Re: [APML] Focusing OM-1 - but what about the Nikon F2?

[Duncan <dmunro@sfu.ca>]

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


Wei-Hao Wang wrote:

>Hi,
>
>I agree with Bill.  Let's look at some real numbers.
>
>Suppose we are using a Traveler-like telescope: D=100mm, f=600mm.
>The diffraction disk on the focal plane has a diameter of about 8 micron 
>( =0.008mm).  Under 2" seeing, the seeing disk diameter will be 6 micron.
>Thus the smallest spot size on the focal plane we can have is about 14 
>micron and we don't want the focusing error to create a spot size larger
>than this.  How good shall we control the position of the focal plane?
>It's the spot size multiplied by focal ratio, 14 micron *6 = 0.08mm in this 
>case.
>
>Such an accuracy might be achieved by a helicoid focuser, or a micrometer 
>attached onto a rack & pinion focuser.  I couldn't imagine a printed focus
>scale can do this.  This is why a knife-edge device (or a high magnification
>microscope) is needed.
>
>My $0.02.
>
>Cheers,
>Wei-Hao
>
>
>On Tue, 16 Nov 2004 14:56:37 -0600, William R. Mattil
><wrmattil@ix.netcom.com> wrote:
>  
>
>>Duncan Munro wrote:
>>
>>    
>>
>>>If your scope has fixed optics its quite a simple matter to create a
>>>focus scale on your printer, paste it on your focus knob, and
>>>calibrate it for temperature. Then you don't even have to bother with
>>>a knife edge, and unlike a knife edge you can adjust the focus
>>>continuously and effortlessly throughout the night. You could even
>>>adjust focus during the exposure, if you have a motorized focuser with
>>>a smooth movement. Try that with a knife edge...
>>>
>>>      
>>>
>>Duncan,
>>
>>I wouldn't waste my time in trying. The tolerances for focusing at f/4
>>simply preclude using such a method as you describe. But you may however
>>use any method you wish. But I think that results speak for themselves.
>>There is a reason everybody ends up using a knife edge.
>>Regards
>>Bill
>>--
>>William R. Mattil  :  http://www.celestial-images.com
>>
>>    
>>
>
>  
>

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