[APML] Need Help Focusing Telephoto Lens

["Kevin Wigell" <kwemail@twcny.rr.com>]

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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