will have
the same altitude gain or loss as the target during the exposure period.  In
the simplest case, if the target is at the meridian, pick a guide star that
is above or below the target.  It gets more complicated if you want to shoot
a target well before or after meridian crossing.

Both FR and ADR are functions of exposure time and FOV.  ADR is also
strongly influenced by local altitude.  The length of elongated star images
is also a function of focal length.  Shooting with a Schmidt Camera at f2 or
less for less than 30 minutes probably won't see ADR.  Try shooting a
piggyback camera with 180 telephoto off axis from the guide scope more than
10° for an hour below 45° altitude to the south and see what happens. It's
not very pretty.  This is why this kind of shot is not recommended.  It is
always best to have the guide star as close to the center of the target as
possible.  But then there's mirror flop, structural deflections, and .....

Don


> > A subject that hasn't been discussed much is Atmospheric Differential
> > Refraction (ADR).
>
> Gene wrote:
>
> That's because it generally isn't a issue.  Large (8" or larger)
> wide-field instruments are rare plus most people tend to shoot at higher
> altitudes above the horizon.
>
> But even still, I for one am curious about your solution to this
> problem.  If I understand the effect you are talking about I always
> assumed it resulted in stars elongated parallel with the center,
> not perpendicular to it as with field rotation.
>
> > I studied the
> > causes of ADR and now have a way to control it.
>
> Gene Horr




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