> This does bring up another issue that I have never understood. What is
the
> point of a parfocal eyepiece?
>
> The goal is to have the image focused at the film plane in order to get a
> sharp picture. That means you focus the telescope so that the image at
the
> camera is sharp first. You don't adjust that focus at all prior to taking
> the expsure or you'll blur the image. Once the image is focused, then you
> slide the 201 in and/or out of the off axis guide port until it achieves
> maximum focus. Correct? Seems to me, then, that a parfocal eyepiece may
> not be of much use since you have to slide both it and the 201 in/out to
> find focus. Ultimately doesn't it come down to finding the exact focus
> point on the 201 and then placing a parfocal ring on it, rather than
> placing the ring on an eyepiece. If you place the ring on the eyepiece,
> wouldn't you still have to slide the 201 in and out to find the sweet
spot?
> One thing you won't do is adjust the focus of the telescope in order to
> get the 201 into focus, because that would cause the film plane to lose
> focus.
>
> Am I right on this? If so, why even make a parfocal eyepiece?
>
> Bill
This depends on whether the focal plane of the camera, is above the 'stop'
on the 1.25" barrel, or below it. Most eyepieces, have their field stop
close to this stop, or slightly below it. Most cameras, have the CCD,
significantly above the stop. Hence to make the to devices 'parfocal', the
eyepiece as to sit higher in the eyepiece holder, and the ring is attached
to the eyepiece. Now many OAG's, have the tube that holds the guide camera
itself adjustable. So you can focus the main camera, then adjust this tube,
so that the guide camera is also in focus _sitting fully down in the guide
tube_. Then fit the eyepiece, and adjust it up, till it too is in focus, and
tighten the ring there.
In the case of a system without adjustment on the guide tube, you would need
_two_ rings. One fitted to the guide camera, and set so it is parfocal to
the main camera, and another for the eyepiece.

Best Wishes

> -----Original Message-----
> Sent: Wednesday, April 21, 2004 10:01 AM
> Subject: RE: [M]: 201XT Problems...
>
> No, the 12mm I used sat about 1/16"-1/8" higher in the guideport than
> the 201. The Meade 9mm IS supposed to be parfocal, a 10mm will be
> close.
>
> Alan
>
> > Does anybody knows if the MA 12mm with reticle eyepiece is parfocal
> > for the
> > 201XT? I ask this beacuse i tried last evening to focuse the 201 and
> > i found out
> > that the focus maches exactly the focus of that eyepiece.

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