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Subject: Re: [M]: confused about zero-image shift
From: Bill Arnett
Reply To: mapug@shore.net
Date: Tue Oct 22 01:30:37 1996

At 11:53 PM -0700 10/21/96, R. Agu. wrote:
>Mark & Cheryl Buettemeier wrote:
>>
>> All the recent talk about JMI NGF-S and other electric focusing devices
>>has me
>> confused. At least a couple of people have claimed that installing one of
>> these on their LX200s has eliminated focus shift. Since focus shift is a
>> matter of the primary shifting or tilting if you will, how does adding a
>>motor
>> to the focus knob fix this? It still moves the primary in and out the
>>same as
>> when I use my hand to turn that little silver knob. So what's the magic? I
>> don't get it. Moving the mirror is moving the mirror no matter if it's hand
>> or motor doing it. Also, there seems to be some play in the focus
>>system. If
>> I change from focusing in to focusing out, there's a slit amount of
>>turning I
>> must do before the images appears to begin to change. Again, I don't
>>see how
>> a motor makes this any better.
>>
>> --Mark
>
>
> Its an external electronic focuser that mounts to the visual back
>similar to a refractor. The main focus knob is still usable, but only
>required for course focus. The enternal focusing mirrors dont move in
>this manner.
>
> Ralph

Ralph is correct about the JMI NGF-S. But (unless I'm confused, too) JMI
also makes a product called "MotoFocus" which does fit on the focus knob
the same as Meade's electric focuser. I have a great deal of difficulty
focusing my 12" LX200 and I've been thinking of getting one of these
puppies. Does the JMI MotoFocus work any better than the Meade one? Does
it have the dynamic braking to prevent overshoot? If you don't use the
LX200's keypad to control it how to you avoid tangling the cords?

---
Bill Arnett              billa@znet.com           http://www.seds.org/billa/

  "I know that I am mortal and the creature of a day; but when I search
   out the massed wheeling circles of the stars, my feet no longer touch
   the earth, but, side by side with Zeus himself, I take my fill of
   ambrosia, the food of the gods." -- Claudius Ptolemaeus (Ptolemy)