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Re: [APML] Knife-edge focuser
Hi Jim
I have been toying with the idea of using a piece of cigarette foil with
a hole puched in it. I remember from my younger days that with a slight
amount of heat from a lighter that the foil and paper can be seperated
and then the foil applied to a surface much like gold foil. I was
thinking of punching a 1/2" hole in a piece and then applying it to a
piece of glass or plastic. The only catch was whether I would need to
add a bit of thickness to the foil where it touches the inner film
rails.
George Anderson
Montreal Canada
Clear skies and good health
Jim Vineyard wrote:
>
> Hello George,
>
> Pardon my jumping in on this thread. As you know I shoot with
> Pentax's on one of those 100 inch focal length SCT's, so it's got a
> pretty long length "range" of good focus. If you're shooting shorter,
> "your mileage may vary" considerably.
>
> My solution was surprisingly easy in that I bought a pack of those
> old style super thin full-size single Gillette double-edged razor
> blades at the local grocery store. But you only need one edge
> half, or they won't fit. Amazingly, they cut pretty easy though, with
> a decent set of scissors, holding their flat shape real well without
> bending even if you're just sort'a careful. You can lay them flat on
> say, a piece of glass or something, after cutting to insure they
> haven't warped. They're easy to cut in half length wise, and have
> most of the center already cut out for you. Then just trim the ends
> of one half so it will lie on the inner set of the camera's film
> rails. They're really thin but do have a bevel on the razor side, I
> can't remember how I oriented the bevel. But this is not a problem
> since as you'll see later you can always turn them back over.&nbs! p;
> And if you do somehow bend one, hey you've got the whole rest of
> the pack to work with.
>
> Since I deal with that curved focal plane of an SCT, I mounted the
> razor edge off by about 1/3 of the distance "away from center frame"
> toward the edge of frame on the right side. I left about 4 to 5mm to
> the left of the razor "open" for actual knifing operations, and simply
> taped regular "non-clear" scotch tape over the remaining left side of
> the frame for a "rough achieve" focusing screen, from which I then
> slew over to the razor edge. The regular focusing screen is used
> at first to get the chosen star onto the scotch tape. I leave the
> regular old camera's back door shut to protect all this during
> any non-use. And I just lock the shutter open with a regular cable
> lock when in use.
>
> The remaining trick is to keep the blade laying in it's naturally
> "tempered steel perfectly flat" shape, but not totally fixed, to allow
> for some slight thermal expansion and contraction with ambient
> temperature to prevent any bowing. How do you do that? Why the
> well-known "astrophotographer's friend" of course! Carefully cut,
> highest quality, about $7 a roll, placed very carefully around the
> blade's edges so as not to impinge....... Duct Tape.
>
> Always works for me. Never had to change a thing since first use
> some years ago. And remember those "Duct Taped Rocks" once yet again
> in place of the counter weights which you forgot and left at home. I
> know you always remember that one! But you only need little tiny cut
> strips to hold that blade... just right. All my photos were done that
> way. But again, shorter focal lengths, I dun'no.
>
> High quality Duct Tape, never leave home without it! And in a
> nice gallon-size zip-lock bag... keeps it from stick'in on the trunk
> carpet.
>
> Jim Vineyard www.deepskyphotos.com
>
> P.S. Very sorry for the apparent "non-plain text" format. This "more
> secure web based e-mail" now appears not to offer it, at least as far
> as I can tell. Gee... now I just have to go back and read the last
> 1,015 APML posts that I'm currently running behind on right now! See
> you later.
>
>
>
> >From: George Anderson
> >Reply-To: Discussion of Film Astrophotography
> >To: Discussion of Film Astrophotography
> >Subject: Re: [APML] Knife-edge focuser
> >Date: Sun, 14 Dec 2003 00:33:28 -0500
> >
> >Stuart
> >
> >Keep your eyes open on e-bay for "nikon parts" you should find some
> dead
> >bodies showing up at reasonable prices. I picked up a dead pentax
> >me-super for $25 canadian pesos. Did a bit of a hack job to it and
> >removed the pentaprism and modified the guts so that I could swing
> the
> >mirror out of the way with a lever as well as removed the shutter.
> The
> >intent is that I can get the focusing close with the screen and then
> >swing the mirror up and use the knife edge on the back. Most of the
> work
> >is complete several home projects got in the way, just have to add
> the
> >knife edge and then get some clear skies to test it out.
> >
> >George Anderson
> >Montreal Canada
> >
> >Clear skies and good health
> >
> >Stuart Heggie wrote:
> > >
> > > Howard, looks good! Now I need a broken Nikon!
> > > Stuart
> > > ----- Original Message -----
> > > From: "Howard Edin"
> > > To: "Discussion of Film Astrophotography"
> > > Sent: Saturday, December 13, 2003 4:23 PM
> > > Subject: [APML] Knife-edge focuser
> > >
> > > > Over the past month or so I have been writing an article on
> building a
> > > > knife-edge focuser. With the recent threads on focusing I
> finished the
> > > > article up and have published it on my website:
> > > >
> > > > http://www.howardedin.com/articles/knife-edge.html
> > > >
> > > > Slightly different method than other articles. Any comments or
> > > > criticisms are welcome.
> > > >
> > > > I certainly agree with Bill's earlier comment that most
> successful
> > > > astrophotographers use knife-edge focusing (well - except for
> you
> > > > schmidt guys 8-).
> > > >
> > > > Howard
> > > >
> > > >
> > > >
> > > >
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