Re: [APML]: Telephoto lenses

[Chuck Vaughn <aa6g@aa6g.org>]

Michael Covington wrote:
> 
> Lenses longer than 50mm should always be focused on the actual stars.
> Once you have done this you can use a piece of tape to record the position
> of true infinity focus, but if the lens is really long (over 200mm),
> you should focus every time, in any case.
> 
The focus on my Olympus 350mm-f/2.8 is so critical that I now have a 
temperature versus focus position chart for each filter. When the 1.4
extender is added there is essentially _zero_ focus tolerance which
makes it nearly impossible to get consistent focus.

There's a dilemma I've never been able to solve with this lens. There
is no way I know of to focus through the lens with an H-alpha filter 
in it. Since you can't see through the filter you can't knife edge it
and
you can't use a substitute clear filter because it doesn't focus at the
same place for white light as for red.

To give you an idea of just how critical the focus is, at 350mm the 
focus ring must be set to better than .01" and at 500mm better than
.005".

Some extreme measures might be to permanently mount a micrometer on it
or use a CCD camera at the focal plane before putting in the film.

Chuck  <aa6g@aa6g.org>