In a message dated 12/22/04 9:57:27 PM, twade@bmi.net writes:
I recently purchased a Mitsuboshi Knife-edge Focusers for my Pentax
67. Tonight was the first night I tried using it (i.e. testing
purposes only). I had it mounted to a Pentax 300 mm EDIF f/4.0
lens. Using a regular lens at least gives you an idea where to
start so I set the "focus" just short of infinity to try to get the
star (i.e. Vega in this case) to appear as stated here:
http://sciencecenter.net/hutech/mitsub/focuser.htm
Unfortunately, I don't really have any idea how the system is
supposed to behave. Is the star supposed to look like a quarter
moon just for an instant while it crosses the knife edge or what?
The star I saw was generally round while it was visible (i.e. within
the FOV but not at the knife edge). Basically what I saw is a
distorted out of focus star. At times, it almost looked like the
picture, although it never was half gone. I did get it to where it
would disappear rather quickly when move away from the knife edge.
I was using the keypad to my AP 900GTO mount. Is this the key (i.e.
you have perfect focus if the star disappears instantaneously when
it moves across the knife edge)? As you can tell, I'm pretty
clueless about what I'm actually supposed to be looking for. Can
the knife edge focuser be used on any lens or telescope no matter
what the focal length (e.g. 45 mm lens)? Any suggestions, comments
or techniques you would like to share would be greatly appreciated.
Thanks in advance,
Wade
Wade:
To focus your 300 f4 I suggest you first have the body on the lens.
Have the lens and body on your mount.
Focus through the viewfinder using a bright star, like Sirius, Capella, Rigel, etc.
Using the mount controller position the star in the middle of the viewfinder.
Remove the body and mount the Mitsuboshi KEF.
Because this KEF is circular, it doesn't matter which way you move the star, but I usually use DEC.
Bring your eye close to the KEF 'eyepiece'.
You should see a 'circle of light' evening illuminated.
Set the mount speed to 12x and, while watching the 'circle of light', press one DEC button.
Stop when the 'circle of light' winks out. With a telephoto lens this will take longer than with a longer focal length instrument.
Reset the mount speed to 1x and bump the opposite DEC button until the 'circle of light' reappears.
Now, repeatedly bump the other DEC button and you should see one of three things.
1. You'll see the knife edge move in from one direction.
2. You'll see the knife edge move in from the opposite direction.
3. You'll see the 'circle of light' slowly appear mottled and grey out.
If you see either number 1 or 2, you need to adjust the focus either in or out and try again.
If you see number 3, you are at or very near focus. You want to see the 'circle of light' turn dark with no straight edge visible.
Depending on the seeing, this can appear mottled or more solid.
The slower you move in DEC the more this is apparant.
If you move faster, it will just wink out and the mottling won't be seen.
This indicates the point of focus.
Remove the KEF and mount the camera body.
Kent Kirkley
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