Re: [M]: OT(a little) Knife edge focuser
| Subject: | Re: [M]: OT(a little) Knife edge focuser |
| From: | Email address hidden |
| Date: | Sun Jun 04 11:52:00 2000 |
When you slide the film on the film plane you are cutting the light beam
with the film like a sliding door. If the light beam is large at this
point (out of focus) the beam will shadow and move left or right
depending on your focus (to far in or to far out of focus). When the
focus is perfect (smallest beam of light cone your telescope can produce)
the film slider (knife edge) cuts the beam which should turn off the
light cone when intercepted and back on when you slide back the film
piece (slider). Better optics makes a small cone and this cone will turn
off and on very quickly. Faster optics with large secondaries is not
that apparent but with practice can be seen easily with experience.
I first get the best focus on a mag. 3 star with the camera view finder
before starting, in this way it will be very close and a minor adjustment
is needed to bring the telescope to camera focus at the best
combination. Practice at a longer focal length to get the hang of what to
look for before moving up to a faster f/ration. i.e. f/10 then try f/6.3.
AV
On Sun, 4 Jun 2000 12:28:58 -0700 "Ron Peterson"
writes:
> Help!
>
> Maybe Im missing something. I made a knife edge focuser out of an
> OM-G
> camera body (my camera is an OM-1). A piece of film sits at the
> film plane
> and is help flat by a piece of clear plastic.
>
> About a half millimeter of film is exposed from under the plastic
> and
> bisects the shutter aperture. Another piece of plastic covers most
> of the
> other side of the shutter aperture with a piece of scotch tape
> behind it for
> rough focus. A millimeter or so of area is left open in order to
> bisect a
> bright star with the knife edge.
>
> When I tried to use this devise last night, I mounted it on my LX50
> and
> trained the scope on a bright star. Looking through the back of the
> focuser
> I saw a bright blob with a central dark spot. I took this to be the
> star I
> was focusing on. As I turned the focus knob the bright ring didnt
> change
> at all and even nudging the scope to cover the ring with the knife
> edge
> produced NO change in what I was seeing. No shadows, no blink out
> of the
> star, even through the full focus range. Nothing.
>
> What am I missing. I thought I understood what I should be seeing.
> I guess
> not.
>
> Thank you for any help.
>
> Ron Peterson
________________________________________________________________
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with the film like a sliding door. If the light beam is large at this
point (out of focus) the beam will shadow and move left or right
depending on your focus (to far in or to far out of focus). When the
focus is perfect (smallest beam of light cone your telescope can produce)
the film slider (knife edge) cuts the beam which should turn off the
light cone when intercepted and back on when you slide back the film
piece (slider). Better optics makes a small cone and this cone will turn
off and on very quickly. Faster optics with large secondaries is not
that apparent but with practice can be seen easily with experience.
I first get the best focus on a mag. 3 star with the camera view finder
before starting, in this way it will be very close and a minor adjustment
is needed to bring the telescope to camera focus at the best
combination. Practice at a longer focal length to get the hang of what to
look for before moving up to a faster f/ration. i.e. f/10 then try f/6.3.
AV
On Sun, 4 Jun 2000 12:28:58 -0700 "Ron Peterson"
writes:
> Help!
>
> Maybe Im missing something. I made a knife edge focuser out of an
> OM-G
> camera body (my camera is an OM-1). A piece of film sits at the
> film plane
> and is help flat by a piece of clear plastic.
>
> About a half millimeter of film is exposed from under the plastic
> and
> bisects the shutter aperture. Another piece of plastic covers most
> of the
> other side of the shutter aperture with a piece of scotch tape
> behind it for
> rough focus. A millimeter or so of area is left open in order to
> bisect a
> bright star with the knife edge.
>
> When I tried to use this devise last night, I mounted it on my LX50
> and
> trained the scope on a bright star. Looking through the back of the
> focuser
> I saw a bright blob with a central dark spot. I took this to be the
> star I
> was focusing on. As I turned the focus knob the bright ring didnt
> change
> at all and even nudging the scope to cover the ring with the knife
> edge
> produced NO change in what I was seeing. No shadows, no blink out
> of the
> star, even through the full focus range. Nothing.
>
> What am I missing. I thought I understood what I should be seeing.
> I guess
> not.
>
> Thank you for any help.
>
> Ron Peterson
________________________________________________________________
YOURE PAYING TOO MUCH FOR THE INTERNET!
Juno now offers FREE Internet Access!
Try it today - theres no risk! For your FREE software, visit:
http://dl.www.juno.com/get/tagj.
