On Friday, April 03, 1998 8:28 PM, R. A. Greiner wrote:

> > Well, I seem to be in the market for an electric focuser, and I need a
> > recommendation.
> >
> > I know about the benefits of the NGF-S style focusers, but for travel and
> > backfocus (not to mention cost) reasons, I may want to use the the type
> > that attaches to the existing focus knob on my LX200. Unfortunately, I
> > don't know if there are better and worse designs out there, so.......
> >
> > For specific focusers that you may have owned or used, can anyone answer
> > some questions like:
> >
> > How much does it interfere with smooth manual focusing?
> > How precisely does it start/stop when used with the scope focuser outlet?
> > Does it present a large physical obstruction (ie., how easily can you still
> > reach the focus knob)?
> > Reliability?
> > Construction, fit, finish?
> >
>
> I have used, some two years ago, the Meade focuser that goes on the focus
> knob. It brevents manual focus when it is in place. The motor can be removed
> easily however so that you can manually focus. I found the Focus to be too
> fast even in the slow mode. The slow mode might not work if you use 12 volts
> for your telescope power supply.

The #1206 Meade electric focuser is easily separated into two parts, a
removable gearmotor and a housing. Installing the focuser is very easy.
Slip a gear over the focus knob and tighten. Install the housing and finally,
slip in the gearmotor which contains another gear that meshes with the
gear clamped on the focus knob. I own several focus knob style focusers
from JMI except the latest with integral counter (which by the way should
be 4 digits to record 40-50 turns). Of the older style JMI knob focus motors,
the Meade is a better fit and finish.

As Doc G states above, the Meade electric focuser can prevent manual
focussing when it is in place, however this focuser can be adjusted to allow
controlled slippage of small miniature clutch contained within for manual
focusing. This is accomplished by loosening a small nut (4/40, 3/48 or
metric, I believe) which adjusts the miniature clutch. Add a second nut
(not included) as a stop-nut to hold the adjustment.

As Doc G states, this focuser is somewhat fast in the slow mode.
Residual inertia decreases the ability to achieve fine focus. The Meade
focus jack outputs around 8 & 18 VDC at 18 VDC input and 0 and 12 VDC
at 12 VDC input for the two available speeds.

Those that are interested in this focuser can build my "test circuit" to control
this focuser quite nicely. This simple circuit uses the inertia of the focus
motor to generate current upon release of the button and thus provides braking
as well as variable speed. The "test circuit" is on Ed Stewart's Topical Archives
with details on all needed parts as well as a schematic. I have assumed it's
buildable as I received a note of thanks from a college observatory (I believe),
that built and is using this control circuit. This provides very fine focuser
movement of the Meade 1206 focuser.

See Ed Stewart's Topical Archives:
http://www.austin.cc.tx.us/astro-ES/AstroDesigns/MAPUG/MtrFocus.htm

>
> I have used the JMI for two years on my 12" and for 1 year on my 10". The
> focuser is perfect especially when used with the DRO.
>
> The advantage of a rear focus unit is that the focus function is not amplified
> by the secondary mirror, the mirror can be locked at infinity, and the mirror
> does not wobble about. The JMI is very accurate, has medium to very slow focus
> action, holds focus, does not drift and is almost zero image shift.
>
> There is just no comparison between the two focusing methods in my opinion.
>
> Doc G <http://www.mailbag.com/users/ragreiner>
>

There is a downside to the JMI focuser on my 12" LX-200. The NGF-S is 2.4"
tall with a fully retracted draw-tube which won't allow room for accessories if
one expects a CCD camera, etc. to clear the fork base. While this is not
usually critical in the equatorial mode, be prepared to abort a slew to prevent
slewing the camera into the base on the 12" LX-200.




--
Michael Hart
Husen Observatory

View index by [date] [author] [subject]
Previous message: [M]: Pictorview 6.4b3 and autoguider control, Email address hidden
Next message: RE: [M]: Help playing around with the 416(long), Michael Hart
Next message in thread: Re: [M]: Electric focuser recommendation, Bob Freeze
Previous message in thread: Re: [M]: Electric focuser recommendation, BJohns7764