Re: [APML] Need Help Focusing Telephoto Lens

["Anna L. Scott & Harry L. Juday" <juday1@gte.net>]

Kevin:

There are several possible reasons for this, the prime one being focus shift
of telephoto caused by the heavy lens elements (I have this problem with a
Nikkor 300 mm).

You should be focused at infinety, unless your lens will focus beyand, as
some do.

Also, perhaps everything is not full tightened down.

What type of camera and lens are you using?

What type of focus screen?

Are you locking the mirror up?

How long are your exposures?

How are you tripping the shutter, and releasing it? (the cable release might
cause a problem).

How steady is your mount?

Many things could be affecting the results.

Do you get good results vs bad one when you are imaging at particular
altitudes?

The list may need more info.

Harry L. Juday


----- Original Message -----
From: "Kevin Wigell" <kwemail@twcny.rr.com>
To: "APML Discussion List" <astro-photo@seds.org>
Sent: Wednesday, December 12, 2001 5:52 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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