Re: [APML]: varimagnifier/FOCUSING

[Chris Vedeler <cvedeler@ix.netcom.com>]

Let me see if I understand how the B&K Astrofocuser works.

You use the Olympus 1-12 screen in your OM1 and prefocus the device on the cross
hairs of the screen.  Then in the field you simply focus the star through the
camera using the 1-12 screen and the device?

Hmmm, if this is right, then I have done a similar technique using the 1-12, on my
own with inconstant results.  Through an eyepiece in the dark, my eye can still
adjust for the focus slightly as the 1-12 screen is too clear.  In fact, under
magnification the fresnel graiting of the 1-12 makes fine focus confusing, and the
clear center of the screen acts like a magnifying glass making point souce
focusing suspect (take a close look at it if you have one).  The 1-12 is designed
for microphotography, and so being able to see the cross hairs is essential for
exact focus using this screen.  This is something that one can't see in the dark
unless you are shooting the bright planets or the moon.

I have no reason to doubt that what you say regarding the accuracy of this device
is true.  It is just that I have tried similar techniques with limited success.
Maybe my eyes are more buggy than most.  :-)

Chris Vedeler

maxx wrote:

>
> Hi Mark,
> It is obvious from your post that you have never used the Astrofocuser.
> The 1-12 screen has a small split crosshair in the center circle. (if I
> remember correctly) . You "pre focus" on the crosshair (which is etched
> on the bottom of the screen) until it is in sharp focus. Then you are
> parfocal to the film plane. Then you center a medium bright star and
> focus the scope until the star is a pinpoint. You are dead sharp at that
> point. Works every time. No guesswork.
> Regards,
> Maxx