Re: [APML] OT: KEF Update Questions

["Steve..." <s-walters@comcast.net>]

Steve,

One refinement that you may know about but didn't mention.

Getting the star to "blink out all at once" is a good start and puts you
very close to focus. Once you can accomplish this, there is a refinement.

Move the KE into the star very very slowly so the star becomes partially
greyed, not totally black. This takes some practice to see. Fine tune the
focus so the star is equally grayed all around the donut. Then you're really
at critical focus (assuming the KE is properly calibrated).

Incidentally, you'll see all the zones in your mirror/system when you are at
this "greyed" position. This is a classic focault test of an optical system.
Due to seeing, the zones will be dancing around quite a bit.

Doing this requires a good smooth slow motion control on declination. Mirror
shift on an SCT means that you want to approach the focus position very
slowly from one side, otherwise when you reverse directions, the star will
no longer be on the KE.

Have fun and Clear Skies!

Steve...

----- Original Message -----
From: "Steve Lindsey" <stevelindsey@earthlink.net>
To: "APML" <astro-photo@seds.org>
Sent: Friday, April 25, 2003 12:38 AM
Subject: [APML] OT: KEF Update Questions


> I tried out the new Mitsuboshi focuser with my SCT and after a bit of
trial
> and error I think I got the idea.  Here's a couple of things I noted with
> some questions:
>
> 1. What I see is actually a donut shaped image with the enlarged star
image
> on the donut and the center blackened out.  I didn't expect the star image
> to look so large but I guess it makes sense since my eye is not at the
focal
> plane.  Is the donut a function of my SCT or the circular KEF
construction?
>
> 2. For dim stars the image was next to non existent for seeing the KE
> pattern.  I'm assuming this was worse with my long focal length scope.  I
> ended up sticking with medium to bright stars to practice on and found
that
> the image blinking out was most distinct on medium brightness stars.  Does
> this make sense?
>
> 3. And a related question...  I had some trouble centering a star so that
I
> could get it in the KEF field of view to even check focus (potentially a
> slight downside of the relatively small circular FOV with this version KEF
> used with a long focal length SCT).  This proved much easier with very
> bright stars, not too hard with moderately bright stars, and impossible
for
> the reason in #2 on dim stars.  I assume it's ok to just slew off the
object
> to a brighter star for focusing.  Sound ok?
>
> Otherwise I think this should work out fine and the device seems very well
> built.  The optional adapter to use with a diagonal turned out to be a
must
> on an SCT.  I'd have been at the chiropractor for sure today with out it!!
>
> Steve
>
>
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