Re: [APML] Need Help Focusing Telephoto Lens

["Bobby Middleton" <bobm@koyote.com>]

Hi Kevin. If you've got a camera with a removable viewfinder then you can
use a magnifier directly on the screen to get reliable focus with. If not
you've got your work cut out for you unless you try a knife edge. The best
way to do that is with a dummy camera that is known to be calibrated with
the camera you use. That will do away with the questions about focus after
the film is loaded. look at chuck vaughn's description of knife edge here:
http://www.aa6g.org/Astronomy/articles.html

Jerry Lodriguss has some tips on focusing also.
http://www.astropix.com/INDEX.HTM
Bobby Middleton

----- Original Message -----

> Hello, APMLer's, I need some help. I'm fairly new at AP and have been
mostly
> lurking here for several months. My problem is getting consistent focus
when
> I'm piggybacking my 35mm camera with a 300mm telephoto lens. Every once in
a
> great while I'll get some in-focus shots, but most of the time I end up
with
> little donuts instead of stars. I think I've finally mastered tracking, as
> the donuts are nice and round.
>
> Ironically, I usually have no problem with focus at prime focus of my 10"
> SCT. I use a Hartmann mask and that seems to work. With my telephoto lens,
> lately I have been trying focusing on a very distant terrestrial object
> during the day, then firmly taping the lens in place, then checking again
> that the distant object is still in focus. The lens in question is stopped
> down during exposure by one stop to f/5.6.
>
> So, can someone please tell me how I can achieve consistent focus on stars
> with this lens?
>
> As an aside, could someone also please explain a little bit about how
> knife-edge focusing works? I understand the principle perfectly, I think
> (when at perfect focus, a sharp edge should extinguish a star image at the
> film plane instantaneously), but how does this work in practice? Does one
> have to do the KE focusing before loading the film? If not, then how the
> heck do you do it with film loaded? And if so, can you do KE focusing a
> second time without removing the film? And how do you know when the
> knife-edge is right at the film plane?
>
> I am *so* confused.
>
> Thanks in advance for any help,
> Kevin Wigell
>
>
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