Re: [APML] Need Help Focusing Telephoto Lens

["cherrio" <cherrio@shocking.com>]

Kevin
Go here for a great web site on everything you ever wanted to know about
astrophoto.
http://www.astropix.com/HTML/I_ASTROP/TOC_AP.HTM
Ray Rochelle
----- Original Message -----
From: "Kevin Wigell" <kwemail@twcny.rr.com>
To: "APML Discussion List" <astro-photo@seds.org>
Sent: Wednesday, December 12, 2001 2:52 PM
Subject: [APML] Need Help Focusing Telephoto Lens


> 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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