In a message dated 11/17/04 9:07:04 PM, dmunro@sfu.ca writes:
Wei-Hao, thanks for the calculations.
My NGF-SE focuser (for example) moves the focal plane about 8mm (8000
microns)with a 360 turn of the focus knob. So one degree of rotation =
22 microns. According to your calculations I need to be able to control
the focus knob to about 3.5 degrees of accuracy (3.5deg=77microns). If
I take a 150mm peanut butter jar lid and glue it to knob I now have a
focuser with a circumference of about 500mm. If I paste a printed scale
with 500 subdivisions on the the lid, I now have a scale accurate to
.72 degrees or about 16 microns of focal plane movement. I have no doubt
that I can move the focuser in .5 mm increments so I can effortlessly
control the focal plane position to about 8 microns, or 10 times the
required accuracy. I'm guessing that most R+P focusers will have
similar amount of focus travel per degree of focus knob rotation, but
even if they move the focus position by 100 microns per degree it should
still be quite easy to control the focus to the accuracy required.
Logically, even with a knife edge focuser you must still be able to move
the focuser in small enough increments to achieve consistent focus. If
can achieve consistent focus, then it follows that you can make a simple
dial indicator to allow you to repeat the process to the required degree
of accuracy, ergo, the knife edge focuser in not needed. All that is
needed is a simple indicator to allow you to control the focus position
to the required accuracy.
cheers
Duncan
Sometimes you can lead a horse to water and still can't get them up on skiis!!!
Kent Kirkley
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