Re: [APML] OT: KEF Update Questions

["Steve Lindsey" <stevelindsey@earthlink.net>]

Good comment Steve.  Yes I was already doing this esp. with a
brighter star as there's a range of uniform "grey out" on th
pattern where I can estimate centering the focus.  I did focault
testing on a 6 in. mirror a while back and this whole procedure
brought back those memories and made the process much more intuitive.
 Too bad I never finished that scope.  So many projects, so little
time.

Steve

>
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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